Moon and Regulus

Here on Earth, the poles are a lot colder than the equator — by a couple of hundred degrees. And it’s the same for most of the other planets. That’s because the Sun shines more directly on the equator than the poles, pumping in more energy.

But for some stars, it’s just the opposite: the poles are much hotter than the equator.

A prime example is Regulus, the bright star that marks the heart of the constellation Leo, the lion. It’s close to the right of the Moon as they climb into good view late this evening.

What we see as Regulus is one of four stars that make up the system. The others are so faint, or so close to the bright star, Regulus A, that they’re invisible without some help.

The poles of Regulus A are almost 10,000 degrees hotter than its equator. And that’s because of the star’s high-speed rotation.

Regulus A spins at about 700,000 miles per hour at the equator — hundreds of times faster than the Sun spins. The fast rotation pushes gas at the equator outward, so Regulus is about a third wider through the equator than through the poles.

That puts the poles much closer to the star’s core, which is where it generates energy. Because of that, the poles are hotter than the equator. And that makes them brighter than the equator, too. So if we could see Regulus A from close range, it would look a little like a lozenge that’s bright at the top and bottom, and much fainter around the middle.

Tomorrow: fact and fiction in the stars.

Script by Damond Benningfield