Astronomy is the one science in which the phrase "the possibilities are endless" really means something. There are all kinds of strange phenomena in space — one of which uses gravity to brighten and darken certain stars.

With so much historical emphasis on establishing the fact that the Earth revolves, rotates, precesses, and otherwise acts very much as if it isn't the center of the universe, we forget that the sun moves as well. It travels through space, along with the rest of the galaxy. It wobbles as the pull of various planets revolving around it moves it this way and that. It also rotates, spinning like a top.

Most stars rotate. Stars are generally formed by matter, pulled together by gravity, swirling like water around a drain. The swirl doesn't go away as the matter collapses inwards, it just translates into spin. Some stars spin very, very fast indeed.

These ultra-fast spinners presented a brief mystery to scientists. They darkened around their equators, and brightened up at the poles. The faster a star spun, the more pronounced the effect was. What about spin caused the poles to burn bright and the equator of a star to darken?'


Nothing — at least not directly. Get on a carousel or merry-go-round. When it spins, stand right at the edge. Then walk to the center. As you move to the center, the force you feel "pushing" you outwards will gradually diminish. You're spinning faster, and feeling greater centrifugal force, on the outside edge of the merry-go-round. Once you get to the center, where you are barely moving, the force disappears. Spinning objects in space, whether they're planets are stars, are three-dimensional versions of that kind of spin. Stand on a star's equator, you'll be standing on the area of the star that's spinning fastest. You'll feel a strong "push" out into space. As you walk to the poles, you'll be spinning more slowly, and feeling less of a push.

All the matter on the star feels that same outward push at the equator, so the star bulges outwards at the equator until it becomes an oblate spheroid — think a beach ball that's being slightly stepped on. Points on the star's poles are much closer to the center of the star than points on the equator. Since the poles are closer to the center of the star, they feel a much higher surface gravity than the equator does. Gravity pulls matter towards the center of the star, so a higher gravity area is a higher pressure area. When you exert pressure on a gas, you bring up it's temperature. Areas at the pole of a fast-spinning star are at a higher temperature, and shine brighter, than the rest of the star. This is known as gravity brightening. Its converse, gravity darkening, happens at the equator, where gravity, pressure, and temperature, are relatively low.

That's how a star, spinning fast enough, can glow at its poles and get dark around its middle.


Image: NASA Goddard Laboratory for Atmospheres.

[Via Gravity Darkening in Rotating Stars]