Jimmy Westlake: The colors of Orion
December 3, 2012
Now that the full moon is out of the way for another month, it's time to do some stargazing. At the top of your list should be the magnificent constellation of Orion the Hunter. Orion rises over the eastern horizon at about 8 p.m. in early December and is high in the southern sky by midnight.
Orion is a very colorful constellation. Its two brightest stars, Betelgeuse and Rigel, present distinctively different colors to the human eye. Betelgeuse, the bright star marking Orion's left shoulder, has a red-orange hue, and Rigel, the bright star at Orion's right foot, looks icy blue. Why do these stars have such different colors?
The palate of star colors is a direct result of their temperatures. Astronomers rank stars by their surface temperature, using an alphabetic scale that might at first seem confusing. From hottest to coolest, the temperature classes are O-B-A-F-G-K-M. The hottest stars, in the O and B classes, look bluish-white to the eye. Rigel is a B-class star. The coolest stars, in the K and M classes, look reddish-orange. Betelgeuse is an M-class star. Our sun lies near the middle of the temperature scale and is classified as a yellow-colored G-class star. The time-tested mnemonic for remembering the correct order of the stellar temperature classes goes like this: Oh, Be A Fine Girl (or Guy), Kiss Me. This memory device has been used by astronomers and students alike for the better part of a century.
The three stars that form Orion's belt are named Alnitak, Alnilam and Mintaka. Alnitak, the left star in the belt, is the brightest O-class star in the night sky and looks very blue. Alnilam and Mintaka, the middle and right stars in the belt, are both slightly cooler B-class stars.
Bellatrix, Orion's right shoulder, and Saiph, his left foot, are hot B-class stars and look bluish-white to the eye.
Below Orion's Belt are three rather faint, compact groups of stars that represent the Hunter's sword, hanging at his side. You might notice that the middle star in Orion's sword looks as if it is surrounded by a fuzzy haze. If you have a pair of binoculars, try aiming them at this object and you'll see the fuzz even better. The Latin word for fuzz is "nebula." This object is the Great Nebula in Orion, also known by its catalog number, M42. The hot stars in the center of this nebula are causing the surrounding gases to glow with a striking hot pink color. A small telescope shows this even better.
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If you have trouble seeing the colors of stars, you're not alone. Faint sources of light sometimes don't trigger the color receptors in the human eye, so they just look white. Binoculars and telescopes gather more light than your eye alone can.
On the next clear night, why not step outside with your binoculars and observe the colors of Orion yourself?
Professor Jimmy Westlake teaches astronomy and physics at Colorado Mountain College's Alpine Campus. Check out Westlake's astrophotography website at http://www.jwestlake.com.