Constellation of the Month: Orion, The Great Hunter
The February Constellation of the Month is one of the most obvious constellations. It is one that is almost universally recognized as a figure of a man in the winter sky, being composed of two first- and two second-magnitude stars forming a vertical rectangle and a diagonal line of three second-magnitude stars in the middle forming a belt. It is Orion, the Great Hunter.
Click on the map for a larger version.
Orion is due south around 8 p.m. This chart, centered about two-thirds of the way up from the horizon, shows not only the Great Hunter, but the Pleiades to his northwest. Following him is Canis Major, the Big Dog, with its brilliant star Sirius.
Mythologically, Orion was the son of Poseidon, the god of the seas, or at least was a gift from Poseidon, depending on which version of the story you read. He grew up strong and handsome, becoming a great hunter. In his travels, he met and fell in love with Pleione and her beautiful daughters. He pursued them for several years, trying to win their affection. Finally, Zeus intervened and transformed the Pleiades into doves and they flew off into the sky to become the open cluster of the same name.
Meanwhile, Orion traveled to the isle of Chios where he fell in love with King Oenopion's daughter Merope. The king kept delaying their wedding, so one night, Orion got drunk and forced himself on Merope. In revenge, Oenopion blinded Orion while he slept.
After a long quest, Orion's sight was restored by Helios, the sun god. On his way home, on the isle of Crete, Orion met his female counterpart, Artemis, the Huntress and goddess of the Moon, and they fell in love. But her brother, Apollo, feared she was just another of Orion's conquests, and plotted against him. Spying Orion swimming far out at sea, Apollo challenged Artemis to hit the tiny speck bobbing out in the ocean. Her arrow made a direct hit, though Artemis didn't realize what she'd done until Orion's body washed up on shore. She placed Orion's body in her silver Moon-chariot and carried him into the darkest part of the sky, where his stars would shine the brightest. She also placed his hunting dogs with him, now the constellations Canis Major and Canis Minor. Southeast of the Pleiades, Orion appears to be continuing to pursue the women across the sky.
Orion is actually composed of what is called an OB Association, an extremely open star cluster. This places most of the stars in Orion about the same distance from us, 800 light years. They are virtually all of spectral type "O" or "B." The exception to all this is the red giant star that is Orion's left shoulder, Betelgeuse (pronounced the same way as the character in the movie of the same name, "beetle juice"). Betelgeuse is roughly half the distance of the stars in the Orion OB Association, and much larger. If it were in place of our Sun, Betelgeuse would stretch to the asteroid belt between Mars and Jupiter. It is possible that one day Betelgeuse will become a supernova, making it as bright as the crescent Moon and easily visible in the daytime.
Below Orion's belt is a grouping of three stars that run downward, often depicted as his sword in constellation drawings. The middle "star" is really not a star and if you look at it with binoculars, you will see that it is a fuzzy patch commonly referred to as the Orion Nebula. This is actually a complex of three or four nebulae. When you look at it with binoculars or a telescope, you may be able to discern a greenish color. This is the glow emitted by oxygen when it is excited. The major color emitted by the nebula is red from excited hydrogen, but your eye is not very sensitive to red. Photographic film and digital cameras are sensitive to red, so most pictures show it as a red-to-white nebula, rather than the greenish one you will see directly.
Not only is the Orion Nebula easily visible, but it is an exciting area for astronomers—a stellar nursery where tens of thousands of new stars have formed within the past 10 million years or so. This is an extremely active region of star formation with all the right ingredients: dust and gas in large quantities, plus other stars to help start the compression that eventually forms new stars. The center of this nebula contains an obvious group of four stars called the Trapezium. What is not so obvious is that there are a thousand stars very close together, all less than a million years old. The sky on a planet circling one of those stars would be amazing, with hundreds of stars visible in the daytime, and a thousand brilliant stars shining in the night.
The Planets for February 2005
The first planet of the February evening is Saturn, low in the east-northeast. It remains in the eastern part of Gemini all month. Just past opposition, the Earth is starting to pull away from Saturn and so it is beginning to shrink and fade slightly at magnitude -0.6. Through the telescope, Saturn's ball is down to 20.1 seconds of arc across. The rings are continuing to open at 23.6 degrees, with the southern face showing, and they are 45.5 seconds of arc across.
Around 10 p.m. Jupiter will rise in the constellation Virgo, where it will be all month. Jupiter is at its stationary point in early February, so it will move very little during the month. Jupiter continues to slowly brighten at magnitude -2.3. In a telescope, you will see its equatorial diameter at 41.2 seconds of arc.
Mars rises around 4 a.m. Right at the beginning of the month, Mars moves from Ophiuchus to Sagittarius, where it remains the rest of the month. Brightening to magnitude +1.3, the Red Planet remains a tiny 4.9 seconds of arc across, a very difficult telescopic object.
The Moon passes 1.1 degrees north of Antares on the morning of Feb. 4. The Moon continues eastward and will be 1.2 degrees south of the Pleiades on the morning of Feb. 16. Finally, the Moon will pass near Jupiter on Feb. 27. That morning the Moon will be west of Jupiter. By the next morning it will already be east of Jupiter. On either day, they will make a beautiful pair in the morning sky, so "keep watching the sky"!
An amateur astronomer for more than 35 years, Bert Stevens is co-director of Desert Moon Observatory in Las Cruces.