Betelgeuse, one of the biggest stars in our region of the galaxy, slides across the southern sky tonight. It marks one of the shoulders of Orion, the hunter, which stands due south around 10 or 11 o'clock.
Betelgeuse hasn't always been this bright. And it might not always have looked orange, as it does now. A study released a few years ago said that until about 10,000 years ago, the star probably was blue-white. As humans built their first civilizations, it turned white, then yellow, and finally orange.
If so, the reddening was caused by changes deep inside the star.
For most of its 10-million-year life, Betelgeuse "fused" hydrogen in its core to make the next-heavier element, helium. The same thing is happening now inside the Sun. But Betelgeuse is about 15 times more massive than the Sun, which is a critical difference. The cores of heavy stars are extremely hot, so the stars "burn up" their hydrogen in a hurry.
As Betelgeuse exhausted the hydrogen in its core, it started to burn hydrogen in a thin shell around the core. The star's interior got hotter, so its outer layers expanded. This cooled the surface, which is why Betelgeuse looks orange. With more surface area, Betelgeuse got a little brighter, too.
Today, Betelgeuse is probably beginning to burn the helium in its core to make carbon and other elements -- a process that'll last no more than a few hundred thousand years. After that, Betelgeuse will blow itself apart. We'll have more about that tomorrow.
Script by Damond Benningfield, Copyright 2004, 2007
For more skywatching tips, astronomy news, and much more, read StarDate magazine.