absolute magnitudesearch for term

The apparent magnitude that a given star would have if that star were placed 10 parsecs, or 32.6 light years, away from an observer. Absolute magnitude is a measure of the intrinsic brightness of an object

absorption linesearch for term

A dark line in a spectrum that represents the absorption of energy at a particular wavelength of light. Each element on the periodic table absorbs energy at unique wavelengths. Astronomers identify which elements are present in an object, and other information about the object, by measuring these absorption lines.

accelerationsearch for term

The rate of change in velocity of an object.

accretion disksearch for term

A swirling disk of gas and/or dust orbiting a star or black hole. The material within the disk may generate heat from friction and glow.

active galaxysearch for term

A "hyperactive" galaxy that emits more energy than the sum of the individual stars within it. These galaxies emit energy at wavelengths across the electromagnetic spectrum, from radio to X-ray, and they have powerful, concentrated, non-thermal energy sources at their cores. Some are also unusually bright at radio, infrared or X-ray wavelengths compared to the Milky Way. Three types of active galaxies are Seyfert Galaxies, quasars, and blazars. Many active galaxies are also radio galaxies.

alt-az coordinatessearch for term

A system of coordinates that uses altitude and azimuth to locate objects in the night sky. A telescope that uses alt-az coordinates is said to have an alt-az mount. Altitude is an up-down measurement, while azimuth is a left-right measurement.

altitudesearch for term

The measurement, usually in degrees, of an object's apparent height above the horizon. An object on the horizon has an altitude of 0 degrees, and an object at zenith has an altitude of 90 degrees. Altitude and azimuth together are one of several types of coordinate systems that astronomers use to locate astronomical objects in the sky.

Andromeda Galaxysearch for term

The closest spiral galaxy to the Milky Way that is visible in the northern hemisphere, and the most distant object that can be seen with the unaided eye. The Andromeda Galaxy is 2 million light-years away and is located in the constellation Andromeda, the Chained Maiden.

Angstromsearch for term

A unit of length used by astronomers and physicists to measure the wavelength of light. One Angstrom equals 0.0000000001 (10^-10) meter, or about the size of an atom. The symbol for Angstrom is Ã….

annular eclipsesearch for term

An eclipse of the Sun in which the Moon is slightly farther away from the Earth than in a total solar eclipse, so it appears too small to completely cover the sun's disk. The result is a ring of light (an annulus) around the moon at the peak of the eclipse.

Antarctic Circlesearch for term

On the Earth, the small circle located at 66.5 degrees south latitude, which is 23.5 degrees latitude north of the Earth's south pole. Below this latitude, the summer Sun never sets, and the winter Sun never rises.

antimattersearch for term

A form of matter that is composed of particles that exhibit opposite quantum mechanical properties from particles of normal matter. As an example, positrons are the antimatter particles that correspond to electrons, which are made of regular matter. A positron is simply an electron with a positive, rather than negative, charge. When matter and antimatter particles encounter each other, they annihilate to become energy according to Einstein's famous equation E=mc^2. Pairs of matter and antimatter particles can also be created from energy. Most of the matter in the universe is normal matter; there has been very little antimatter present in the universe since the Big Bang.

aphelionsearch for term

The point of greatest orbital separation between a planet and the Sun. At aphelion, the planet experiences its minimum orbital velocity around the Sun.

apogeesearch for term

The point of greatest separation between the Earth and an orbiting satellite, including the Moon. At apogee, the orbiting satellite experiences its minimum orbital velocity around the Earth.

apparent magnitudesearch for term

The magnitude of a star or other celestial body as measured from Earth. Apparent magnitude depends upon the intrinsic brightness of the object and on its distance; that is, near-by objects appear brighter than more distant objects of the same intrinsic brightness.

Arctic Circlesearch for term

On the Earth, the small circle located at 66.5 degrees north latitude, which is 23.5 degrees latitude south of the Earth's north pole. Above this latitude, the summer Sun never sets, and the winter Sun never rises.

asteroidsearch for term

A small, irregularly shaped rock of iron and nickel, many of which are located in the Asteroid Belt. The largest asteroids are Ceres (900-km diameter), Pallas (500-km diameter), and Vesta (500-km diameter).

Asteroid Beltsearch for term

A ring of asteroids containing perhaps thousands which measure several kilometers in diameter, and millions of which measure several meters in diameter or smaller. The asteroids travel in orbits between 2 and 3.5 A.U. from the Sun, locating them between the orbits of Mars and Jupiter.

astrologysearch for term

The art of studying the relative positions of the Sun, Moon, planets, and constellations in the zodiac in order to explain and predict events on Earth. Not to be confused with astronomy.

Astronomical Almanacsearch for term

A book of astronomical facts and tables published annually by the United States Naval Observatory. Astronomers use the information, for example, to locate stars, planets, and asteroids, and to forecast eclipses and lunar phases.

astronomical unitsearch for term

A unit of length used by astronomers to measure distances in space. One astronomical unit, or A.U., is equal to the average distance between the Earth and Sun, which is 92,955,806 miles or 149,597,870 kilometers.

astronomysearch for term

A scientific discipline devoted to the study of the non-terrestrial universe. Not to be confused with astrology.

atmospheresearch for term

The layers of gas that are gravitationally bound above the surface of a planet, moon, or outer layers of a star.

atomsearch for term

The smallest part of an element of matter that retains the basic characteristics of the element. An atom consists of a tightly packed nucleus composed of protons and neutrons surrounded by a "cloud" of electrons that arrange themselves in an orderly pattern according to the rules of quantum mechanics. Atoms with equal numbers of protons and electrons are electrically neutral; ions are simply atoms that contain more or fewer electrons than protons. Atoms are very small -- much smaller than the wavelength of visible light. The simplest and most abundant atom in the universe is hydrogen, which contains one proton and one electron.

aurorasearch for term

Beautiful ribbons of light caused by the interaction of high-energy particles in the solar wind and the Earth's magnetic field. These are common near Earth's poles, in both extreme northern latitudes (aurora borealis or Northern Lights) and extreme southern latitudes (aurora australis).

autumnal equinoxsearch for term

The equinox that occurs on or near September 21 each year.

azimuthsearch for term

The measurement, usually in degrees, of an object's apparent angular separation from north. An object that lies due north has an azimuth of 0 degrees, an object that lies due east has an azimuth of 90 degrees, one that lies due south has an azimuth of 180 degrees, and one that lies due west has an azimuth of 270 degrees. Altitude and azimuth together are one of several types of coordinate systems that astronomers use to locate astronomical objects in the sky.

Balmer Seriessearch for term

A distinct pattern of emission or absorption spectral lines that are caused by the transition of electrons to the n=2 energy level of hydrogen. These spectral lines appear at visible wavelengths, and since stars are mostly composed of hydrogen, the Balmer series of spectral lines is one of the most prominent features in the visible spectrum of a star. The first four Balmer lines of the Balmer series are:

H-alpha: 656.3 nm (red)

H-beta: 486.1 nm (green-blue)

H-gamma: 434.0 nm (blue)

H-delta: 410.1 nm (blue-violet)

barred spiral galaxysearch for term

A distinct type of galaxy distinguished by a straight bar of stars, gas and dust that cut through the center of the galaxy and then trail off in a spiral pattern.

Big Bangsearch for term

A theoretical model of the "birth" of the observable universe, which suggests that the universe began as a rapid expansion of space and time from a single point, or singularity.

Big Crunchsearch for term

A potential fate of the universe in which the observed expansion of the universe is halted by gravity; gravity then pulls the universe back into a singularity.

binary starsearch for term

One of a pair of stars that orbit around a common center of mass. Three types of binary stars include:

visual binary stars

Binary stars that are identified visually, that is, observers can see two stars orbiting each other.

eclipsing binary stars

Binary stars that are identified by changes in their brightness due to the stars passing in front of one another.

spectroscopic binary stars

Binary stars that are identified by Doppler shifts in their spectrum. These Doppler shifts indicate the presence of an otherwise unseen companion star.

black bodysearch for term

A perfect absorber and radiator of energy, and an object whose spectrum is totally dependent on its temperature. Colder black bodies appear red, and hotter black bodies appear blue, or white. Stars closely mimic blackbodies; cool stars are red while hot stars are blue or white.

black holesearch for term

A singularity in space-time from which light and matter cannot escape; an object so massive that the escape velocity is greater than the speed of light.

blazar (BL Lac Object)search for term

A type of active galaxy that varies in brightness over several years. The peculiar appearance and variability associated with these objects may be a result of our viewing perspective. Blazars seem to eject material at superluminal speeds, between 5 and 10 times the speed of light. This apparent violation of the laws of Special Relativity is caused by the angle at which we view the superluminal matter.

blue shiftsearch for term

A displacement of emission or absorption line patterns toward the blue end of the spectrum as a result of the Doppler effect. As a star travels toward an observer, the wavelength of the star light decreases. The observer sees the star as "bluer" than the same star at rest, and the magnitude of the shift corresponds to the velocity of the source.

Brahe, Tychosearch for term

A Danish astronomer who lived from 1546 to 1601. In 1576, the King of Denmark granted him the island of Hveen on which to establish an observatory. Since this was before the invention of the telescope, he designed his own equipment for unaided eye observations. Until 1597 he made very accurate observations at Hveen, then moved to Prague. His very detailed records were later used by Johannes Kepler to discern Kepler's three laws of planetary motion.

brightnesssearch for term

A measure of the amount of energy that reaches Earth from a source, such as a star, galaxy, or planet. Brightness depends on both the intrinsic luminosity (or power output)

of the object, and how far away it is. For instance, at the orbit of Pluto, the Sun appears much less bright than at Earth.

brown dwarfsearch for term

A "failed" star, larger than a planet but smaller than a star, that is not massive enough to ignite a nuclear fusion reaction at its core.

calcium H and Ksearch for term

Two broad absorption lines caused by singly ionized calcium (Ca II), located between 380 nm and 400 nm. These lines are very prominent in cool stars and in almost all galaxies.

carbon cyclesearch for term

A nuclear fusion reaction cycle that occurs in the cores of stars with masses greater than that of our Sun, with temperatures exceeding 16 million degrees. The reaction cycle involves carbon (C), nitrogen (N), oxygen (O), hydrogen (H) and helium (He). Hydrogen is fused into helium with the help of carbon, which acts as a catalyst to the reaction.

carbonaceous chondritesearch for term

A rare type of meteorite containing water and complex organic compounds. These meteorites may be fossil remnants of our early Solar System.

Cassegrain focussearch for term

A style of focus for reflecting telescopes for which the focal point lies behind the primary mirror. In order to achieve this focus, a reflecting telescope must have a secondary mirror and a hole cut in the center of the primary mirror. Such an optical arrangement compacts the size of the telescope tube and centers the mass close to the primary mirror.

Cassini divisionsearch for term

A 5000-km gap in Saturn's ring system observable through small telescopes. G.D. Cassini discovered this dark gap in Saturn's rings in 1675.

celestial equatorsearch for term

The projection of Earth's equator onto the sky. For an observer standing on Earth's equator, the celestial equator would extend from the eastern horizon, through the zenith, then to the western horizon. The celestial equator is part of the celestial sphere.

celestial polesearch for term

The north or south projection of Earth's rotation axis onto the sky. In the northern hemisphere, the north celestial pole is within a degree of Polaris, the North Star.

celestial spheresearch for term

A coordinate system similar to Earth's latitude and longitude used to locate planets, stars, star clusters, nebulae and galaxies. The celestial sphere is centered on Earth, and it includes the celestial equator, the north and south celestial poles, and lines of right ascension and declination.

center of masssearch for term

Within a system of masses, the point that can be used to represent the entire mass of the system. Massive bodies in space, such as binary stars, orbit around a center of mass.

centimetersearch for term

A unit of linear measurement that is equal to 1/100 of a meter, or about 0.4 inch. Centimeters are abbreviated cm.

Cepheid variablesearch for term

An unstable star whose brightness changes periodically. In 1912, Henrietta Leavitt discovered what is known as the Period-Luminosity Relationship for Cepheid variables, by which the period of the brightness change is related to the luminosity, and therefore the distance, of the star. An astronomer can record the changing brightness of a Cepheid variable and plot the brightness change over time to create a light curve for the star. The distance to the Cepheid variable is then obtained by measuring the period of the light curve.

Ceressearch for term

With a 900-km diameter, the largest of the asteroids. Ceres was the first asteroid discovered, and so is often referred to as "1 Ceres." It was discovered on January 1, 1801, by Giuseppe Piazzi in Palermo, Italy. Ceres' orbital period is 4.6 years, at an average distance of 2.7 A.U.

Chandrasekhar limitsearch for term

Equal to 1.4 solar masses, the maximum mass a dying star may have and still turn into a white dwarf star. Dying stars with masses greater than 1.4 solar masses collapse into neutron stars or black holes. Subrahamanyan Chadrasekhar, at age 19 in 1930, worked out this limiting mass while on a steamship to England, where he planned to present his astrophysical work.

