Gemini, the Twins
Look straight south in the early evening, and you’ll see the familiar constellation Orion high in the sky. Above Orion, to the northeast, the constellation Gemini, the Twins, will be easily visible.
Gemini consists of two long lines of stars, each of which is topped with a bright star.
The constellation takes its name from the two bright stars that are close together and farthest to the north — Pollux on the left and Castor on the right and farther to the north.
Castor is a famous binary, or double, star. Even a small telescope used at high power will show that Castor is really two stars, not one as it seems to the unaided eye.
The two stars of Castor are similar in brightness, but careful examination will show that one is slightly brighter than the other. Castor is a good test of the optics of a small telescope. If repeated attempts to split it into two stars fail, the optics of the ‘scope aren’t very good and you should return it for a refund. But keep trying for a while. Often our Central Ohio atmosphere is so turbulent that Castor will look like a fuzzy blob.
In 1804, the great British astronomer William Herschel noticed that the fainter of the two stars had shifted slightly in its position with respect to the brighter one. That discovery suggested for the first time that one star could revolve around another one in a true binary system. It takes about 350 years for the faint star to orbit once around the brighter one.
Pollux is similar to our sun in some ways. Their colors and temperatures are about the same, but Pollux is brighter and larger, suggesting that we might be seeing what our own sun will look like in a few billion years when it stops being a friendly yellow-orange star and expands into a dying red giant.
Or maybe even sooner. The ancients considered Castor and Pollux to be equally bright. However, even an untrained eye can see that Pollux is a bit brighter. Stars measure their life spans in billions of years, so it’s very unusual to see such a big change in only a few thousand years. According to 365 Starry Nights by Chet Raymo, Pollux may have increased in brightness in historical times. Such rapid changes suggest that Pollux may in fact be very near the end of its life. So check it out now. In 100 million years, it might be too late.
It is unusual to have two stars of such similar brightness so close together in the sky, so the ancients named them after a pair of storied mythological twins. Look for their story next week.
Is it any wonder that the ancients thought that Venus was the goddess of love and beauty? There she sits, blazingly white, against the azure-blue backdrop of evening twilight. Look low in the west. You can’t miss her.
Up and to the left of Venus and high in the southwest just after dark is Jupiter.
Mars is high in the south just before morning twilight. Look for it under the back end of the constellation Leo. Its color ranges from yellow to orange depending on your color sensitivity. The “red planet” is never really red and it’s still pretty faint right now.
Get up just before morning twilight to see Saturn and the star Spica right next to each other and parallel to the horizon. The planet is to the left.
The star and planet are almost identically bright. By late spring, they will both have migrated to the evening sky. Come see Saturn in a really big telescope at one of our Friday– ight programs.
Tom Burns is director of Ohio Wesleyan University’s Perkins Observatory in Delaware. firstname.lastname@example.org