09 December 2003

Astronomers Re-measure the Universe

with Hubble Space Telescope

AUSTIN, Texas—University of Texas at Austin astronomers are using Hubble Space Telescope (HST) to improve measurements of vast distances in space, which could greatly increase the accuracy of knowledge in all areas of astronomy from understanding how stars evolve to the size and age of the universe itself.

Fritz Benedict, Barbara McArthur, Tom Barnes and colleagues are shoring up the wobbly "extra-galactic distance ladder" by measuring the tiny apparent motions, or "parallax," of a particular kind of star called "Cepheid variables."

"HST is the only telescope on Earth or in space that can do this with the required precision right now," Benedict said. "Obtaining these parallaxes is extremely difficult, equivalent to measuring the size of a quarter seen from 3,000 miles away."

The project, which ranked first among 1,100 proposals by astronomers for use of HST this year, continues later this month with more Hubble observations.

Cepheid variable stars are one tool that astronomers use to measure vast astronomical distances. They work well for this because the rapidity with which their light-output varies tells scientists their intrinsic brightness. This interdependence is called the "period-luminosity (PL) relationship." So astronomers can measure the period of variation for a Cepheid variable star in a galaxy and deduce that galaxy’s distance from knowledge of the luminosity of a Cepheid with that period.

Cepheids make up one rung on the extra-galactic distance ladder that astronomers use to measure distance to objects outside our own Milky Way galaxy. In this ladder, each rung is a type of distance measurement that is the basis for the next rung above it, to measure out to farther distances.

One of the lower rungs is knowledge of the distance to the Large Magellenic Cloud (LMC) -- one of the satellite galaxies of the Milky Way. Astronomers’ knowledge of the LMC’s distance is based in large measure on Cepheids inside that galaxy. The problem is, those Cepheids are not made up of the same stuff as the ones in our galaxy. So astronomers aren’t sure if the P-L relationship really works right on them.

The team is working to eliminate the LMC rung from the distance ladder and to replace it with something sturdier. They’re using HST to directly measure the distance to 10 Cepheid variable stars inside our own Milky Way galaxy.

"By doing this we can compare the direct distance measurement with the one predicted by astronomers’ best calculation of the Cepheid P-L relationship -- revealing any discrepancies and allowing for necessary adjustments in that calculation," said Barnes. McArthur added, "Cepheids will then become a better yardstick."

For this study, the team is using HST to make extremely precise measurements of the location of each of the 10 Cepheids at various times over two years. In comparing earlier observations to those taken later, each star appears to have moved. This apparent motion is called "parallax."

"Trigonometric parallax -- watching a star seeming to move from side to side because the Earth orbits around the Sun -- is the only fundamental method of getting Cepheid distances and luminosities free from complicating assumptions," Benedict said.

His team is making the measurements using HST’s Fine Guidance Sensors (FGS) -- the instruments whose primary reason for being is to enable HST’s cameras and spectrographs to lock onto their targets. However, this "bonus science" with FGS was planned from the start. Benedict helped ensure that the FGS could be used for parallax work, and has helped in planning their use for more than two decades.

"Because of the great demand for HST time, we can do these measurements only for a small number of stars in the Milky Way, fewer than a dozen," Benedict said. "In the future, SIM can do this for a lot more stars."

SIM, the Space Interferometry Mission, is a future NASA space observatory, but one whose final results will not be available until 2015.

With this HST project, Bendict says, "I’m happy that we’ll have good results in two years instead of 12."

— END —