Tuesday, December 15, 2009


During the winter months, the constellation of Orion dominates the northern sky. This group of stars is among the easiest to recognize because of the three bright stars in a straight line that make up his belt. Orion and the surrounding area contains a number of remarkable stars and other celestial objects.

In Greek mythology, Orion was a mighty hunter who fell in love with Metrope, daughter of the king of Chios, and sought her hand in marriage. The king constantly deferred his consent to the marriage, and eventually had Orion blinded. Orion consulted an oracle who told him he could regain his sight by going to the far east and letting the rays of the rising sun fall on his eyes. His sight restored, Orion went to live on the island of Crete, where he was killed by the sting of a scorpion. After Orion's death, he was placed in the heavens as a constellation.

The orange star at he top left is the star Betelgeuse, 400 light-years from Earth and shining more brightly than 10,000 Suns. This is a star in decline, pulsating in the red supergiant stage. The color of a star gives information about its temperature, size, and life expectancy. Stars change color like a piece of heated metal, becoming progressively red, orange, yellow, white, and blue. For this reason Betelgeuse appears to flash red, contrasting with the pale blue of the surrounding stars.

Orion also contains a number of notable nebulas. Until 1610, nebulas were thought to be stars. Now they are recognized as giant clouds of gas and dust, sometimes luminous and other times dark. Nebulas fascinate astronomy enthusiasts because of their wispy, spiral shapes.

Clearly visible to the naked eye as a bright fuzzy patch, the Orion Nebula is the closest nebula to Earth (1,200 light-years away). Its location in Orion's sword, just south of the leftmost star in the belt, also makes it easy to find. Several thousand stars were probably born here. A few of the youngest are still inside the nebula, and they heat the ambient gas to over 18,000oF.

The Horsehead Nebula is one of the most admired objects in the sky, first observed at the Harvard University Observatory in 1888. The familiar horse head shape is actually a small extension of a much larger cloud, invisible against the night sky's black background. Stellar winds are gradually eroding the magnificent scrolls of this famous nebula, so that the head will lose its familiar shape over the next several thousand years. Enjoy it while you still can.

Adjacent to the Orion Nebula is a cloud of microscopic, bluish gas known as the Witch Head Nebula. The color is derived from the reflected light of the blue supergiant star Rigel, which shines more brightly than 40,000 suns. The nebula is actually a more vibrant blue than the star itself. Like the particles in the Earth's atmosphere, the nebula reflects more blue radiation because it absorbs blue less than other colors.

Next to Orion is the constellation Taurus, which contains the Pleiades, the most visible open cluster in the night sky. Single stars like our sun are relatively rare: stars usually come in groups of two or more that orbit one another. The Pleiades appears to contain seven stars in tight formation, although there are actually more than seven. They have been celebrated since ancient times: Homer's Odyssey mentions the Pleiades, as do the Old Testament books of Job and Amos. The cluster will dissipate over the next few million years, and each star will follow its own path through the galaxy.

All these objects near Orion are visible during December and January. For a clear view, use a telescope, but most can be spotted simply by looking up at the night sky on a clear night.

If you're interested in outer space, check out Astronomy: A Visual Guide, by Mark A. Garlick, and Cosmos, by Sylvia Arditi and Marc Lachieze-Rey. Both contain fascinating astronomy facts, along with lots of gorgeous glossy pictures.

For my previous posts on outer space, click here.

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