Chandrasekhar masssearch for term

Equal to 1.4 times the mass of the Sun. This particular measure of mass is applicable to the Chandrasekhar limit, for which the fate of a star depends on its mass being more or less than this amount.

charge-coupled device (CCD)search for term

A light-recording device that has revolutionized modern astronomy. A CCD is a silicon wafer, about the size of a thumbnail, that converts light into an electronic signal which may be manipulated and stored on a computer. CCD recording efficiency is very high, about 80 to 95 percent, compared to its predecessor, photographic plates, which have an efficiency of about one percent. Astronomers mount CCD chips into cameras on telescopes, allowing them to see 9 to 10 times farther out into space than they could with photographic plates.

chromospheresearch for term

Literally meaning "sphere of color," the chromosphere is an outer layer of the solar atmosphere sandwiched between the photosphere and the corona. Prominent features of the chromosphere include spicules, bright hydrogen alpha emission lines, and calcium H and K emission lines. The emission lines suggest that the chromosphere is

thousands of degrees hotter than the photosphere.

cluster (star or galaxy)search for term

A grouping of the same types of astronomical objects. For example, stars in the Milky Way can group together into open clusters or globular clusters. Galaxies also group together into clusters of galaxies, and the clusters of galaxies group together into superclusters of galaxies.

collimating mirrorsearch for term

A special type of mirror that causes beams of light that strike the mirror at various angles to reflect off it parallel to each other. This type of mirror is said to collimate the light.

cometsearch for term

An icy, "dirty snowball" (Fred L. Whipple) that orbits the Sun. The majority of comets orbit well beyond Pluto in halos known as the Kuiper Belt and the Oort Cloud that surround the Solar System. Comet size ranges from a few meters to a few kilometers in diameter. Upon close encounters with the Sun, comet ices vaporize, creating a coma, or cloud, around the comet, and a long tail that always points away from the Sun.

conjunctionsearch for term

A geometrical arrangement of the Earth, Sun and one or more planets in a line, with the Sun between the Earth and the planet(s). The complementary arrangement is called opposition, when the Earth is between the Sun and the planet(s).

Conservation of Energysearch for term

The principle that states that energy is neither created nor destroyed, merely exchanged from one form to another. Thus for any closed system (for example, a star or the universe), the total energy is constant, but the amount of energy in any one form may change.

Conservation of Masssearch for term

The principle that states that mass is neither created nor destroyed, merely exchanged from one form to another. Thus for any closed system, the total mass is constant, but the amount of energy in any one form may change.

constellationsearch for term

A meaningful pattern of stars visible with the unaided eye, literally meaning "group of stars" in Latin. Americans know Northern Hemisphere constellations by the names given them by ancient Babylonians and Greeks. Seafaring explorers named many of the constellations in the Southern Hemisphere.

continuous spectrumsearch for term

A smooth-looking spectrum without absorption or emission lines that indicates a thermal source of radiation, such as a light bulb, star, or other piece of glowing matter.

continuumsearch for term

The constant background of a spectrum against which emission or absorption lines can be seen.

Copernicus, Nicholassearch for term

A Polish astronomer who lived from 1473 to 1543. Copernicus is most famous for inventing the Copernican system, which is also known as the heliocentric theory. The Copernican system is a model for our Solar System in which the Earth and all other planets orbit around the Sun and the Sun is the center of the universe. In contrast, scientists before Copernicus ascribed to the Ptolemaic system, also known as the geocentric theory. The Ptolemaic system stated that all the planets, the Moon, and the Sun orbited the Earth, which was the center of the universe.

coronasearch for term

Literally meaning "crown," it is the outer layer of the Sun made of very thin plasma heated to 1,000,000 Kelvin. The corona is only visible during a total solar eclipse.

Cosmic Background Radiation (CBR)search for term

An electromagnetic radiation field at a black body temperature of 2.7 Kelvin that fills the entire universe uniformly to 0.00001 Kelvin. Also known as the Cosmic Microwave Background (CMB), it is thought to be the residual glow from the very hot early universe that followed the Big Bang.

Cosmic Microwave Background (CMB)search for term

See Cosmic Background Radiation (CBR).

cosmic rayssearch for term

High-energy particles that fly through the universe at speeds approximating the speed of light. Cosmic rays are mostly the atomic nuclei of hydrogen, helium, carbon, nitrogen, and oxygen that were possibly ejected from supernovae.

cosmologysearch for term

The study of the past, present, and future of the universe, including the structure and evolution of energy, matter, space, and time.

Coudé focussearch for term

A style of focus for reflecting telescopes for which light is brought to a focal point in a location remote from the telescope. In order to achieve this focus, a telescope must have secondary and tertiary mirrors to direct light into a "Coudé room" or a "spectrograph room" near the telescope, where it is passed though optical components and recorded.

cratersearch for term

A bowl shaped blemish left on the surface of a planet or moon as the result of a meteoroid impact. Several craters exist on the Earth, and thousands litter the surface of the Moon, Mercury, and other Solar System bodies.

dark mattersearch for term

Theoretical non-luminous matter that has eluded detection by all present means, except through gravitational interaction with luminous objects. Astronomers and astrophysicists calculate that dark matter comprises more than 90 percent of the universe. Possible sources of dark matter include undetected clumps of interstellar dust and gas, stellar remnants such as white dwarfs and neutron stars, neutrinos that have mass, and some as yet unknown form of matter.

declinationsearch for term

The measure in degrees of the position of a celestial object north or south of the celestial equator. If an object is on the celestial equator, it has a declination of 0 degrees, and if it is on the north or south celestial pole, it has a declination of +90 degrees or –90 degrees. Declination is analogous to latitude on the Earth, which measures terrestrial positions north or south of the equator. right ascension and declination together comprise a coordinate system that allows astronomers to locate objects in the sky.

degreessearch for term

A unit of angular measure equal to 60 arc minutes or 3600 arc seconds. One degree is equal to the diameter of two full Moons.

densitysearch for term

The amount of mass per unit of volume.

deuteriumsearch for term

A form of hydrogen that consists of a proton, neutron, and electron. Because it has one more neutron than most hydrogen, which is merely a proton and an electron, it is often called "heavy hydrogen," and it is denoted by D.

diffractionsearch for term

The bending of light around an opaque physical edge that demonstrates the wave nature of light.

diffraction gratingsearch for term

A glass surface into which fine grooves have been cut, allowing light to be "spread" into its component colors, creating a spectrum. The groove dimensions correspond to wavelengths of light, usually around 500 nm (green).

domesearch for term

In astronomy, the building that houses a telescope. The roof of a telescope dome has a dome shape, and there is a shutter that opens and closes, allowing the telescope to view the sky.

Doppler effectsearch for term

A measurable shift in the wavelength of a traveling wave caused by the relative motion of the source and observer. If a source and observer are approaching each other, the wavelength of incoming light is shifted to shorter, or bluer, wavelengths; if a source and observer are receding from each other, the wavelength of incoming light is shifted to longer, or redder, wavelengths. Stars with relative motion toward an astronomer appear bluer than they would otherwise, i.e. the entire spectrum of star light shifts toward the blue end of the spectrum or is blue-shifted. Stars speeding away from the astronomer appear redder than they would otherwise, i.e. the entire spectrum is red-shifted. The expansion of the universe can be detected because the farther away a celestial object is from the Earth, the more it tends to be red-shifted.

Earthsearch for term

The third planet from the Sun in the Solar System, and the planet on which people live. The Earth orbits the Sun at an average distance of 1.0 A.U. and has a mass of 6x10^24 kg. It is made of solid, rocky materials such as iron, nickel, and silicon, and it has an atmosphere composed mainly of nitrogen, oxygen, argon, and carbon dioxide. Liquid water oceans cover 75% of the planet's surface. Earth has one moon.

eccentricitysearch for term

A measure of the shape of an ellipse, such as the orbit of a body around a center of mass, that compares the lengths of the semi-major, or longest, axis and the semi-minor, or shortest, axis. The eccentricity of a circle is zero, while a highly elliptical orbital path of a comet might be 0.9.

eclipsesearch for term

Total or partial masking of a celestial body by another along the line of sight. Solar eclipses result from the Moon blocking the Sun relative to the Earth; thus Earth, Moon and Sun all lie on a line. Lunar eclipses work the same way in a different order: the Earth is between the Sun and the Moon. In this case the Earth's shadow hides the Moon from view.

eclipsing binarysearch for term

A system of two stars that periodically eclipse one another from our point of view on Earth. Astronomers cleverly use these observations to calculate mass, size, and distance. One noted eclipsing binary is called Algol, with a period of 2.87 days and a change in magnitude of 1.2.

eclipticsearch for term

The Sun's path across the sky. The Moon and the planets follow this path closely, since their orbital planes are nearly aligned with Earth's orbital plane. The ecliptic is tilted 23.5 degrees from the celestial equator.

effective temperaturesearch for term

The temperature of a star's outermost visible layer, the photosphere. Temperatures vary throughout a star, from millions of degrees in the core to thousands of degrees in the outer stellar atmosphere. The overall color of a star is related to its effective temperature. The dominant color of the star's spectrum is the wavelength range in which the star radiates most of its energy. For example, stars with cooler stellar atmospheres than our Sun appear orange and red. Those with hotter temperatures are blue and white. In addition, astronomers can combine the effective temperature of a star with other information to calculate the temperature of the stellar core.

Einstein, Albertsearch for term

A Swiss mathematician and physicist who lived from 1879 – 1955. Einstein was born in Germany to Jewish parents, and after moving to Switzerland, he took a job at a patent office. When he began publishing scientific papers, he quickly rose to fame and was recognized as a leading thinker of the time. He held several different professorial positions in Switzerland, Germany, and the United States during his lifetime. In 1922, Einstein won a Nobel Prize for work he had done with the Photoelectric Effect. Over the course of his life, Einstein made major advances in physics, such as developing the Theory of Special Relativity and the Theory of General Relativity. He was arguably the most important physicist of the 20th century.

electromagnetic radiationsearch for term

Energy that travels through space at the speed of light. The total range of electromagnetic

wavelengths and frequencies is called the electromagnetic spectrum. Radio waves have the lowest energies and longest wavelengths of all types of electromagnetic radiation. X-rays and gamma-rays have the highest energies and shortest wavelengths, about the size of atoms. Wavelengths of light are measured in microns, millionths of meters. For all forms of electromagnetic radiation, the speed of light must remain constant, regardless of wavelength or frequency.

electromagnetic spectrumsearch for term

The full range of electromagnetic radiation in order of wavelength from longest to shortest, or frequency from lowest to highest. In order, the types of radiation that make up the electromagnetic spectrum include radio, microwave, infrared, visible, ultraviolet, x-ray, and gamma-ray. Our eyes are only sensitive to the sliver of the electromagnetic spectrum we call visible light. Astronomers use instruments that can observe as much of the electromagnetic spectrum as possible to learn about the nature and history of the universe.

electronsearch for term

A fundamental subatomic particle that carries a negative electric charge. It participates in electromagnetic interactions, and its mass is less than one thousandth of that of the smallest atom. Its electric charge is defined by convention to be negative, with a value of −1 in atomic units. Together with atomic nuclei (protons and neutrons), electrons make up atoms; their interaction with adjacent nuclei is the main cause of chemical bonding.

ellipsesearch for term

A closed shape resulting from the intersection of a circular cone and a plane, resembling an oval. In space, orbiting bodies follow elliptical paths.

ellipticitysearch for term

A measure of the amount an ellipse deviates from a perfect circle.

elongationsearch for term

The angle between a planet and the Sun in the sky. Because Mercury and Venus are inside the Earth's orbit, their elongation angles are never more than 23 degrees (for Mercury) or 46 degrees (for Venus), which is why they appear as "morning" or "evening" stars, never as "midnight stars." All other planets, though, with orbits outside the Earth's, can appear at any elongation angle. When a planet reaches the greatest angular separation east of west of the Sun in the sky, it is said to have reached greatest elongation. The maximum elongation for Venus and Mercury is 46 and 23 degrees, respectively. The maximum elongation of planets outside the orbit of the Earth is 180 degrees, when the planet is said to be at opposition. At these points in the sky, planets are visible for the longest period of time.

emission linesearch for term

Within a spectrum, an excess amount of energy that is emitted at a specific wavelength. Emission lines in a spectrum usually appear as slender slivers of light on a dark background. The extra light at these wavelengths results from an electron "falling" from a high atomic energy state to a lower energy state. When this happens, the atom emits a photon, or discrete unit of energy in the form of light. The difference in energy between the two energy states determines the wavelength and frequency (color) of the emission line.

