StarDate: May 11, 2009

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The view from a star cluster known as M3 would be spectacular. The cluster is more than 30,000 light-years above the disk of the Milky Way galaxy. In fact, from our position inside the galaxy's disk, M3 is almost straight up. So looking "down" from M3, the galaxy would fill the sky, with its spiral arms and the big bulge of stars in its middle shining brightly.

M3 consists of half a million stars, all of which are billions of years old. Some of those stars are entering their final stages, which makes them a bit cranky. It also makes them good markers for measuring the cluster's distance.

They're known as RR Lyrae stars. They're all roughly half as massive as the Sun, but a few dozen times brighter. They've burned through the hydrogen in their cores to make helium. Now, they're burning the helium to make carbon. That process makes their outer layers pulse in and out like beating hearts.

And that's what makes them good markers.

There's a relationship between the length of each "beat" and the star's true brightness. And if you know how bright a star really is, and compare that to how bright the star looks, it's a simple matter to calculate its distance. There are about 170 RR Lyrae stars in M3. They tell us that the cluster is 33,900 light-years away.

M3 is high in the east at nightfall. It's to the upper left of the bright yellow-orange star Arcturus. Through binoculars, it looks like a fuzzy round blob of light with a bright center.

Script by Damond Benningfield, Copyright 2009

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