You are here


May 16, 2011

The "twins" of Gemini aren't twins at all. But one of them is a triplet of twins -- three pairs of stars that are all gravitationally bound to each other.

Pollux and Castor are high in the west as night falls, with Pollux to the left and Castor to the right.

Although they represent the heads of the mythological twins, Pollux and Castor aren't related. In fact, they don't even look like twins. Pollux is about twice as bright as Castor, and shines orange to Castor's white. And the two are separated by more than 15 light-years, which means there's no physical relationship between them at all.

But Castor itself is a system of at least six stars, divided into three close pairs. The light from two of the pairs blends together to form the pinpoint that we see as Castor. The third pair is so faint that it doesn't add much to the overall glow.

Each of the bright pairs includes a star that's much bigger, brighter, and hotter than the Sun, plus a star that's much smaller, fainter, and cooler than the Sun. The stars in each pair are so close together that it takes them just a few days to orbit each other.

The stars that make up the third pair are both faint, cool cosmic embers known as red dwarfs. They're quite close to each other, but a long, long way from the other pairs -- so far that it takes thousands of years to complete a single orbit around them -- making the red dwarfs distant cousins of their double twins.


Script by Damond Benningfield, Copyright 2011


Get Premium Audio

Listen to today's episode of StarDate on the web the same day it airs in high-quality streaming audio without any extra ads or announcements. Choose a $8 one-month pass, or listen every day for a year for just $30.