An absorption line is just the opposite of an emission line. Instead of emitting a photon, an absorption line represents an atom absorbing a photon at a specific wavelength and energy. In a spectrum, absorption lines appear as darkened slivers, where light is missing from the spectrum.

energysearch for term

The ability to do work on another object. Energy comes in many forms, including kinetic, potential, and thermal. It is what causes things to move and to have temperature. On Earth, we create electrical energy for our homes at power plants, we turn gasoline into energy to make our cars run, and we eat foods so that our bodies have energy to move. Plants use energy from the sun to grow, and gravitational energy causes objects to fall to the ground. In space, stars shine because of the energetic reactions that take place in their cores and galaxies form and rotate with the energy of the objects that they contain. In his famous equation E=mc^2, Einstein showed that mass and energy are equivalent. Like ice and liquid water, mass can be thought of as a solidified version of energy.

equatorial coordinatessearch for term

A system of coordinates that uses right ascension and declination to locate objects in the night sky. A telescope that uses equatorial coordinates is said to have an equatorial mount.

equinoxsearch for term

The time of year when the Sun is located in the sky at the intersection of the ecliptic and the celestial equator. Two such points exist: the vernal equinox, which is approximately March 21, marks the start of spring, and the autumnal equinox, approximately September 20, marks the start of autumn. At each of these points, the Sun rises due east and sets due west, and every place on Earth receives 12 hours of daylight and 12 hours of darkness.

ergsearch for term

A unit of energy commonly used by astronomers. Ergs have units of gcm^2/s^2, and one erg is equal to 0.0000001 Joule.

escape velocitysearch for term

The minimum speed necessary to escape the gravitational pull of a celestial body. For example, a rocket launched from Kennedy Space Center must accelerate to at least 11.18 km/s, or 18,300 m.p.h. to escape Earth's gravitational pull.

event horizonsearch for term

The radius of a black hole within which no light or matter can escape, because the escape velocity necessary to overcome the gravitational pull exceeds the speed of light.

extrasolar planetsearch for term

A planet that orbits around a star other than the Sun.

focal lengthsearch for term

The distance between a lens or mirror and its focal point. This length is usually stated in terms of the diameter of the lens or mirror; for example, a reflecting telescope of focal length f/4 has a focal length that is four times larger than the diameter of the primary mirror. Generally, longer focal lengths permit higher magnification, but smaller fields of view.

focal pointsearch for term

The point in an optical path where beams of light converge to a point, creating a focused image of an object.

focussearch for term

1) To bring beams of light into a convergent point, or focal point. An image created by beams of light at the focal point is said to be "in focus." Focused images have sharp edges, as opposed to being fuzzy or blurry. In smaller telescopes, astronomers place an eyepiece close to the focus in order to project the telescope focal point onto an observers eye. In larger telescopes, astronomers place a CCD camera at the focal point to capture the focused light.

2) One of two fixed points inside an ellipse. The foci of an ellipse help to define the curve of the ellipse itself.

forcesearch for term

The result of an event that produces pressure or causes an object of non-zero mass to accelerate. Everyday examples of force include a person pushing a door open, pulling a rope, or tightening a screw. There are four fundamental forces that cause such actions to happen: gravity, the electromagnetic force, the weak force, and the strong force. Gravity and the electromagnetic force are the two forces with which people are usually familiar; the weak force and the strong force apply to subatomic particles.

frequencysearch for term

The number of oscillations, or cycles, within one second. Frequency refers to anything that exhibits the properties of a wave. For example, sound waves are produced when a piano or guitar string vibrates; when this happens at a frequency of 440 times per second, we hear the specific musical note middle C. Likewise, a "vibrating" electron emits electromagnetic waves. When the electron vibrates about 600 thousand billion times in one second, it emits green light.

Ocean waves also have frequencies that you can measure by counting the number of times a floating buoy bobs up and down. With a stop-watch, count the bobs for one minute (one "bob" = one cycle), then divide by 60 to calculate the frequency of the buoy in bobs per second (cycles per second).

galaxysearch for term

A vast island of stars, gas, and dust, billions of which populate the universe. Galactic size and structure vary greatly and range from enormous elliptical and grand spiral galaxies to tiny irregular galaxies. Galaxies are classified according to their structural properties, as can be seen on the Tuning Fork Diagram, originally drawn by Edwin Hubble in the early 1900s. Our own galaxy, the Milky Way, is a medium-sized spiral galaxy made of about 100 billion stars. Galaxies tend to exist in groups, called clusters, and the clusters also clump together into groups called superclusters. Clusters and superclusters of galaxies are not randomly scattered throughout the universe, but instead stay together in web-like or foam-like structures. These structures form giant filaments and walls, leaving great empty voids in between.

Galilei, Galileosearch for term

An Italian scientist who lived from 1564 to 1642. Among many other interests, Galileo studied astronomy and constructed the first telescope for astronomy. He used the telescope to discover sunspots, craters on the Moon, and the four largest moons of Jupiter. Galileo supported the Copernican system for the Solar System, postulated by Nicholas Copernicus in 1543. As a result, he was brought up before the Catholic Inquisition, where he was forced to recant.

gamma-ray radiationsearch for term

The most energetic wavelength, or frequency, of light that is known. Gamma-rays have wavelengths smaller than 10^-11 meters and in frequencies larger than 10^20 Hz. They can be quite harmful to life because they are strong enough to ionize atoms and thus destroy cells. Fortunately, the Earth's atmosphere shields us from all astronomical gamma-ray radiation.

Gausssearch for term

A unit used by astronomers to measure the strength of magnetic fields, abbreviated G. One Gauss is equal to 0.0001 Tesla.

General Theory of Relativitysearch for term

"Space tells mass how to move" while "mass tells space how to curve" -- J.A. Wheeler. A mathematical model created by Albert Einstein in the years 1909-1916. It describes gravity as curvature in space-time, the four-dimensional fabric of our universe. His theory is the best model for gravity so far, and has been confirmed in experiments and observations. According to the theory, measurements made in time and space are not absolute, but relative to an observer's particular point of view, or reference frame. However, regardless of point of view (as measured by speed and direction), the speed of light is unchanged. The consequences of the laws of General relativity include concepts such as black holes, parallel universes, worm holes, and space-time.

geodesicsearch for term

A path between two points that follows the shortest possible distance between those two points. In a curved space, geodesics are curved lines, and on the surface of spheres, geodesics are great circles.

globular clustersearch for term

A ball of one hundred thousand to one million stars that resides in the halo of our galaxy and other galaxies. Because globular clusters contain stars that have been physically separated from the main part of the galaxy, they have evolved in relative isolation. As a result, globular clusters still hold the original mix of material that made the galaxy. There are about 250 globular clusters that form a spherical halo around the Milky Way. Messier 13, a bright globular cluster in the constellation Hercules, is a classic example easily seen with binoculars or a small telescope.

gramsearch for term

A unit of measure for mass.

gravitysearch for term

One of the four fundamental forces in the universe that affect all matter. It is the weakest of these four forces, and it is an attractive force. The magnitude of gravitational attraction depends directly on the mass of the two objects being attracted and inversely on the square of the distance between them. For instance, the gravitational attraction between a person and the Earth is much greater than that between a person and the Sun. Even though the Sun is 330,000 times more massive than the Earth, it is about 100 million times farther away than the Earth. This distance weakens the gravitational attraction between the person and the Sun, so that the Earth's gravitational pull on a person at Earth's surface is 1653 times greater than the Sun's.

gravity wavessearch for term

Ripples in space-time created by the stretching and squeezing effects of massive objects,

notably supernovae, binary black holes, and binary neutron stars. Gravity waves are

transverse waves, meaning that they stretch and squeeze space-time in directions perpendicular to the wave's motion. Einstein's Theory of General Relativity predicts the existence of gravity waves, but they have not yet been detected.

great circlesearch for term

A circle on the surface of a sphere whose center is also the center of the sphere. Lines of longitude and the equator are examples of great circles on the Earth.

Great Red Spotsearch for term

A raging red swirling storm located in Jupiter's upper atmosphere that can be seen as a red spot on Jupiter from Earth. The storm has been observed for the last 300 years, and appears to rotate counterclockwise, like a hurricane, once every six days. During the 1979 Voyager II encounter with Jupiter, the Red Spot measured 10,000 by 20,000 kilometers- larger than the Earth!

greatest elongationsearch for term

The maximum separation between Mercury or Venus and the Sun in the sky. Mercury and Venus peak as "evening stars" at their greatest eastern elongations, and as "morning stars" during their greatest western elongation. Geometrically, the greatest elongation of Venus occurs when a line tangent to the orbit of Venus intersects Venus and the Earth, and the greatest elongation of Mercury occurs when a line tangent to the orbit of Mercury intersects Mercury and the Earth.

H I regionsearch for term

Neutral hydrogen regions located in interstellar space. H I regions are vast, cold clouds of hydrogen that line the spiral arms of our own galaxy. Astronomers detect dim but distinct H I radiation at the "21-cm line" with radio telescopes. This is the wavelength of energy radiated during a hydrogen quantum spin flip. Astronomers gauge the mass and structure of our galaxy and other galaxies with the luminosity of H I regions.

H II regionsearch for term

Ionized hydrogen regions located in interstellar space. H II regions are visible as a part of nebulae, where hot young stars ionize their nearby hydrogen womb. As hydrogen recombines, it radiates at several specific wavelengths as electrons descend through atomic energy levels. From the 3rd to 2nd energy level, an electron radiates a photon at 656.0 nm (red light) giving such H II regions a red glow. Astronomers use the luminosity of H II regions to gauge the rate of star formation within a nebula.

halosearch for term

In astronomy, the spherical mix of a globular star cluster and dark matter that surrounds a galaxy.

heliumsearch for term

An element, usually in the form of a gas, whose nucleus is made of two protons and two neutrons. Helium is the second-most abundant element in the universe, after hydrogen, and accounts for about 25% of the composition of the universe. Helium is also the product of hydrogen fusion in stars.

Hertzsprung-Russell Diagramsearch for term

A plot of stellar luminosity versus effective temperature that reveals an orderly pattern of stellar evolution. The H-R Diagram provides astronomers the stellar "big picture," a handful of conceptual hooks on which to hang billions of stars. The most prominent feature is the main sequence, where stars spend the majority of their luminous life. The main sequence represents a stable zone where the temperature, luminosity, and lifetime of the star are dependent on its mass: large stars are bright and blue, but only shine

for tens of millions of years before running out of fuel, while small stars glow a dim red for many billions of years. Another area of interest on the diagram stretches horizontally off the main sequence and is called the Giant Branch. Stars in this region of the diagram have outer atmospheres that have expanded to be close to the size of the orbital radius of Mercury. Above the Giant Branch is the area where supergiant stars are located. These stars are of enormous size. If the Sun were the size of Betelgeuse, an orange supergiant in Orion's left shoulder, it would swallow the Earth. Another important group of stars called white dwarf reside in the low-luminosity, high-temperature corner of the H-R diagram. These stellar remnants glow with heat generated from their collapse from the size of the Sun to the size of the Earth.

Hubble Classification Systemsearch for term

A classification of galaxies based on their visual appearance in a telescope, invented by Edwin Hubble in 1936. In his paper, "The Realm of the Nebulae," Hubble introduced a "tuning fork" diagram of galaxy types that included:


Pinwheel-shaped galaxies with a bright compact center, or bulge, surrounded by graceful spiral arms. Our galaxy is considered a spiral, as is the Andromeda Galaxy.

Barred spiral

Contains two sickle shaped spiral arms extending from a central bulge.


Smooth football-shaped galaxies with little gas and dust between stars. These look like glowing cotton balls in a telescope. Most of the Local Group galaxies are ellipticals.


Similar to spirals, but without any spiral structure.


Galactic blobs without definite shape. These could represent casualties of galactic collisions. Two nearby examples are the Large and Small Magellanic clouds.

In 1959, Dr. Gerald de Vaucouleurs suggested expanding the classification of spiral galaxies to incorporate a greater variety of spirals than Hubble allowed in his tuning fork diagram. He also organized galaxy types in a three dimensional scheme that formed a continuous boundary for all galactic types, instead of the two dimensional tuning fork form. This three dimensional shape looks like a lemon, with ellipticals and irregulars on the ends and spirals in the center.

Hubble Constantsearch for term

The present expansion rate of the universe, symbolized by H and measured in units of km/s/Mpc or kilometers per second per megaparsec. In 1929, Edwin Hubble discovered that the Doppler red shifts of other galaxies are directly proportional to their distances from the Milky Way. The constant of proportionality in the relationship is called the Hubble Constant, and it has been measured to be between 50 and 100 km/s/Mpc. In order to check its validity, astronomers use many various observation techniques that each independently derive H.

