Want to Know a Star's Age?  Ask for its Birth Certificate

We all know that stars are burning balls of hot gas. The problem is, those balls of hot gas just keep on burning for most of their lives. When astronomers look at an individual star, they have no way of knowing how old it is, unless it's either very old or very young. 

Dr. Soren Meiborn thinks he may have found a new technique to measure stellar age. As he said in the Boston meeting of the American Astronomical Society last month, he can now ask for a star's birth certificate, and he does it with the Kepler satellite. Kepler has the on-going task of monitoring the brightness of 100,000 stars, with the aim of finding transiting planets in other star systems. But for astronomers like Meiborn, it's the stars themselves that are really interesting.

In the video, you can see spots form (around 0:20) and rotate along the sun's surface - astronomers have been using them for centuries to measure the sun's rotation. Kepler can now help astronomers do the same with far-away stars. Kepler can't make out the individual spots on a stars' surface, but it can do the next best thing by giving very precise brightness measurements. 

Meiborn monitored a group of young stars with lots of surface activity, measuring each star's rotation rate by looking for the brightness changes caused by its spots. Stars start out their lives spinning like a top, but just like tops, as time goes on, the spinning slows. Our sun spins once every 30 days, but the younger stars Meiborn monitored were spinning much faster - from 1 to 11 days. Using this data, Meiborn was able to prove that this method of measuring a star's age, called gyrochronology, really works.