Two Suns in the Sky!

Fall has just begun, but already we can see the first star of winter: Capella. Its gleam is usually taken as a sign that winter is on the way. Really, though, it's never been absent. It's so close to the North Star that it's always above the horizon, like the stars of the Big Dipper, but because of the mountains surrounding us, we can't always see it. Now, though, it's above those mountains.

In Greek mythology, Capella was the goat that suckled the infant Zeus; and if you look carefully, you'll see three pinpoints of light under her; one is Zeus, the other two are her "kids." They look like diamonds through a pair of binoculars.

Capella is a yellow star, like our Sun, which means that it's just as hot: about ten thousand degrees Fahrenheit. But that's where the similarity ends. Capella is much bigger than our Sun -- more than ten times bigger, so even though it's the same temperature, the total heat from it would be a hundred times as great. And it has a companion -- red -- closer to Capella than we are to our sun.

Let's move to a safe distance -- say, ten times further than we are now from our Sun, to about the orbit of Saturn -- and see what this pair would look like. At this distance, Capella looks about as big as our Sun does now; its red companion, about half as big. As they circled each other they would appear in the sky, at most, as far apart as the width of your hand held at armıs length. They would draw together for about a month, then apart.

Most stellar systems are like that: pairs or whole groups of stars circling each other of which we see only the brightest member. Our Sun is different from most stars, being a loner. Itıs special -- but only to us.

(10/09/09)