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Invader from Space

Week of February 9, 1998

Our Galaxy, called the Milky Way, is a vast collection of gas, dust and about a half trillion stars. Like the solar system on an immense scale, the stars and other denizens slowly orbit the center of the Galaxy, with our Sun taking about 200 million years or so to make one circuit.

But there is more to our system than that. The Milky Way has other, smaller galaxies that also orbit it, galactic moons, if you will. Two of these dwarf galaxies were first "discovered" by the explorer Magellan, and are called the "Magellanic Clouds" (they look fuzzy and cloudy to the naked eye). In recent times as many as a dozen other, fainter galaxies have been found orbiting the Milky Way as well. The orbits of these satellite galaxies are varied, with some always staying far away, and others apparently with orbits that send them plunging through the Milky Way itself.

Until now, no one had seen one of these galaxies actually passing through our own (the movements are so slow that it would take millions of years to see any orbital progress). In 1994, Rosemary Wyse, an astrophysicist at Johns Hopkins University, and her team found a dwarf satellite galaxy actually in the disk of ours, on the far side of the Milky Way (someone tell Captain Janeway!). They calculate that its orbital period is about a billion years in length, and they were surprised to see that the satellite galaxy was relatively intact, given that the much larger mass of the Milky Way should have caused some major disruptions of its shape. This companion galaxy has passed through the Milky Way several times, and evidently will continue to do so. As it does, the Milky Way will strip more and more stars from it. Its eventual fate may be to merge completely with the Milky Way, as some of its stars already have.

The image above is of the two larger of Magellanic Clouds, and was taken from the Astronomy Picture of the Day Site. I highly recommend perusing that site. There are many pictures of the Clouds in the archives there.

©2008 Phil Plait. All Rights Reserved.

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