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.

Plan­ets

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. tlburns@owu.edu

Posted by Tom Burns on Feb 13 2012. You can follow any responses to this entry through the RSS Feed. Comments can be made below.