Hubble, Edwinsearch for term

An American astronomer who lived from 1889 to 1953. Hubble discovered the expansion of the universe by measuring the red shifts of many galaxies. He also discovered that the recession velocity of a given galaxy is proportional to its distance from the Milky Way.

hydrogensearch for term

An element, usually in the form of a gas, whose nucleus is made of one proton. Hydrogen is the most abundant element in the universe, accounting for about 75% of the composition of the universe.

hydrostatic equilibriumsearch for term

The balance maintained within a star between the explosive pressure of the core caused by heat and the gravitational attraction of the mass of the star itself. This equilibrium allows the star to have a controlled nuclear fusion reaction and a steady radiation rate or luminosity. Stars achieve and maintain hydrostatic equilibrium during their main sequence life on the H-R Diagram.

inertiasearch for term

The property of objects with mass to remain at rest if they are already at rest, or to stay in motion if they are already moving.

inflationsearch for term

A sudden and dramatic expansion of the universe after the Big Bang.

infrared radiationsearch for term

A wavelength, or frequency, of light that is slightly less energetic than visible light, but more energetic than radio or microwave radiation. Infrared radiation ranges in wavelength between 10^-6 to 10^-4 meters and in frequency between 10^12 and 10^14 Hz. Infrared radiation is not strong enough to ionize atoms.

interstellar dustsearch for term

Pockets of micron-size grains of carbon, iron, and iron-magnesium silicates scattered at varying densities between the stars in galaxies.

interstellar gassearch for term

Mostly hydrogen and helium scattered at varying densities between the stars in our galaxy and other galaxies. The proportions of the gases are similar to those in the Sun. Interstellar gas supplies the raw material for star formation. Carbon monoxide and hydroxyl molecules (CO and OH) have also been detected within interstellar gas, along with highly ionized oxygen, nitrogen, carbon and silicon, which are likely remnants of past supernovae.

interstellar mediumsearch for term

Regions of space between stars populated by gas and dust. Astronomers presently estimate that the interstellar medium accounts for 10 percent of galactic mass.

ionsearch for term

A negatively or positively charged atom. Normally, atoms are electrically neutral; the amount of positive and negative charge is equal. Energetic photons may excite and eject electrons from the atom, thus leaving a net positive charge. This process is called ionization.

isotopesearch for term

A species of a given element that has more or fewer neutrons per atomic nucleus than other species of atoms of that element.

Joulesearch for term

A unit of energy commonly used by physicists. Joules have units of kg m^2/s^2, and they are abbreviated J.

Jupitersearch for term

The fifth planet from the Sun and the largest planet in our Solar System. Jupiter orbits the Sun at an average distance of 5.2 A.U.. The planet has a mass 318 times the mass of the Earth and a radius 11.2 times the radius of the Earth. Jupiter is a gas planet and has no solid surface. It is made of molecular hydrogen and helium, with trace amounts of ammonia, water vapor, and methane. The most recognizable features of the planet are its red and white bands of clouds and a giant storm visible from Earth called the Great Red Spot. Jupiter has 16 moons, the largest of which are Io, Europa, Ganymede, and Callisto.

Kelvin (K)search for term

A unit of temperature equal in magnitude to Celsius (1 K = 1 degree C) and abbreviated with K. Zero degrees Celsius, the freezing point of water, equals 273.15 K. Absolute Zero is defined as 0 K. The surface of the Sunis about 6000 K, while nuclear fusion reactions deep in the center of the Sun raise the temperature to about 15 million K.

Kepler, Johannes & Kepler's Lawssearch for term

A German mathematician and astronomer who lived from 1571 - 1630. Kepler was the first person to model planetary orbits as ellipses instead of circles. He tested his theory of elliptical orbits using Danish astronomer Tycho Brahe's observations of planetary motion in the sky. In 1609, he published his findings in a book called Astronomia nova or The New Astronomy, where he summarized planetary motion with three rules, or laws.

Kepler's First Law, the Law of Ellipses

A planetary orbit is an ellipse with the Sun at one focus.

Kepler's Second Law, the Law of Equal Areas

As a planet orbits the Sun, it sweeps out equal areas of the ellipse in equal times. This behavior means that the planet's orbital velocity varies with distance from the Sun. At perihelion, the planet is at maximum speed and at aphelion the planet crawls along at minimum speed.

Kepler's Third Law, the Harmonic Law

Even though the orbital velocity of a planet changes constantly, one relationship does remain constant. The orbital period is directly related to the average distance between the planet and the Sun. This law implies that planetary orbital velocity decreases with increasing distance from the Sun. For instance, the orbital velocity of Mercury (47.9 km/s) is far greater than Pluto (4.7 km/s).

kilometersearch for term

A unit of linear measurement that is equal to 1000 meters, or 0.6 miles. Kilometers are abbreviated km.

Kuiper Beltsearch for term

A ring of small, icy objects that surrounds the Solar System beyond the orbit of Neptune. Postulated by American astronomer Gerard P. Kuiper in 1952, it is thought to be the birthplace of comets whose periods are less than 200 years, such as Halley's Comet.

Large Magellanic Cloud (LMC)search for term

A satellite galaxy of our galaxy that lives "next door," only 169,000 light-years away. The LMC contains more than 30 million stars, and stretches more than 50,000 light-years from end to end, but is visible only from the Southern Hemisphere, 70 degrees south of the celestial equator. In 1987, astronomers detected a supernova in the LMC, called 1987A. The close proximity of 1987A gave astronomers a front-row seat to study supernova evolution and measure the distance to the LMC.

lightsearch for term

Electromagnetic radiation of all wavelengths and frequencies. The familiar "rainbow" of light spans a very narrow range in the electromagnetic spectrum, from 700 nm (red) to 400 nm (blue). While the wavelengths of red and blue light differ by less than a factor of two, the entire electromagnetic spectrum spans more than a factor of 10^18, (1 followed by 18 zeroes) from radio to gamma-ray wavelengths. Radio wavelengths can be the size of mountains while gamma ray wavelengths are the size of an atomic nucleus.

light curvesearch for term

Brightness or intensity of light from a celestial source plotted against time on a graph. Astronomers use light curves, for example, to discover dark companions of stars. As a dark object orbits in front of a star, thus partially eclipsing it, the brightness falls, producing a dip on the light curve. Careful analysis of the light curve, together with other information, reveals the masses of the star and dark companion plus the distance to the eclipsing binary system.

light gathering powersearch for term

A measure of a telescope's ability to collect light. Light gathering power is proportional to the telescope's lens or mirror surface area. It is advantageous for astronomers to observe through large aperture telescopes because the increased light gathering power allows them to see faint, distant objects and details of nearby objects.

light-yearsearch for term

A unit of length used by astronomers to measure distance. One light-year is equal to the distance light travels in one year, which is about 5.88 trillion miles or almost 800 times the diameter of our Solar System. The nearest star is a mere four light-years away, while the nearest spiral galaxy lies 2.2 million light-years from Earth.

Local Groupsearch for term

Our galactic neighborhood, including the Milky Way, the Andromeda Galaxy, M33, and more than 25 smaller galaxies. The Local Group appears to be a suburb of a supercluster of galaxies called the Virgo Supercluster that lies 60 million light-years away, in the direction of the constellation Virgo.

luminositysearch for term

A measure of the rate of energy flowing from a source, such as a galaxy, star, or light bulb. Luminosity tells astronomers how fast energy "leaks" from the star. It is a measure of power, and it is measured in units called watts. Given the luminosity of a star, an astronomer can calculate the distance to the star by measuring the star's brightness. Luminosity is also related to temperature, mass and size of a star.

luminosity classsearch for term

A shorthand description of a star based on spectral line widths denoted by the Roman numerals I to V. For a given spectral type, the luminosity of the star decreases from I through V. Our Sun is a G2 V star. The Roman numeral V denotes the luminosity class, which is the main sequence, while G2 refers to a spectral class. Thus, the luminosity of a B8 I supergiant star is far greater than that of a B8 V main sequence star, yet the surface temperatures are equal. The prominent hydrogen absorption-line patterns are the same for both stars, but the width of the absorption lines differ. The B8 V star lines are wider than the B8 I lines, indicating a vast difference in luminosity.

lunarsearch for term

Having to do with a moon.

lunar eclipsesearch for term

A spectacular, though relatively common, celestial event that occurs when the Moon, Earth and Sun are aligned in space. The full Moon travels through Earth's shadow and grows much darker, but it does not disappear entirely.

lunar phasessearch for term

The apparent shapes of the Moon as seen from the Earth, which are caused by the Moon's orbit of the Earth once every 29.5 days. The same phase will fall on the same date of the year according to the Saros cycle, every 18 years, 11 days, and eight hours. The phases of the Moon are:

New Moon

In conjunction with the Sun; rises and sets with it. Zero percent illuminated, 0 degrees elongation.

Waxing Crescent

Prominent just after sunset; less than 50 percent illumination.

First Quarter

Rises at noon and sets at midnight; 50 percent illuminated, 90 degrees east elongation.

Waxing Gibbous

Lunar illumination increasing between first quarter and full.

Full Moon

In opposition with the Sun; rises at sunset and sets at sunrise; 100 percent illuminated, 180 degrees elongation.

Waning Gibbous

Decreasing illumination decreasing between full and last quarter.

Last Quarter

Rises at midnight and sets at noon; 50 percent illuminated, 90 degrees west elongation.

Waning Crescent

Prominent just before sunrise; less than 50 percent illumination.

Lyman seriessearch for term

A distinct pattern of emission or absorption spectral lines that are caused by the transition of electrons to the ground state of hydrogen. These spectral lines appear at ultraviolet wavelengths. Since more electrons are in the ground state than in higher energy levels, Lyman series lines are usually more prominent in stellar spectra than lines of the Balmer Series.

Magellanic cloudssearch for term

Two companion galaxies of our own Milky Way, called the Large Magellanic Cloud (LMC) and the Small Magellanic Cloud (SMC). They are both irregular galaxies that are visible only from the Southern Hemisphere.

magnetic fieldsearch for term

A force field generated by moving electrical charges, measured in units of Tesla or Gauss. An electrical current running through a loop of wire generates a magnetic field. The strength of the field depends on the current and area of the wire loop. Plasma churning through the atmosphere of the Sun drives powerful magnetic fields that sometimes produce relatively cool magnetic knots called sunspots. A magnetic dynamo in Earth's core generates a magnetic field around Earth.

magnificationsearch for term

In a telescope, an increase in the apparent size of an object. The process of magnification

expands the apparent size of an object by spreading the image, or light, across a large area. A large primary mirror or objective lens of a telescope focuses incoming light toward an eyepiece lens that magnifies what the telescope sees.

magnifying powersearch for term

The ratio of the apparent image size as seen through optics to the actual size of an object. Telescope magnifying power is equal to the focal length of the objective lens or primary mirror divided by the focal length of the eyepiece. This is why the high power eyepieces are tiny, and the low power eyepieces are large.

magnitudesearch for term

A measure of brightness, or faintness, as perceived by the human eye. In the system used by astronomers, the higher the magnitude, the fainter the object. The magnitude and apparent brightness of a star are related in a logarithmic fashion. For every five steps in magnitude, the apparent brightness of a star, galaxy, or nebula changes by a factor of 100. For example, we receive 100 times more light energy from Vega -- a zero-magnitude star -- than from Eta Ursa Minor -- a fifth-magnitude star in the Little Dipper. Under the clearest, darkest skies, your eye cannot see stars fainter than sixth magnitude. With the aid of binoculars, the human eye can detect 10th-magnitude stars.

The magnitude scale is organized logarithmically because that's the way our human eyes perceive brightness, whether it's light bulbs or stars. For instance, your eye would perceive the same brightness difference between a 25- and 50-watt light bulb as it would between a 100- and 200-watt light bulb. Likewise with stars, your eye would detect the same brightness difference between a first- and a second-magnitude star as it would between a second- and third magnitude star. If you repeat this exercise down to a sixth-magnitude star, the first-magnitude star's brightness (amount of light received on Earth) is 100 times the sixth-magnitude star.

main sequencesearch for term

A group of stars on the H-R Diagram that all shine as a result of hydrogen thermonuclear fusion and are all in a state of hydrodynamic equilibrium. Stars spend the greatest portion of their luminous (nuclear fusion) life on the main sequence.

maresearch for term

Latin for "sea," it is an area of basalt rock on the surface of the Moon created relatively recently by oozing lava. The plural form of mare is maria. It is thought that most maria formed as a result of impacts on the Moon; lava bled from cracks made in the Moon's surface and filled craters to form the dark "seas" visible from Earth. Maria are the youngest and most crater-free regions on the Moon's surface.

Marssearch for term

The fourth planet from the Sun in the Solar System. Mars orbits the Sun at an average distance of 1.52 A.U.. The planet has a mass 0.1 times the mass of the Earth and a radius 0.5 times the radius of the Earth. It is made of solid, rocky material, and has a thin atmosphere of carbon dioxide. Iron oxide, or rust, on the surface of Mars gives it its characteristic red color. The planet has carbon dioxide ice (dry ice) caps at its poles. Mars also has two small moons, Phobos and Deimos.

masssearch for term

The amount of matter contained in an object and a physical measure of inertia. Mass is measured in units called grams, and one gram is equal to the mass of one cubic centimeter of water at 4 degrees Celsius. Newton's Third Law states that mass is equal to the ratio of force to acceleration. Furthermore, Einstein's Theory of General Relativity states that mass and space are related, because mass warps space and space directs the motion of mass.

mattersearch for term

The material out of which all ordinary material is made. Protons, neutrons, and electrons are examples of matter, as is a piece of wood, a puddle of water, or an animal or plant. All matter has the following properties: it occupies a volume and has inertia. The measure of the amount of matter in an object is that object's mass.

