Measuring the distances to other galaxies is a tough problem. In fact, it would be all but impossible without a discovery that was reported 100 years ago.
The discovery was made by Henrietta Leavitt, an assistant at Harvard College Observatory.
She was studying stars in two small galaxies that orbit the Milky Way. She found hundreds of stars that regularly brightened and faded. Leavitt discovered that there was a relationship between how bright the stars got and how long it took them to brighten.
The discovery was one of the most important of 20th-century astronomy. That's because it provided a way to measure the relative distances to these stars, and therefore to their parent galaxies. Just measure the length of a star's pulse, then see how bright the star looks. Stars that look fainter must be farther away.
Later, astronomers turned the relative distance into absolute distance when they realized that there are similar stars in our own galaxy -- close enough to measure their distances with other techniques. With the distances to a few of the stars, astronomers could then calculate the distances to any of them just by measuring the star's brightness and the length of its pulses.
These types of stars are bright enough to see in galaxies that are up to 50 million light-years away. So these "œflickering" stars provide a way to measure distances to the closest galaxies -- a first step in measuring the distances to all galaxies.
Script by Damond Benningfield, Copyright 2008
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