The hearts of galaxies are crowded and busy. They contain lots of densely packed stars and big clouds of gas and dust, and they generate strong magnetic fields. And supermassive black holes fling stars around, and occasionally pull a star apart and ingest its gas.
This process is a lot more active in some galaxies than others.
In a galaxy with an "active" nucleus, a continual supply of gas encircles the black hole with a wide, thin disk. The gas is heated to millions of degrees as it spirals close to the black hole. That generates X-rays and ultraviolet light. Magnetic fields funnel some of the gas into powerful jets, which produce radio waves. So the cores of active galaxies are bright and noisy across the entire energy spectrum.
A survey a couple of years ago by an X-ray satellite found that there are about 200 active galaxies within 400 million light-years -- a tiny fraction of the total number of galaxies.
Supermassive black holes probably inhabit the hearts of most all of the galaxies. But in most galaxies, they're quiet; they have little or no gas around them, so they produce almost no energy. That includes our own galaxy, the Milky Way. The black hole at its heart is about three million times as massive as the Sun. It occasionally flickers a little, but most of the time, it sits quietly -- a dark blob in the busy heart of a galaxy.
We'll have more about black holes tomorrow.
Script by Damond Benningfield, Copyright 2008
For more skywatching tips, astronomy news, and much more, read StarDate magazine.
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