Rivals

Low in the southern sky tonight is ruddy Antares, the heart of the scorpion. At about 10:30, its "rival," the planet Mars, rises. Both are deep red, which gave rise to the name "Antares," which means "rival of Mars."

Why do they look so similar? The ancients had no answer, but we do, because we've learned the real difference between stars and planets.

Mars is just a big rock, like the Earth, but only half its diameter: about four thousand miles. On the other hand, Antares is a gleaming red supergiant almost a thousand times larger than the Sun.

Stars are really very strange objects. Logs in a fireplace, and all fires on the Earth, burn from the outside in. The heat, the kindling, the other burning logs are on the outside of the log, and it's this outside that begins to burn first.

Not so with stars. They all "burn" from the inside out. That's because the source of their energy is the fusion of atoms forced together by the tremendous gravitational force compressing the gasses of the star, and this force is greatest near the center. Only the innermost part of any star is actually burning; the rest is "waiting in the wings" for this core to be exhausted.

And eventually it will be exhausted. Now, for Antares. Not for another four billion years for our Sun. The fire moves outward from this incredibly hot core. Its heat will make the star expand, and this heat, though great, now has to be spread over the star's enormous bulk. Dying stars, like Antares, Betelgeuse, and Aldebaran are cool, and red.

Mars, though, is red because of a very different, and much simpler reason. It's rusty. Iron in the soil, oxygen in its atmosphere, water, were all there on early Mars. The same ingredients that make our tools rust here on Earth made Mars the red -- rusty -- planet.

(06/05/09)