Our galaxy, the Milky Way, is fairly big, bright, and heavy. So is its closest big neighbor, the Andromeda galaxy, which is two and a half million light-years away. Yet they're both tiny compared to the biggest galaxies of all, known as "cD" galaxies. If you dropped the center of one of these monsters between Andromeda and the Milky Way, its outskirts would engulf both galaxies, and extend another million or so light-years beyond.
CD galaxies are found only in the centers of dense clusters of galaxies. One of the closest examples is NGC 4881, which is about 350 million light-years away. It's a member of the Coma Cluster -- a collection of thousands of galaxies.
CD galaxies can be up to 50 times wider than the Milky Way. They're also much more massive, and they contain many more stars. A cD galaxy has a bright, dense ball of stars in the middle, but its outskirts are spread pretty thin -- you can easily see background galaxies right through it.
About 90 percent of a cD's mass consists of dark matter -- matter that produces no detectable energy, but that exerts a gravitational pull on the visible material around it.
Astronomers haven't solved the mystery of how cD galaxies took shape. They may have formed from the mergers of smaller galaxies, and added to their great heft by pulling in the gas that permeates galaxy clusters. They are likely to pull in even more material -- adding to their already impressive dimensions.
Script by Damond Benningfield, Copyright 2008
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