Mercurysearch for term

The first planet from the Sun in the Solar System. Mercury orbits the Sun at an average distance of 0.4 A.U.. The planet has a mass 0.055 times the mass of the Earth and a radius 0.4 times the radius of the Earth. It is made of solid, rocky material that is heavily cratered, and it has no significant atmosphere. Mercury has no moons.

meridiansearch for term

An imaginary line in the sky that goes from the north pole, through an observer's zenith, to the south pole. Astronomical objects are best observed when they are crossing the meridian, because along that line, the least amount of Earth's atmosphere is in between the observer and an object.

Messier cataloguesearch for term

A catalogue of about 110 astronomical objects compiled by French astronomer Charles Messier at the end of the 18th century. The catalogue includes objects such as nebulae, star clusters, and galaxies, and the objects are denoted by an M followed by a number from 1 to 110.

meteorsearch for term

A meteoroid traveling between about 10 and 70 km/s that vaporizes within Earth's atmosphere because of intense friction between it and the air. From the ground, meteors are seen as shooting stars.

meteor showersearch for term

A spectacular display of meteors streaking through the sky at rates between several to

hundreds per hour. Whenever Earth intercepts a stream of comet debris in orbit around the Sun, some of the debris falls into Earth's atmosphere, producing the meteor shower. Meteor velocities usually fall within a range of about 10 km/s to 70 km/s.

meteoritesearch for term

A space rock that strikes the surface of the Earth. Most meteorites are classified as stony

because of their mineral composition, which may include olivine, pyroxene, serpentine, sulfates, organic compounds, iron, and nickel.

meteoroidsearch for term

A tiny grain of silicate or metal between 1 and 10 millimeters in size that orbits the Sun.

Sometimes meteoroids fall into Earth's atmosphere and become meteors.

metersearch for term

A unit of measurement of length. One meter is equal to 1.1 yards. Meters are abbreviated m.

micrometersearch for term

A unit of length used by astronomers and physicists to measure the wavelength of light. A micrometer is equal to one millionth of a meter, or 0.000001 meter. Micrometers are abbreviated ìm, and are also known as microns.

micronsearch for term

A micrometer, or 0.000001 meter. Micron is simply a nickname for micrometer.

microwave radiationsearch for term

A very unenergetic wavelength, or frequency, of light. Microwaves are a type of radio radiation, meaning that they are less energetic than infrared radiation. They are not harmful to life because they are not strong enough to ionize atoms or destroy cells. While the Earth's atmosphere shields us from some microwave radiation, it does allow some frequencies to pass through. Astronomers study these microwaves with large radio telescopes or antennas, which resemble giant satellite dishes.

Milky Waysearch for term

Our galaxy.

The Milky Way is a spiral, or disk-shaped, galaxy with four major arms containing young bright stars, gas, and dust. From the Earth, the Milky Way is seen edge-on as a swath of patchy white glow in the sky. The mass of the entire Milky Way is estimated to be between 400 billion and one trillion solar masses, and the luminous diameter of the Galaxy is some 80,000 light-years across. The entire Solar System orbits the center of the Milky Way once every 200 million years at a distance of 25,000 light-years from the Galactic center, which is located in the direction of the constellation Sagittarius. Sometimes, astronomers refer to the Milky Way as the Galaxy.

millimetersearch for term

A unit of measure that is equal to 1/1000 of a meter, or about 0.04 inches. Millimeters are abbreviated mm.

minute of arcsearch for term

A unit of angular measurement used by astronomers to describe angles in the sky. One minute of arc is equal to 1/60 of a degree, or about the diameter of a quarter as seen from 75 yards away. Minutes of arc are also known as arc minutes.

moonsearch for term

A natural satellite of a planet.

Moonsearch for term

The largest natural satellite of the Earth.

nanometersearch for term

A unit of length used by astronomers and physicists to measure the wavelength of light. A nanometer is equal to one billionth of a meter, or 0.000000001 (10^-9) meter. Nanometers are abbreviated nm, and one nanometer is equal to 10 Angstroms.

nebulasearch for term

A generic term for a fuzzy, diffuse astronomical object. Astronomers have observed four different types of nebulae: H II regions, reflection nebulae, planetary nebulae, and supernova remnants.

HII regions

A "stellar womb" composed of hydrogen, helium, interstellar gas and dust that is illuminated with the light and energy of new born stars. An example is the Orion Nebula (M 42), located in the constellation Orion.

reflection nebulae

A nebula that is mainly composed of cool interstellar dust that reflects and scatters light from nearby stars. These nebulae are mainly bluish in color. The blue color provides a clue to the size of the dust grains, because in order to scatter the blue light, the dust grain size must be close to the wavelength of blue light.

planetary nebulae

Contrary to their name, these nebulae have nothing to do with planets. A planetary nebula is created when a star in the last stage of its life puffs off its outer atmosphere. The nebula usually looks like a donut, sometimes with the small, hot, rapidly evolving star visible in the center. The Ring Nebula (M 57) in the constellation Lyra is an example.

supernova remnant

This type of nebula is the result of a supernova explosion ripping apart and scattering the stellar atmosphere of its progenitor. The radiation and shock wave produced in the explosion illuminate the remnants of the stellar atmosphere. The shock wave, which plunges through the gas like a snowplow, causes the gas to glow all across the electromagnetic spectrum. An example is the Crab Nebula (M 1), which is located between the horns of Taurus the Bull.

Neptunesearch for term

The eighth planet from the Sun in the Solar System. Neptune orbits the Sun at an average distance of 30.0 A.U.. The planet has a mass 17.1 times the mass of the Earth and a radius 3.9 times the radius of the Earth. Neptune is a gas planet made of hydrogen, helium, and methane and has no solid surface. It has a small ring system and 8 moons, the largest of which is Triton.

neutrinosearch for term

A subatomic particle with very small mass and zero charge, that rarely interacts with matter. Many quadrillions of neutrinos emanating from the core of the Sun pass through your body every second, but you'd never know it. Because of their ghostly

properties, neutrinos are extremely difficult to capture and measure, but several super-sensitive neutrino detector arrays are now in operation around the world. Unlike optical telescopes, which can only see the surface of the Sun, neutrino telescopes might soon let us directly observe the solar core, so we can test our theories of thermonuclear fusion reactions.

neutronsearch for term

A subatomic particle with zero charge and rest mass of 1.6749286 x 10^-27 kilograms. Neutrons, like protons, are found in atomic nuclei. By themselves, neutrons are unstable and decay into a proton, electron, and antineutrino through the beta minus decay process.

neutron starsearch for term

A gravitationally compressed stellar core, made almost

entirely of neutrons. They are formed in supernova explosions, when a

massive star (8 to 20 times the mass of the Sun) runs out of fuel and

collapses upon itself, or when a white dwarf accumulates too much

material from a binary companion, exceeds the Chanrdasekhar mass,

and collapses. The force of gravity squeezes protons and electrons

together to form neutrons. Since neutrons are electrically neutral and

don't repel each other like protons do, neutron star densities are

extremely high: 10^14 grams/cubic centimeter, or 100 trillion times as

dense as water. A full bathtub of neutron star (instead of water)

would weigh as much as two Mt. Everests. A neutron star with the same

mass as the Sun is between 10 and 15 km (6-10 miles) wide, with a

liquid neutron core and an atmosphere of iron. Some neutron stars,

called pulsars, spin rapidly at speeds of 1 to 1000 revolutions per

second and sustain a powerful magnetic field, which produces radio

pulses which can be detected with radio telescopes.

Newton, Isaac & Newton's Lawssearch for term

A British mathematician and physicist who lived from 1643 - 1727. Newton made major advances in the studies of optics, mathematics, astronomy, and physics, most of which were published in his two major works, Opticks and Principia. Among other innovations, he constructed the first reflecting telescope (now known as a Newtonian style telescope), invented integral calculus, and postulated a Universal Law of Gravity. Newton's first three laws and his Law of Gravity are listed below:

Newton's First Law

Objects in motion stay in motion, and objects at rest stay at rest, unless acted upon by an outside force.

Newton's Second Law

For every action, there is an equal and opposite reaction.

Newton's Third Law

The force exerted on an object and the acceleration of the object are directly proportional. The constant of proportionality is the mass of the object.

Newton's Law of Gravity

The gravitational force exerted on one object by another object is proportional to the product of the masses of the two objects and inversely proportional to the square of the distance between them. The constant of proportionality is the Universal Gravitational Constant, G.

Newtonian focussearch for term

A style of focus for reflecting telescopes for which the focal point lies to the side of the telescope. Light enters the telescope and is reflected from the primary mirror to a secondary "pickoff" mirror in the middle of the telescope tube. The secondary mirror directs light out of the side of the telescope. Newtonian focus telescopes, also known as Newtonian telescopes, are popular for telescopes with eyepieces, because they provide for convenient viewing positions.

non-thermal radiationsearch for term

Electromagnetic radiation that is not caused by a black body. The energy of non-thermal radiation is not related to the temperature of the body that emitted it, and it is not caused by electrons changing their orbital levels around atoms. An example of non-thermal radiation is synchrotron radiation. Synchrotron radiation occurs when electrons are accelerated through a magnetic field and spiral around the magnetic field lines. As they are accelerated, they emit photons of synchrotron radiation.

novasearch for term

An explosive increase in stellar brightness by a factor of one hundred thousand to one million. Most novas occur in closely bound binary star systems in which one member is a white dwarf. The white dwarf draws material (mostly hydrogen) that lies outside the companion star's Roche lobe from the companion star. As hydrogen falls onto the white dwarf, the surface temperature and pressure increase until the hydrogen shell suddenly explodes in runaway thermonuclear reaction. Most of the material falls back onto the white dwarf. This is a recurring process that can continue as long as a steady stream of hydrogen flows to the white dwarf.

nuclear fissionsearch for term

A reaction that occurs in the nuclei of radioactive isotopes of elements, during which the nucleus splits into two pieces, resulting in a tremendous release of energy.

nuclear fusionsearch for term

A reaction that involves two atomic nuclei merging to create the nucleus of a new atom. The reaction is accompanied by a tremendous release of energy. The Sun's energy, as with all stars, comes from nuclear fusion reactions that happen in its core. The reactions fuse hydrogen nuclei into helium nuclei.

nucleussearch for term

1) The center and most massive component of an atom, consisting of positively charged protons and electrically neutral neutrons. The number of protons in the nucleus of an atom is the property that makes every element unique.

2) The very central region of a galaxy that sometimes contains a massive black hole.

objective lenssearch for term

The primary light gathering optic of a refracting telescope, located opposite of the eyepiece.

observable universesearch for term

All that the largest telescope can see. The cosmological theory of inflation suggests that very early on in the history of the universe, just fractions of a second after the Big Bang, the universe went through a period when it was effectively expanding faster than the speed of light. As a result, there may be regions which are within our universe, but are so far away that light emitted there just hasn't had enough time to reach the Earth yet.

observatorysearch for term

The entire complex of buildings, telescopes, equipment, and staff involving scientific astronomical observations. Small observatories might consist of just one telescope situated in a dome, one instrument and computer, and one observer. McDonald Observatory is an example of a major observatory, and consists of five major telescopes in five different domes, numerous instruments, computers, living quarters, outbuildings, astronomers, and engineers.

occultationsearch for term

An eclipse of a celestial object by the Moon or another Solar System body.

Olber's paradoxsearch for term

An apparent disagreement between cosmological assumptions and observations first noted by German astronomer Heinrich Olbers in 1826: if the universe is infinite and static, as Newton had declared, every line of sight leading from the Earth should intercept the surface of a star. Therefore, the sky should always be bright. However, we observe that the night sky is dark. This paradox is resolved if the universe is expanding and evolving.

Oort Cloudsearch for term

A halo of debris left over from the formation of the Sun and planets that surrounds our Solar System. The Oort Cloud has a diameter of about 100,000 A.U. and contains some 100 billion fossils of Solar System formation. It is likely that comets originate from this cloud.

open clustersearch for term

A gravitationally bound group of approximately 100 to 1,000 stars born of the same material at about the same time.

oppositionsearch for term

A planet's position when it is 180 degrees east or west (i.e., directly opposite) from the Sun in the sky.

orbitsearch for term

1) The motion of a massive body around another massive body, governed by the force of gravity. Planets in our Solar System follow elliptical orbits around the Sun, as first noted by Johannes Kepler. An orbit is actually composed of two motions: one that points toward the other body (for example, a planet or star) and another that points tangentially to the direction of motion. In the case of a circular orbit, these two component motions are at a 90 degree angle. According to Einstein's General Theory of Relativity, an orbit is actually a geodesic in space-time.

