Gemini is in prime viewing time during these early weeks of winter. The constellation is low in the east-northeast at nightfall, and remains in view all night. It’ll remain visible for most of the night throughout winter. And tonight, the Moon is passing through the constellation’s middle.

Gemini is best known for the bright “heads” of the twins, the stars Pollux and Castor. Pollux is the brighter of the two, with slightly fainter Castor above it during the evening hours.

Both stars are impressive, but for different reasons.

Pollux is nearing the end of its life, so it’s puffed up to giant proportions. It’s many times wider than the Sun, and dozens of times brighter. Eventually, the nuclear reactions in its core will shut down, and its outer layers will puff out into space. Only the star’s dead core will remain.

Castor is impressive not for its size, but for sheer numbers. What we see as Castor is actually at least a half-dozen individual stars — one of the largest families of stars yet seen. The stars span a wide range — from much smaller and fainter than the Sun, to much bigger and brighter.

Because of that range, the stars will live to reach different ages. The heavier ones will burn out within a few million years. But the little ones will continue to shine for billions of years longer. They’ll remain faint, though. So the heads of both celestial twins are destined to fade from sight.
 

Script by Damond Benningfield

Support McDonald Observatory