The Invisible Giant

One of the strangest objects the sky in the sky tonight is an invisible star. Or one that's almost invisible. Here's what it is and how we know it's there:

That brilliant star in the northeast is Capella, sixth brightest star in the sky. Capella is close enough to the Pole Star that it is never really absent from the night sky, but in the summer the mountains in the north obscure it. (If there were no mountains, we could always see it, but then we couldn't go skiing.) Starting in the fall, though, and continuing through spring, it can be seen, first in the northeast, as it is tonight; then becoming more prominent as fall turns into winter and spring approaches.

In mythology, Capella was the goat that nursed the infant Jupiter. Under Capella are three dim stars known as "the kids." The "kid" closest to Capella has no proper name at all but is known simply as "epsilon Aurigae" -- the fifth brightest star in the constellation of Auriga. Every twenty-seven years this star gets a little dimmer and then returns to its regular brightness.

Last week we spoke about another star that brightens and dims periodically, Algol, in Perseus. But there's a major difference between Algol and the kid. When Algol is bright, we see it; when it's dim, we see its companion. This makes sense because Algol is one of a pair of stars rotating around each other. When the other star rotates around Algol, it partially eclipses it, and Algol appears dimmer.

Now the kid wobbles in time to its dimming, so we know it's circled by another star, pulling it to and fro. But when it dims, astronomers see only it (dimly,) not the other star. We see the kid right through its companion. Its companion is invisible!

This stellar companion is so cool that it emits no visible light. Its surface temperature is less than fifteen hundred degrees, compared with the Sun's six thousand. It has never been seen. It is so vaporous that we can see right through it. A supergiant larger than the orbit of Saturn, its density is only a billionth that of the Sun's. It is close to a perfect vacuum. It is the largest star there is, and it almost isn't there.

(09/24/08)