Bootes, the herdsman, passes high overhead on May evenings. It's best known for its brightest star, yellow-orange Arcturus, which is high in the east at nightfall. It's easy to pick out because there are few other bright stars in that region of the sky.
Bootes is also home to a double star that's teaching astronomers about some of the faintest stars in the galaxy.
The star is GU Bootis, and it consists of two red dwarfs -- stars that are smaller, fainter, and cooler than the Sun. The two stars orbit each other twice a day. And by good luck, their orbit is perfectly aligned to our line of sight. As a result, the stars periodically eclipse each other.
Eclipsing binaries provide valuable basic information about stars.
The gravitational attraction between the stars depends on their masses, so the faster they revolve around each other, the heavier they must be. So monitoring the system reveals the stars' masses. This information is especially important for red dwarfs. They account for most of the galaxy's stars, but astronomers have measured the masses of only a handful of them.
GU Bootis is valuable for another reason. When two stars eclipse each other, the length of the eclipse reveals the sizes of the stars.
In this way, astronomers have learned that each star in GU Bootis is about 60 percent as massive as the Sun, and about two-thirds as big -- rare measurements for the most common stars in space: red dwarfs.
Script by Ken Croswell, Copyright 2008
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