Standard Candles
One of the hardest things astronomers do is measure the distances to celestial objects. Yet it's also one of the most important. Only by knowing the distance of a star or galaxy can we learn how big and bright it is.
Measuring distances is difficult because the sky looks flat. Some stars appear brighter than others, but the brighter ones aren't necessarily closer -- they may just be more powerful.
To help, astronomers use what they call "standard candles." These are objects whose true brightness is known.
The best-known standard candles are a class of stars called Cepheids, named for a star in Cepheus, the king. These giant stars puff in and out like a beating heart. As the stars beat, they brighten and fade.
The nearest Cepheid is Polaris, the North Star, but Cepheids are so bright they can be seen in other galaxies.
A century ago, in 1907, Henrietta Leavitt was studying Cepheids in a nearby galaxy. She discovered that the longer a Cepheid took to pulsate, the brighter it looked. Since the stars were all in the same galaxy, they were all about the same distance from Earth. That meant the slower-pulsating Cepheids must emit more light than their peers.
Today, astronomers measure the apparent brightnesses of Cepheids in galaxies tens of millions of light-years away. This reveals the galaxies' distances -- and shows how Cepheids are among the most useful "standard candles" in the sky.
More about Cepheus tomorrow.
Script by Ken Croswell, Copyright 2007
For more skywatching tips, astronomy news, and much more, read StarDate magazine.
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