Red Giants

By chart time, the two red giants of winter will have risen in the east: ruddy Aldebaran and blazing red Betelgeuse. The bigger and the brighter is Betelgeuse, but they show us the path all stars must follow, even our own Sun.

At the center of every star is a blazing nuclear inferno, converting hydrogen to helium at a temperature of over fifteen million degrees. What we see, though, is the much cooler surface. It takes millions of years for the heat to rise from this nuclear furnace. By the time this heat reaches the surface, it has cooled to several thousand degrees. For example, the surface of our Sun, the yellow ball we see, is only about six thousand degrees.

Near the end of a star's life, most of their hydrogen fuel in its core has been used up. Other, hotter, reactions begin. Since the core is hotter, the star expands. But because the heat is spread over a larger surface, the surface of the star cools. The star becomes a red giant.

Betelgeuse and Aldebaran are old stars and have expanded in this way. Betelgeuse has expanded more; its surface is cooler. The red we see from it is much deeper. Aldebaran is almost orange. (Like iron bars heated in a furnace, the color of a star tells its temperature. Red stars are the coolest, then orange, then yellow, like our own Sun.)

Four billion years from now, the Sun will begin to die. When it expands it will be larger than the orbit of the Earth. Caught in the atmosphere of the expanding Sun, the Earth will slow down. It will spiral into the Sun, and end in flames.

But all that will happen a long time from now. A very long time. And if men and women still live on the Earth at that time, they probably will be able to take care of themselves.