2) The act of one body revolving around another.

orbital periodsearch for term

The duration of one orbit. For instance, Earth's orbital period around the Sun is one year, and the Moon's orbital period around the Earth is 27.3 days.

orbital planesearch for term

A plane containing at least two masses orbiting around a common center of mass. The Earth and the Sun orbit each other in an orbital plane, as do the Earth and the Moon. However, the Earth-Moon orbital plane is tilted relative to the Earth-Sun orbital plane at a five-degree angle.

outer planetssearch for term

The planets in the Solar System that orbit the Sun beyond the orbit of Mars. Jupiter and Saturn are examples of outer planets.

parallaxsearch for term

The apparent shift in position of an object relative to background objects when observed from two different locations. The distance between the two measurement points is called the baseline, and the angle formed by measuring the position of the object this way is called the parallactic angle. An everyday demonstration of parallax is easy to try: hold a finger in front of your face, and without moving it, wink one eye and then the other. When you see the position of your finger change with respect to the background, you are seeing parallax.

Astronomers measure the parallax of a stars close to the Earth using Earth's orbital diameter as a baseline. The astronomer observes a star at a six-month interval, when the Earth is at opposite ends of an imaginary baseline defined by the width of its orbit around the Sun. Astronomers look for a parallax shift in the two images of the star, and if they find one, they can use simple geometry to estimate the star's distance from Earth. The greater the shift with respect to background stars, the closer the star. This sort of measurement is applicable to only nearby stars, no more than a few hundred light-years away. At 300 light-years, the angular shift is equivalent to a U.S. quarter seen at a distance of 300 miles, or 500 kilometers.

parsecsearch for term

A unit of length used by astronomers to measure distance. One parsec is defined as the distance from the Earth to a star that exhibits a parallax angle of one second of arc; the word parsec is a combination of the words parallax and arc second. A parsec is equal to 3.26 light-years, and it is abbreviated pc.

penumbrasearch for term

1) A partial shadow cast by the Earth or Moon that surrounds the umbra, or total shadow. During a total lunar eclipse, the Moon passes through the Earth's penumbra, then into the umbra where the Moon "disappears". Similarly, during a total solar eclipse, the Earth passes through the Moon's penumbra, and then into its umbra.

2) The region immediately surrounding the darkest portion, or umbra, of a sunspot. This region is darker and cooler than the surrounding photosphere, but lighter and hotter than the center of the sunspot.

perigeesearch for term

The point of minimum orbital separation between the Earth and an orbiting satellite, including the Moon. At perigee, the orbiting satellite experiences its maximum orbital velocity around the Earth.

perihelionsearch for term

The point of minimum orbital separation between a planet and the Sun. At perihelion, the planet experiences its maximum orbital velocity around the Sun.

phases of the Moonsearch for term

See lunar phases.

photonsearch for term

A discrete energy unit of light. Photons behave like waves or particles, depending on the

experiment and observer.

photospheresearch for term

The outer layer of a star where most photons escape. Even though stars are not made of solid material, the photosphere can be thought of as the "edge" or "visible surface" of the star. The temperature of the photosphere is called the effective temperature of the star. Astronomers can use spectrographs to analyze the composition of the photosphere in order to model the interior of a star.

planetsearch for term

An object that is larger than a moon but smaller than a brown dwarf and that orbits a star. The Earth is a planet, and it is the third of a series of planets in our Solar System. In order of increasing distance from the Sun, the planets in our Solar System are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto. While Pluto is conventionally listed as one of the planets, it is debatable whether its small size and odd orbit allow it to be classified as such. Planets have also been discovered orbiting around other stars; these planets are called extrasolar planets.

planetariumsearch for term

A dome-shaped room with reclining seats and equipment that can project simulated sky images onto the interior of the dome. Planetariums and planetarium shows are often associated with science museums.

plasmasearch for term

A phase of matter in the form of hot, ionized gas. Plasma is created when energetic photons strip electrons from atoms, resulting in a hot soup of free electrons and ionized atoms. The Sun and other stars are made mostly of hydrogen plasma.

Plutosearch for term

A Solar System object that orbits the Sun at an average distance of 39.5 A.U. Pluto is considered by some scientists to be the ninth planet from the Sun and by others to be a Kuiper Belt object. Pluto has a mass 0.002 times the mass of the Earth and a radius 0.18 times the radius of the Earth. It is a small body made of water ice, and temperatures there do not go above 50 K, or -223 degrees Celsius. Pluto has 1 moon called Charon.

polarization of lightsearch for term

A property of light involving the direction of its electric field vectors. Most regular light, such as natural Sunlight and most artificial lights, can be described as a wave with electric field vectors that lie within planes that are perpendicular to the direction of propagation of the light. However, it is possible to filter light so that only waves with electric field vectors that lie in a certain direction get through. Light that has passed through such a filter is said to be polarized. Astronomers can study the extent to which light is polarized and in what direction it is polarized in order to learn more about astronomical sources and processes.

precessionsearch for term

The "wobble" of the Earth's rotational axis, which sweeps out an imaginary cone, much like a spinning top, once every 26,000 years. Precession changes the pole star as seen from Earth. Thuban, the brightest star in the constellation Draco, was the pole star while the Egyptians built the Pyramids in Egypt. Since that time, the motion of precession has rotated the Earth's axis away from Thuban and towards Polaris, the current pole star. In 13,000 years, Earth's rotational axis will point towards Vega, the new pole star.

primary focussearch for term

A style of focus for reflecting telescopes for which the focal point lies at the open end of the telescope tube. Light enters the telescope from the sky and is reflected from a primary mirror to a focal point at the telescope aperture. Astronomers place cameras at prime focus to record bright, wide field images.

prismsearch for term

A three-dimensional shape with identical polygon bases and parallelogram sides. A familiar example is a triangular glass prism used to refract light into a spectrum of color.

proper motionsearch for term

The slow motion of a celestial object across the sky, relative to other objects, due to the actual velocity of the object, for example, in its orbit around the center of the galaxy. While all celestial objects appear to move across the sky from hour to hour because of the rotation of the Earth, a few stars shift their position relative to the others when observed year after year. This relative shift in their position is their proper motion. Stars that exhibit proper motion tend to be relatively nearby and are whizzing through the solar neighborhood. Even though they are moving at many thousands of miles per hour (hundreds of km/s) relative to us, they are still far enough away that they only appear to move a few arc seconds per year or less.

protonsearch for term

A positively charged subatomic particle found in the nucleus of an atom. A proton's electrical charge has the same magnitude as that of an electron. A single proton, however, is 1,836 times more massive than an electron. Usually, the number of protons balances the number of electrons within an atom. In this case, the atom is electrically neutral. When the balance is tipped, the atom becomes electrically charged and is called an ion.

pulsarsearch for term

A spinning neutron star with a very strong magnetic field, on the order of one trillion Gauss. This magnetic field accelerates electrons around its magnetic field lines, and the electrons then radiate photons that form a beam of light projecting along the poles. As the neutron star spins, the beam of light from its poles can sweep across an observer's point of view, much like the spinning light of a lighthouse. Astronomers detect pulses in radio, visible, x-ray, and gamma-ray light from pulsars. An example of a pulsar is the Crab Nebula pulsar, which pulses 30 times a second. The light curve for this pulsar shows a strong pulse followed by a weak pulse, a pattern that is caused by the orientation of the pulsar's magnetic field.

Quasar, quasi-stellar object, or QSOsearch for term

A type of active galaxy that exhibits luminosity of 10 to 100,000 times that of the Milky Way from a very compact power-source inside the core of a galaxy. Such a compact source may be a supermassive black hole surrounded by an accretion disk of matter falling into the black hole. Quasars are perhaps the most distant of all objects in the universe, with red shifts indicating recession velocities approaching 90 percent of the speed of light.

radial velocitysearch for term

The line-of-sight velocity of a star or other celestial object towards or away from an observer. Astronomers can calculate the radial velocity of a celestial objects by recording a spectrum with a spectrograph and measuring the Doppler shift of several spectral lines. Measurements of radial velocity have revealed how our Solar System moves through space and orbits the center of our galaxy. Astronomers have also used radial velocity measurements to observe that most objects beyond our galaxy travel away from us at speeds that increase with the object's distance.

radio galaxysearch for term

A particular type of active galaxy that emits more light at radio wavelengths than at visible wavelengths, also known as a radio-luminous galaxy or radio-loud galaxy. Radio galaxies are driven by non-thermal emission. Radio telescopes show that some radio galaxies, called extended radio galaxies, have lobes of radio emission extending millions of light-years from their nuclei. Centaurus A is a nearby example of an extended radio galaxy that features two outer lobes 650,000 and 1,350,000 light-years in diameter. In contrast, compact radio galaxies emit radio lobes not much larger than the galactic nucleus.

radio radiationsearch for term

A very unenergetic wavelength, or frequency, of light. Radio waves are the least energetic form of radiation known. They have wavelengths longer than 10^-3 meters and frequencies less than 10^11 Hz. Radio waves are not harmful to life because they are not strong enough to ionize atoms or destroy cells. While the Earth's atmosphere shields us from some radio radiation, it does allow radio waves in the vicinity of VHF, UHF, and FM frequencies to pass through. Astronomers study waves that pass through this "radio window" with large radio telescopes or antennas, which resemble giant satellite dishes. Often, many radio antennas are coordinated together to synthesize even bigger telescopes, such as the Very Large Array in New Mexico. In addition to astronomy, radio waves have many useful applications on Earth, such as television and radio broadcasts and RADAR.

radio telescopesearch for term

A type of telescope that gathers and focuses radio wavelength light. Radio telescopes are huge dish-shaped antennae. Because of the long wavelengths of radio radiation, it is possible, and quite useful, to use many radio telescopes in different locations together in arrays. By combining telescopes in this way, astronomers are able to study celestial objects with much higher resolution than they could with a single antenna. An example of a radio telescope array is the Very Large Array, or VLA, which lies 80 kilometers west of Soccoro, New Mexico. VLA consists of 27 radio telescopes that are arranged in a "Y" pattern; each antenna has a parabolic dish shape and measures 25 meters across. Radio telescopes can probe the Galaxy and the universe where optical telescopes cannot. Astronomers have used radio telescopes to map the hydrogen content of our galaxy and to discover structures and processes in other galaxies that are invisible to optical telescopes.

red giantsearch for term

A phase of stellar evolution that occurs after the main sequence life of a star. The core of a red giant is degenerate ionized helium, and it is surrounded by a shell of hydrogen fusion. In response to higher core temperatures, the shell of hydrogen fusion expands the outer atmosphere of the star and eats through it, depositing the helium it creates onto the shrinking core. The ballooning atmosphere cools and glows red, hence the name red giant. Once the helium core reaches 100 million degrees, it explosively begins fusing helium. The birth of the active helium core is called the helium flash. The Sun will become a red giant the size of Earth's orbit in five to six billion years, and it will fuse helium for about 2 billion years after the helium flash.

red shiftsearch for term

A displacement of emission or absorption line patterns toward the red end of the spectrum as a result of the Doppler effect. As a star travels away from an observer, the wavelength of the star light increases. The observer sees the star as "redder" than the same star at rest, and the magnitude of the shift corresponds to the velocity of the source.

Astronomers use red shifts, among many other things, to study the expansion of the universe.

reflectionsearch for term

A property of light that causes it to bounce off of a shiny surface, such as a mirror. Such a surface is called a reflective surface. When light encounters such a reflective surface, it bounces off at the same angle with which it struck the surface, with respect to a line perpendicular to the reflective surface.

refractionsearch for term

A change in the direction of light that occurs at the boundary between two different, transparent substances. The amount of directional change depends on the incident angle of the light, the wavelength (color) of the light, and the material of which the substance is made. For example, raindrops and glass prisms refract white light into rainbows. The many colors that make up white light are each a different wavelength, and they all refract at unique angles to form a band of color in order of wavelength: red, orange, yellow, green, blue, indigo, and violet.

resolutionsearch for term

The ability of an instrument, such as a telescope, a pair of binoculars, or the human eye, to distinguish between separate objects that are closely spaced on the sky. Resolution is also the ability of a spectrograph to separate wavelengths of light. The higher the resolution of a piece of equipment, the greater its ability to distinguish between two closely spaced objects or between wavelengths of light.

retrograde motionsearch for term

The temporary apparent backward motion of a planet in the sky, from east to west, caused by the geometry between the Earth and planet. Due to this geometry, only planets that orbit outside the orbit of the Earth are observed to have retrograde motion.

right ascensionsearch for term

The measure in hours, minutes, and seconds of arc of the position of a celestial object east of the vernal equinox. The 360 degrees along the celestial equator are divided into 24 hours of sidereal time, just like lines of longitude divide the 360 degrees of the equator into sections east or west of the prime meridian. One hour of right ascension equals 15 degrees. Right ascension and declination together comprise a coordinate system that allows astronomers to locate objects in the sky.

satellitesearch for term

A natural or artificial object in orbit around another body. For examples, the Moon is the largest natural satellite of the Earth, and the planets are satellites of the Sun. In addition, there are satellite galaxies that orbit the Milky Way.

