Brought to you by Starry Night

Wednesday, July 19

Yesterday I described the star Erakis (Mu Cepheii), one of the largest and reddest in the heavens. Today, I want to talk about another distinctive luminary adjacent to Erakis, one that you can see vary in brightness if you watch it long enough. Moreover, it's an important tool in research astronomy.

At 10:30, Cepheus stands in the northeast directly above the W-shaped constellation Cassiopeia the Queen. Just a little over 2 degrees east of the 3rd-magnitude star Zeta Cephei is the fainter Delta Cephei. Precisely every 5.4 days, Delta varies by nearly a magnitude, from 3.6 to 4.3, and you can actually follow these brightness changes in a small telescope.

Delta Cephei is the prototype for a class of stars that astronomers use in measuring distances to nearby galaxies. Cepheid variables, as they are called, periodically swell and contract in size, struggling with gravity, which wants to collapse the star, and radiation, which wants to expand it.

In the early 1900s, astronomers discovered that the longer it took for a Cepheid star to vary in brightness from maximum through minimum and back to maximum again, the more intrinsically luminous it was. Like recognizing a zebra by its stripes, astronomers now had an unfailing means of assessing a Cepheid's true brightness using its period.

By locating Cepheids in nearby galaxies and correlating their periods with known Cepheids in our galaxy, astronomers were able to accurately determine their distance by using the inverse square law of light: luminosity varies inversely with the square of the distance from the source.

With this new standard candle, astronomer Edwin P. Hubble, in late 1923, was able to confirm that the Andromeda "nebula" was in fact extragalactic in nature. It lay over 2 million light-years away. It could now be called, the Andromeda Galaxy.

Old habits die slowly, though. As late as the early 1960s, even astronomers still referred to it as the "Andromeda Nebula."

** Put the sky in the palm of your hand. Download SPACE.com's Skywatch, along with the latest space news, into your Palm Pilot or other handheld device. **

Jeff Kanipe is the author of A Skywatcher's Year, an astronomy guide just published by Cambridge University Press. He is a former editor at Astronomy and StarDate magazines and a writer for the Earth & Sky radio series.

The images in Skywatch are produced by Starry Night software.