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The Milky Way is a member of the Local Group of galaxies, which in turn is a part of the Virgo supercluster (see Figure 03-06a). It is centered on the Virgo cluster and extends some 150 million light years across. The Virgo cluster itself contains thousands of galaxies including M87, which is known to surround a gigantic black hole. Virgo's gravity affects the movement of its neighbors, including the Local Group. The supercluster is the last outpost before a space traveler would enter a nearly galaxy-free region called a cosmic void. Actually, even the supercluster has a mass equaling some thousand trillion suns, virtually all its volume is empty in such a vast space. The Local Group of galaxies extends some 4 million light years across. Most galaxies in the group are considered dwarfs, but the two largest - the Milky Way and the Andromeda galaxy - are giant spirals moving toward each other at a speed of about 100 km/sec. All the galaxies of the Local Group are traveling together through space - indicating a common origin.
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