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What Are Astronomers Doing? Week of July 26
Using Binary Stars to Measure a Cluster's Distance
Jocelyn Tomkin will be using the Otto Struve Telescope to study "spectroscopic binary stars" to help pin down the exact distance to the Hyades Cluster -- the closest star cluster to our Sun.
Binary stars are pairs of stars that orbit each other. Sometimes, even when two stars appear as one in a telescope, their combined spectrum reveals that the apparent star is actually a binary pair. Binary stars that can be identified by their spectra are called spectroscopic binary stars.
Jocelyn is studying spectroscopic binary stars in the Hyades cluster, a young open cluster of stars that is relatively nearby in our galaxy. His goal is to accurately measure the radial velocities, or velocities towards and away from an observer, of each of the stars in the binary systems.
Some of the stars he studies have already been discovered as spectroscopic binaries, but they don't have accurate radial velocity measurements. In other cases, the second star has never been detected before because it is too faint. With the instrumentation available at McDonald Observatory, Jocelyn is able to find the second star and measure the radial velocities for both members of the binary system.
Measuring the radial velocities of binary stars is important because it allows astronomers to determine the masses and physical separations of the two stars. Moreover, new types of high resolution telescopes are turning many spectroscopic binaries into visual binaries. That is, the resolution on these new telescopes is so high that they can image both stars in a binary pair, where older telescopes could only image the two stars blurred together into one.
An image of the two stars provides the angular distance between them, and together with the mass and physical separation measurements from Jocelyn's spectroscopy work, it is possible to calculate the distance to the pair. Because Jocelyn's stars are in the Hyades cluster, this means he can find out how far away the cluster is.
Jocelyn uses the Sandiford Cassegrain Echelle Spectrometer on the 2.1-meter Otto Struve Telescope for this project. The spectrometer provides the high resolution he needs to separate the spectral lines from each star. Without a high degree of resolution, the spectral lines of each star fall on top of each other most of the time, and it is very difficult to make good measurements of the stars' motions. In addition, the 2.1-meter diameter mirror of the telescope allows him to collect enough data in a reasonable time to make measurements.
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