Stellar Yardsticks

StarDate: March 27, 2009

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One of the most important markers for measuring astronomical distances is a class of stars that "pulse" in and out like beating hearts. The stars are called Cepheids, and they're used to get the distances to relatively nearby galaxies. With those distances in hand, astronomers can then use other techniques to measure galaxies that are much farther.

Cepheids are good markers because they're bright, so you can see them from a long way away. And there's a direct relationship between the length of the pulses and the star's true brightness. So if you measure the length of a pulse and how bright the star looks, then you should be able to calculate its distance.

For the technique to work, though, you have to know the true brightness of the stars -- and that means you have to use other techniques to pin down the distances to some of the closer ones. But the Cepheids in our galaxy are so far away that it's hard to do.

A team led by Texas astronomer Fritz Benedict is using Hubble Space Telescope to make the most accurate measurements of Cepheids to date.

The team measures tiny back-and-forth motions in each star's position in the sky. The shift is caused by Earth's motion around the Sun. The size of the shift reveals a star's true distance.

With the distances to a few Cepheids in hand, astronomers can then calibrate the distances to others -- including those in other galaxies.

Script by Damond Benningfield, Copyright 2009

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