Saturnsearch for term

The sixth planet from the Sun in the Solar System. Saturn orbits the Sun at an average distance of 9.5 A.U. The planet has a mass 95.2 times the mass of the Earth and a radius 9.45 times the radius of the Earth. Saturn is a gas planet and has no solid surface. It is made of hydrogen, helium, ammonia, and methane. Saturn is most famous for its beautiful ring system. In addition to the rings, the planet has 18 moons, the largest of which is Titan.

Schmidt telescopesearch for term

A telescope that has a camera fixed at prime focus. Schmidt telescopes are able to take bright, wide-angle pictures of the sky.

Schwarzschild Radiussearch for term

The distance between the central singularity and event horizon of a black hole. The length of the Schwarzschild Radius depends on the mass of the black hole. Anything inside this radius will not escape the black hole.

scientific notationsearch for term

A shorthand method used by all types of scientists for writing especially large or small numbers. Scientific notation consists of two parts: a real number between 1 and 10 and the number 10 raised to a certain power. When these two numbers are multiplied together, the product is the number being represented. For example, the number 694 can be written in scientific notation as 6.94 x 10^2, which is 6.94 x 100, or 694. As another example, the number 0.003 can be written in scientific notation as 3 x 10^-3, which is 3 x 0.001, or 0.003. Scientific notation is especially useful to astronomers because they deal with extremely large and extremely small numbers. It is much easier for an astronomer to write the mass of the Sun as 1.99 x 10^33 grams rather than 1990000000000000000000000000000000 grams, or the mass of an electron as 9.1 x 10^-28 grams, rather than 0.00000000000000000000000000091 grams.

semimajor axissearch for term

The distance from the center, through a focus, to the edge or perimeter of an ellipse. This is half of the longest dimension of an ellipse. As an example, the semimajor axis of the Earth's orbit is 149,597,870.7 km, or 1.0 A.U.

Seyfert Galaxysearch for term

An energetic galaxy with an exceptionally bright core that emits non-thermal radiation. Carl Seyfert first observed these peculiar galaxies in the early 1940s. The majority of Seyfert galaxies have a spiral structure and vary in brightness over several months. This indicates a small energy source (perhaps a black hole) at the nucleus that is responsible for the non-thermal radiation. Seyfert galaxy luminosity varies between one-tenth and ten times the luminosity of our galaxy.

sidereal timesearch for term

Measurement of time based on the position of the stars, rather than the Sun. It takes Earth 23 hours, 56 minutes, 4.091 seconds to complete one full turn relative to the stars, versus exactly 24 hours relative to the Sun. Therefore, a solar day, which is measured with respect to the Sun, is slightly longer than a sidereal day, which is measured with respect to the stars. The difference between solar and sidereal time is due to Earth's motion; as we move along our orbit around the Sun, the relative position of the Sun compared to the background stars changes. As a result, the stars rise about four minutes earlier each night.

singularitysearch for term

A theoretical infinitely small point where the curvature of space-time is infinite. Einstein's General Theory of Relativity predicts that such points may exist in the space-time of our universe, such as at the core of a black hole. The curvature of space-time inside the event horizon of a black hole is so great that nothing, not even light, can escape it; this curvature increases to infinity toward the singularity. As a result, the gravitational tidal forces near the singularity of a black hole are unimaginably strong and will tear anything that enters into thin strands of super-dense spaghetti.

small circlesearch for term

A circle on the surface of a sphere whose center is not the center of the sphere. Except for the equator, lines of latitude on the sphere of the Earth are examples of small circles.

Small Magellanic Cloud (SMC)search for term

An irregular satellite galaxy of the Milky Way galaxy, orbiting at a distance of 65 kiloparsecs (kpc). The SMC is small next to our galaxy, having a mass of only two billion solar masses, compared to the 200 billion solar masses of the Milky Way. A gaseous umbilical cord of neutral hydrogen, or H I, connects both the Small Magellanic Cloud and the Large Magellanic Cloud (LMC) to our Milky Way. Both the LMC and SMC are visible only in the Southern Hemisphere.

solarsearch for term

Having to do with the Sun.

solar eclipsesearch for term

A dramatic celestial phenomenon in which light from the Sun is blocked from the Earth by the Moon. In order for this to occur, the Earth, Moon, and Sun must be in a line in that order, which means that the phase of the Moon must be new. The Moon's shadow sweeps across the Earth's surface as the Moon moves over the face of the Sun. Totality occurs when the Sun is completely blocked by the Moon. The darkest part of the shadow is called the umbra, and it is surrounded by an area of partial shadow called the penumbra. Because the Moon's shadow stretches from the Moon to the Earth in a cone shape, only a small fraction of the Earth's surface experiences the eclipse. The solar eclipse alignment of Earth, Moon and Sun does not occur every month, because the Moon's orbit is tilted five degrees from a plane containing the Earth and Sun, so the Moon's shadow usually passes above or below the Earth.

solar flaresearch for term

A violent eruption of plasma from the chromosphere of the Sun that is whipped up by intense magnetic activity. During the eruption, flares rise thousands of kilometers above the chromosphere, and the plasma temperatures quickly soar to 20 million degrees. Large flares release 10^25 Joules, or about the energy of a few million volcanic eruptions on the Earth. Sunspot and solar flare frequency are strongly related. In addition, flares often disturb the Earth's atmosphere electrically, thus interfering with radio transmissions. The aurora borealis and aurora australis are results of flare activity that injects energetic particles into Earth's magnetic field.

Solar Systemsearch for term

The system of our Sun and its planets, including moons, asteroids, and comets.

solar windsearch for term

A flow of atomic nuclei, electrons, and other subatomic particles from the Sun that travels at an average speed of 400 km/s.

solsticesearch for term

An event in the Earth's orbit during which the tilt of the Earth's axis is pointed most directly towards or away from the Sun. The summer solstice for the northern hemisphere occurs within a few days of June 21 every year. It is on this day that the position of the Sun in the sky at noon is at its highest altitude of the year, and the position of the Sun at Sunrise and Sunset is farthest north for the year. The winter solstice is around December 21, marking the date on which the Sun is lowest in the sky at noon and rises and sets farthest south. The day of the summer solstice is the longest day of the year, and the day of the winter solstice is the shortest day of the year.

space-timesearch for term

A four-dimensional coordinate system, or reference frame, that physicists and astronomers use to describe the universe. Space-time has three spatial axes (x, y, z) and one time axis (t). A point in this reference frame is called an event, because it is something that happens in space and time. Einstein's General Theory of Relativity explains that the speed of light is constant between all reference frames, regardless of their motion relative to each other.

Special Theory of Relativitysearch for term

A theory postulated by Albert Einstein in 1905 that rejects the Newtonian notion of absolute space and time, based on the observation that the speed of light is independent of the motion of an observer. According to the theory, no matter what the speed of an observer, the speed of light will always measure 3 x 10^8 m/s, often abbreviated c. From this foundation, Einstein constructed a revolutionary model of gravity and was able to predict such surprising phenomena as black holes, gravity waves, time dilation, and the equivalence of mass and energy: E=Mc^2. Astronomers and astrophysicists regularly make use of the theoretical tools of special relativity to interpret and analyze light. For instance, astronomers rely on Doppler shift measurements to calculate the motions of stars and galaxies. The most distant galaxies show Doppler shifts which might imply that that they are moving faster than the speed of light. But using the proper formula from Special Relativity, instead of the simpler Newtonian equation, we find that the galaxies

are speeding away from us at 90%, or 95%, or 99% the speed of light.

spectral classsearch for term

The classification of star according to the appearance of their spectra. Spectral classification is indicated with the letters and the numbers 0 to 9. The letters used are O, B, A, F, G, K, M, often remembered with the mnemonic "Oh be a fine girl (guy), kiss me," and they are assigned to stars in order of decreasing effective temperature.

The colors of stars are dictated by their effective temperature, so that an O-star spectrum is brightest at the blue end, while the M-star spectrum is brightest at the red end.

spectral linesearch for term

An especially dark or bright line present in an electromagnetic spectrum that represents the absorption or emission of energy at a particular wavelength. Dark lines are called absorption lines because they represent the absorption of energy, and bright lines are called emission lines because they represent the emission of energy. The study of spectral lines is called spectroscopy.

spectrographsearch for term

An astronomical instrument placed on a telescope that diffracts light with a grating or prism into its component colors. The different colors, or wavelengths, can then be analyzed for chemical absorption or emission lines. Usually, a CCD camera records the spectrum, transmits the image to a computer that displays it for astronomical analysis.

spectroscopysearch for term

A technique used by astronomers that allows them to determine the properties, such as composition, temperature, and motion through space, of celestial objects by analyzing the spectra of celestial objects. For example, because each atomic element absorbs and emits light in a unique set of wavelengths, the astronomer can sift through the spectrum of a star and determine what elements are present in the star's atmosphere. From the shapes and depths of spectral lines, the astronomer can calculate fundamental qualities of a star, such as how fast the gases churn through the stellar atmosphere or the star's effective temperature. An astronomer may also be interested in correlations between the abundances of certain elements and the physical behavior of the star, the age of the star, or the abundances of other elements. For instance, compared with the Sun, stars with low amounts of iron are also low in almost every other element with respect to hydrogen. Spectroscopy is performed by astronomers with instruments called spectrographs.

spectrumsearch for term

Light that has been spread out into its component colors or wavelengths. Electromagnetic radiation that has been spread out into a spectrum allows an observer to study its individual colors or wavelengths. A rainbow is an example of the visible spectrum. Astronomers use spectra to study absorption and emission lines and their relative brightness to learn about the temperature, composition, speed, and distance of celestial objects. Spectra of such objects often provide very detailed information about them. To obtain spectra, astronomers use instruments called spectrographs on their telescopes. Spectrograph use diffraction gratings or prisms to refract the light into a spectrum of color.

speed of lightsearch for term

The speed at which light travels, which is equal to 186,000 miles a second or 299,000 km/s. At this speed, light travels from the Earth to the Moon and back again in less than three seconds. According to Einstein's Theory of Special Relativity, the speed of light is always constant, regardless of the reference frame of the observer, and nothing in the universe can travel faster than the speed of light.

spiculesearch for term

A structural component of the chromosphere of the Sun that erupts like a plasma geyser driven by dense magnetic fields. Spicules rise between 3,000 and 10,000 kilometers above the magnetic mesh at the photosphere. At the boundary between the photosphere and chromosphere, spicules look like peach fuzz. Spicules live short, dynamic lives of a few minutes but spew plasma at speeds of more than ten kilometers a second.

starsearch for term

A dense, glowing ball of hydrogen, helium, and trace quantities of heavier elements that shines with energy released from a hydrogen thermonuclear fusion reaction in the center, or core. Our Sun is an example of a star. Stars can appear red, orange, yellow, blue, or white, according to their masses and effective temperatures. Stars are born from clouds of gas and dust, live for millions or billions of years, depending on their masses, and die sometimes spectacular deaths. Stars usually exist in very large collections called galaxies.

subatomicsearch for term

Having a size smaller than the size of an atom, which is about one angstrom, or 10^-10 meters.

Sunsearch for term

The closest star to the Earth and the center of the Solar System.

sunspotsearch for term

A cooler, and therefore darker, region of the Sun's photosphere caused by a solar magnetic disturbance. Strong, dense magnetic fields generated by circulating plasma sometimes become entangled and surge through the photosphere, creating the sunspot. The knot causes the temperature to fall to around 1000 K in the sunspot region, which darkens the photosphere. The dark center of the sunspot is called the umbra, and it is surrounded by a dim filamentary area called the penumbra. Sunspots range from Earth-size "pimples" to swollen scars halfway across the surface. Sunspot activity generally follows an 11-year cycle, called the "sunspot cycle."

superclustersearch for term

An enormous chain of clusters of galaxies linked by their mutual gravity. Superclusters look as if they ride on the surfaces of bubbles. Our galaxy is a peripheral member of the Virgo Supercluster, centered around a giant elliptical galaxy M87. Our galaxy seems to be drifting toward the Virgo Supercluster at about 250 km/s. The gravity that binds the superclusters together is not luminous, but dark matter; in the Virgo Supercluster, there is ten times more dark matter than luminous matter.

supergiantsearch for term

A star of at least eight solar masses that has evolved off the main sequence of the H-R Diagram and depleted a significant portion of its hydrogen fuel supply. Helium becomes the new fuel for powering the star, and it is fused together at much higher temperatures in the core while hydrogen continues to fuse in a surrounding shell. This shell expands the outer atmosphere of the star to 10 to 1000 solar radii, thus making the star larger.

supernovasearch for term

A violent stellar explosion that releases energy exceeding the luminosity of an entire galaxy or the radiated energy from the Sun over one billion years. Astronomers divide supernovae into two groups: Type I and Type II. Low mass stars most likely produce Type I supernovae, which are caused by white dwarf stars in binary systems that blow off material accumulated from the binary companion. Stars of eight or more solar masses become Type II supernovae, which are simply the explosion at the end of the life of massive stars. Both neutron stars and black holes can result from Type II supernovae.

