RR Lyrae 
Most stars are pretty steady — their size and brightness seldom change by more than a tiny bit. But a star in the constellation Lyra undergoes major changes every few hours. Its diameter changes by hundreds of thousands of miles. And that causes its brightness to change dramatically, too — at its brightest, the star is twice as bright as when it’s faintest.
RR Lyrae changes because it’s nearing the end of its life.
Nuclear reactions have created a shell of helium around the star’s core. That traps the core’s heat like a lid atop a pot of boiling water. But the energy is still trying to get out, so it pushes the helium outward, causing the star’s outer layers to expand as well. As they do, the energy escapes into space, the helium cools, and the outer layers fall back inward. Each of these in-and-out “beats” takes less than 14 hours.
There are many other stars like RR Lyrae. And it turns out there’s a relationship between the length of each beat and the star’s true brightness. Astronomers can use that relationship to measure how far away these stars are. But first, they have to use other techniques to get the distances to a few of them, including RR Lyrae itself. And astronomers at the University of Texas have done just that. More about that tomorrow.
In the meantime, look for Lyra in the west this evening, marked by its brightest star, Vega. RR Lyrae stands above it, but you need binoculars or a small telescope to spot it.
Script by Damond Benningfield, Copyright 2012
This program was made possible in part by a grant from NASA.