Active Galaxies

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The constellation Virgo glides across the south on these warm spring nights. Look for its brightest star, blue-white Spica, well up in the southeast at nightfall, and due south a couple of hours later.

A big cluster of galaxies spreads out to the north and west of Spica. Two of its most imposing members are M84 and M87. Both are big and heavy, with supermassive black holes at their centers -- black holes that may be billions of times as massive as the Sun.

M84 and M87 are both classified as active galaxies. Their centers are pumping out unusually large amounts of radiation -- from radio waves to gamma rays. And oddly enough, that's probably because of the black holes.

The black hole in the center of a galaxy can surround itself with a disk of gas. As the gas spirals toward the black hole, it gets hotter and hotter, so it glows brightly. It may even get hot enough to create particles of matter and antimatter, which cancel each other out in bursts of gamma rays; more about gamma rays tomorrow. At the same time, powerful magnetic fields direct some of the particles into "œjets" that shoot far into space.

There are different kinds of active galaxies, depending on what kinds of energy astronomers observe from them. One variety is known as blazars. Their jets aim directly our way, so it's like we're looking down the barrel of a galactic weapon -- a powerful energy beam from a galaxy's active heart.

Script by Damond Benningfield, Copyright 2008

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