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Spinning Horsehead

January 8, 2011

The brilliant constellation Orion, the hunter, climbs high across the southern sky tonight. Its most prominent feature is its "belt" of three bright stars. Not far from the belt is an eerie dark cloud that, through a telescope, looks like the knight in a chess game: the Horsehead Nebula. Its striking appearance makes the nebula a popular subject for books, TV, and web sites.

Astronomers in Europe recently discovered that the Horsehead is rotating.

They measured radio waves that are produced by the nebula's gas. Like sound and visible light, radio waves exhibit a Doppler shift. When a source of radio waves is moving toward us, the waves get scrunched together, shortening their wavelength. And when it's moving away from us, the waves get stretched out, increasing their wavelength. It's the same effect you hear when a train or ambulance races by: You first hear a high pitch, which comes from sound waves with a short wavelength, then a lower pitch, which comes from sound waves with a longer wavelength.

By measuring the Doppler shift of different parts of the Horsehead, the astronomers discovered that the nebula spins about once every four million years. The spinning produces a centrifugal force that may have flung two parts of the cloud away from the main body. One part we see as the horse's nose, the other as its mane. So if the Horsehead Nebula weren't spinning, it probably wouldn't look like a horse -- and its unique figure wouldn't be such a media favorite.

Script by Ken Croswell, Copyright 2010

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