Brought to you by Starry Night

Monday, May 29

Last week I pointed out some variable stars that vary in brightness over a period of months. Tonight you can locate one that changes in only a matter of days. It's bright, easy to find and conveniently located in the evening sky.

By 10 o'clock, the constellation Lyra the Lyre hangs well above the eastern horizon. The pattern consists primarily of its brightest star, Vega, trailed by a parallelogram of four stars, which form the body of the mythical harp of Orpheus. As you look at this parallelogram, note the two easternmost stars, and in particular the one that lies furthest south of the two.

This is Beta Lyrae, also known as Sheliak, an eclipsing variable star that fluctuates a little over a magnitude from 3.45 to 4.36 every 13 days (or, to be as accurate as possible, every 12.939637 days). Being an eclipsing variable, Beta Lyrae itself does not vary intrinsically, rather it is periodically occulted by a fainter companion.

When the companion passes between brighter Beta and Earth, it dims the light of the brighter star. The period from minimum to minimum marks the orbital period of the companion.

Fortuitously, there is an easy way to calibrate Beta's maximum and minimum magnitudes. As you study the star from night to night, compare its brightness to Sulafat (Gamma Lyrae), which lies to Beta's left (or north), and Zeta Lyrae, which lies above Beta, nearest Vega. At its brightest, Beta should be only slightly fainter than Sulafat and at its dimmest it should equal Zeta.

Tonight's moon phase.

** 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.