Strange Stars

The southernmost star of the summer triangle is the bright star Altair. Altair marks the head of the eagle Aquila.

First, Altair itself, brightest star in Aquila. Most stars are round, like our sun. The force of gravity, acting equally on all sides, forms the star in the most compact shape possible: a sphere. That's why snowballs are round. Patting the snow equally on all sides takes the place of gravity.

Large moons, too, molten when they first form, become spheres before they have time to cool and solidify. Not so asteroids. They cool and are fixed in shape long before gravity can act to make them round. Usually asteroids are irregularly shaped.

There is another factor, though. Most stars and planets rotate, and this causes them to bulge out at the equator. This bulge is usually only slight, and can barely be noticed. But Altair rotates once in six hours, compared with 25 days for our Sun.

But at the equator of Aquila, the gas is moving at over a hundred fifty miles a second! Because of this, Altair is not a sphere, but, squashed like a hamburger bun, is almost twice as wide as it is high. If our Sun were like Altair, we would see an oval in the sky.

The other strange star in Aquila is Eta Aquilae, due south of Altair. (To find Eta, hold your closed fist out at arm's length. If the top of your fist just covers Altair, the bottom of your fist will be by Eta.) Every few days this star flares up and becomes twice as bright. Then it returns to its previous brightness. This entire cycle takes a week. No one knows the cause of this pulsation.

What a strange universe! Oval stars. Stars that weekly grow and shrink in brightness. How ordinary our own Sun is by comparison. And yet, how central it is in our lives.

(07/04/07)