Astronomers can use data from supernovae to estimate the distance to them. Most observed supernova lie within other galaxies, thus allowing astronomers to check distance calibrations outside of the Milky Way and the expansion rate of the universe. In addition, supernovae observations can reveal the relative abundances of elements blown into the interstellar medium as a result of the explosion. This information helps astronomers understand how materials are recycled in galaxies.

supernova remnantsearch for term

The remainder of a star's atmosphere that is blown away into interstellar space by a supernova. Astronomers tend to see emission lines of elements such as nitrogen, oxygen, and neon in supernova remnants. These elements, which are processed in the late stages of stellar evolution, are still hot and rush through space at hundreds of kilometers per second.

synodic monthsearch for term

The period of the Moon's orbit measured relative to the its appearance in Earth's sky, instead of relative to the background stars. The synodic month (29.53 days) is longer that of a sidereal month (27.3 days), because the Earth travels 70.6 million kilometers, or 1/12 of its orbit, around the Sun as the Moon orbits the Earth once. The sidereal month is the true measure of the Moon's orbital period, but the synodic period, such as from one full moon to the next, is what we on Earth measure as the lunar month.

telescopesearch for term

An optical instrument that gathers and focuses light for use in astronomical study. Telescopes are usually shaped as tubes, and they use a series of either mirrors or lenses to focus light onto a camera, CCD, photographic plate, or eyepiece. Two major types of telescopes dominate astronomy: refracting telescopes and reflecting telescopes.


A refracting telescope uses an objective lens to gather and focus light to a focal point. The aperture size of the telescope is limited by the purity and weight of the glass lens. Several of the most magnificent telescopes are refractors. Yerkes Observatory's 40-inch refractor is the largest. Not only are the instruments mechanically and aesthetically beautiful, but they were also responsible for some of the pioneering astronomical research in the first part of the 20th century that paved the road for present-day discoveries.


A reflecting telescope uses a primary mirror to gather and focus light to a focal point. These telescopes are used today almost exclusively in astronomical observatories. The telescope aperture can range from a few centimeters to several meters, since the mirror diameter is limited by the flexible nature of glass, mirror weight, and design of the telescope. Some telescopes overcome these problems with a segmented primary mirror.

In order to help point a telescope in the correct direction, astronomers have developed two systems of coordinates for locating objects in the sky. The first, called the alt-az system, uses altitude and azimuth coordinates. Telescopes that use this coordinate system are said to have alt-az mounts and are used mostly by amateur observers. Professional astronomy demands telescope motion to be able to track stars and faint objects in the sky for long hours. To accommodate this, the telescope must turn opposite the direction of the Earth. The equatorial system of coordinates, used by telescopes with equatorial mounts, solves this problem. The coordinates in an equatorial system are right ascension and declination.

temperaturesearch for term

A measure of the average kinetic energy of the atoms or molecules that make up an object. Temperature can be measured in degrees Fahrenheit (oF), degrees Celsius (oC), or in Kelvin (K). The respective freezing and boiling points of water are 32 degrees and 212 degrees F, 0 degrees and 100 degrees C, and 273.16 and 373.16 K. One degree Celsius and one Kelvin measure the same amount of change in temperature.

When referring to the temperature of stars, astronomers usually refer to the effective temperature, or temperature of the photosphere, of a star. In the core however, the temperature is highest around 15 million degrees. In between the core and the surface, the temperature is about 5 million degrees, and at the photosphere, where almost all photons escape, the temperature is 6000 degrees.

terminatorsearch for term

A distinct shadow edge that marks the boundary between the night and day side of a moon or planet. Because of shadows cast along this jagged boundary on the Moon, it is easy to see the three-dimensionality of lunar mountains and craters. Sometimes, towering crater rings still catch a bit of sunlight as they slip behind the terminator.

terrestrialsearch for term

Having to do with the Earth.

terrestrial planetssearch for term

The four inner planets of our Solar System: Mercury, Venus, Earth and Mars. They consist mainly of rocky material, including iron, sulfur, magnesium, silicon, and nickel. Unlike the outer planets, no hydrogen nor helium exists in the atmospheres of the inner planets. The inner planets are also much smaller and more dense than the outer planets. Daytime temperatures range from 600 K (621 degrees F) on Mercury to 300 K (81 degrees F) on Mars.

Teslasearch for term

A unit used by physicists to measure the strength of magnetic fields, abbreviated T. One Tesla is equal to 10,000 Gauss.

Theorysearch for term

In science, a theory is a proposed description, explanation, or model of a set of natural phenomena, capable of predicting future occurrences or observations of the same kind, and capable of being tested through experiment or otherwise falsified through empirical observation. It follows from this that for scientists "theory" and "fact" do not necessarily stand in opposition. For example, it is a fact that an apple dropped on earth has been observed to fall towards the center of the planet, and the theory which explains why the apple behaves so is the current theory of gravitation. In scientific usage, a theory does not mean an unsubstantiated guess or hunch, as it often does in other contexts. In common usage, people often use the word theory to signify a conjecture, an opinion, or a speculation. In this usage, a theory is not necessarily based on facts, in other words, it is not required to be consistent with true descriptions of reality. True descriptions of reality are more reflectively understood as statements that would be true independently of what people think about them.

thermal radiationsearch for term

Energy that is output from a source as a result of its temperature.

thermonuclear fusionsearch for term

The process by which two atomic nuclei are merged under extremely high temperatures and pressures to make one nucleus of another element. Usually, thermonuclear fusion refers to two hydrogen nuclei that are combined to make a helium nucleus. The result of this type of reaction is a large release of energy. Thermonuclear fusion is the process that powers a star. Every star on the main sequence converts mass into energy through thermonuclear fusion.

tidal forcesearch for term

A stretching force that is caused by the difference between gravitational forces on opposite sides of an object, such as a planet or moon. For example, because the Moon pulls on opposite sides of the Earth with different strengths, water on the Earth is pulled either toward or away from the Moon, resulting in the ocean tides. Elsewhere in the Solar System, the magnitude of gravitational force on Jupiter's moon Io is greatest on the side of Io that faces Jupiter and least on the back side. The difference between the two forces compresses and stretches Io, which in turn heats up Io's interior. As a result, Io is the most geologically active body in the Solar System.

total solar eclipsesearch for term

A type of solar eclipse in which the Sun is completely blocked by the Moon.

transitsearch for term

The act of one celestial body passing in front of another. For example, when astronomers observe Mercury to be moving in front of the Sun, they say that Mercury is transiting the Sun. Most often, though, transit is used to refer to a celestial body crossing the meridian. The time at which a celestial object crosses the meridian is called the transit time. At the transit time, the object is highest in the sky, and light from the object travels through the thinnest possible layer of Earth's atmosphere. Thus an object's transit time marks the best time to view the object.

ultraviolet radiationsearch for term

A wavelength, or frequency, of light that is more energetic than visible light, but less energetic than x-ray radiation. Ultraviolet, or UV, radiation ranges in wavelength between 10^-8 and 10^-7 meters and in frequency between 10^15 and 10^17 Hz. UV rays can be very harmful to life because they are strong enough to ionize atoms and destroy cells. Fortunately, the Earth's atmosphere shields us from most UV radiation. Astronomers who want to study UV light from celestial sources must do so from space-borne telescopes, such as the Hubble Space Telescope.

umbrasearch for term

The darkest part of a shadow. In astronomy, umbra can refer to the darkest areas of phenomena such as eclipses and sunspots. For example, the umbra is the darkest region of shadow cast by the Sun or Moon during an eclipse. If an observer stood within the Moon's umbra during a solar eclipse, the Sun would "disappear" behind the Moon. Likewise, the Moon disappears from view when it passes through Earth's umbra during a total lunar eclipse. The umbra is also the darkest region of a sunspot, where the sunspot temperatures are lowest.

Universal Time (UT)search for term

Official Earth time, defined as local Greenwich Meridian time at 0 degrees longitude. Astronomers depend on a consistent time standard in order to accurately record and report their observations. The corrections to Universal Time from time zones in the United States are listed in the chart below:

Local Time to Universal Time conversion table (DST indicates Daylight Savings Time)

Atlantic: +4 hours

Atlantic DST: +3 hours

Eastern: +5 hours

Eastern DST: +4 hours

Central: +6 hours

Central DST: +5 hours

Mountain: +7 hours

Mountain DST: +6 hours

Pacific: +8 hours

Pacific DST: +7 hours

Alaska: +9 hours

Alaska DST: +8 hours

Hawaii-Aleutian: +10 hours

Hawaii-Aleutian DST: +9 hours

universesearch for term

All matter, energy, space, and time.

Uranussearch for term

The seventh planet from the Sun in the Solar System. Uranus orbits the Sun at an average distance of 19.2 A.U.. The planet has a mass 14.5 times the mass of the Earth and a radius 4.0 times the radius of the Earth. Uranus is a gas planet made of hydrogen, helium, and methane and has no solid surface. The planet's axial tilt is an extreme 98 degrees, causing one day on Uranus to equal 84 years, equal to the time it takes Uranus to orbit the Sun once. It has a small ring system and 17 moons, the largest of which are Titania and Oberon.

variable starsearch for term

A star whose brightness changes periodically. There are many types of variable stars and their brightnesses vary for many different reasons. For example, the brightness of a Cepheid variable star changes because its luminosity changes; the luminosity change is related to the internal structure of the star and the processes that occur there. Variable stars can also be caused by eclipsing dark or bright companions

velocitysearch for term

The speed and direction in which an object moves.

Venussearch for term

The second planet from the Sun in the Solar System. Venus orbits the Sun at an average distance 0.7 A.U.. The planet has a mass of 0.08 times the mass of the Earth and a radius 0.95 times the radius of the Earth. It is made of solid, rocky material and has a predominantly carbon dioxide atmosphere. Clouds of sulfuric acid cover the entire planet. Venus has no moons.

vernal equinoxsearch for term

The equinox that occurs on or near March 21 each year.

visible radiationsearch for term

A wavelength, or frequency, of light that is more energetic than infrared light, but less energetic than ultraviolet light. Visible radiation is the only form of electromagnetic radiation visible to the human eye. It ranges in wavelength between 10^-7 and 10^-6 meters and in frequency between 10^14 and 10^15 Hz. These wavelength and frequency intervals include the colors red, orange, yellow, green, blue, indigo, and violet. Visible light is not harmful to life because it is not strong enough to ionize atoms or destroy cells. The Earth's atmosphere is transparent to visible light, allowing us to "see" our Earth, the Sun, the Moon, and the stars. Astronomers study visible light from celestial sources with ground-based and space-based telescopes.

voidsearch for term

An enormous empty space between superclusters of galaxies that is seen in the large-scale structure of the universe. Voids are separated from each other by walls and filaments, causing the large-scale structure of the universe to resemble a foam of bubbles.

wallsearch for term

A long arrangement of superclusters of galaxies, surrounded by voids and filaments in the large-scale structure of the universe. Together, walls, voids, and filaments cause the large-scale structure of the universe to resemble foam bubbles. Sometimes walls, such as one dubbed the "Great Wall", are the result of the intersection of several filaments.

wavelengthsearch for term

The length, usually measured from crest to crest or from trough to trough, of one oscillation of a wave. As an example, the distance between two consecutive peaks of ocean waves is a measure of wavelength. For light, wavelength measurements indicate both the color and energy of the light. For example, x-rays have very short wavelengths, around 0.1 nm, and very high energies. Visible light ranges in wavelength from 400 nm to 700 nm, and is of intermediate energies. Radio wavelengths measure on the order of meters, and radio waves are not very energetic. For light, wavelenth and frequency are inversely proportional.

white dwarfsearch for term

A dense, degenerate, Earth -size stellar remnant that does not exceed 1.4 solar masses. White dwarfs spin rapidly (1 revolution every 10 sec) and radiate at high temperatures (4000 - 100,000 degrees), properties that are the result of previous rapid gravitational collapse. White dwarf magnetic fields are very strong and can reach a billion times that of the Sun.

x-ray radiationsearch for term

A very energetic wavelength, or frequency, of light. X-rays are more energetic than ultraviolet light, but less energetic than gamma-ray. They range in wavelength from 10^-8 meters to 10^-12 meters, and in frequency from 10^17 to 10^20 Hz. X-rays can be quite harmful to life because they are strong enough to ionize atoms and thus destroy cells. The Earth's atmosphere shields us from all astronomical x-ray radiation. X-rays produced by people do have useful applications, however; for example, they are used to help identify broken bones and to treat certain types of cancer.

zenithsearch for term

The point in the sky directly above an observer. Because the atmosphere is thinnest at the zenith and thickest at the horizon, astronomers prefer to observe objects when they are closest to the zenith.

zodiacsearch for term

A ring of constellations that lie along the ecliptic, including Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpius, Sagittarius, Capricornus, Aquarius, and Pisces in order east of the vernal equinox. The constellations in the zodiac are often used by astrologers to help explain and predict events on Earth. Because the zodiacal constellations were defined 2,000 years ago, precession of the Earth's axis has shifted the position of the Sun relative to the zodiac signs so that they are off by one month, as quoted in daily newspapers.