Study GuideAstronomy–Galaxies1.Clusters of GalaxiesGalaxies are rarely found alone. Just like stars often form groups, galaxies tend to gather in smallgroups or very large clusters.Some clusters are small and contain only a few galaxies. A famous example isStephan’s Quartet,which has just four galaxies. Other clusters are enormous. TheVirgo Cluster, for example, containsabout3,000 galaxiesand has a total mass of about10¹⁵times the mass of the Sun.Small clusters usually have galaxies scattered in irregular patterns across a relatively small region ofspace. In contrast, very large clusters tend to look more organized. Their galaxies are spread outsymmetrically around a center. This orderly shape suggests that the galaxies’ motions are balancedby the cluster’s gravity. In other words, gravity is strong enough to hold the cluster together.1.1The Large-Scale Structure of the UniverseFor a long time, astronomers thought clusters were the largest structures in the universe. Modernobservations show that the universe is even more complex.Galaxies are arranged in hugesheets or wallsthat surround vast regions calledvoids, where veryfew galaxies exist. Clusters and long chains of galaxies appear where these walls intersect. Whenastronomers map this structure, it looks a lot like the inside of a sponge—dense regions surroundinglarge empty spaces.1.2The Local GroupOur home neighborhood in space is called theLocal Group. It contains about30 galaxiesspreadacross roughly1 megaparsec (Mpc)of space.Most of the mass in the Local Group is concentrated in two large spiral galaxies: theMilky WayandtheAndromeda Galaxy. These two galaxies are close enough to each other that they are bestdescribed as abinary galaxy system.Each of these large galaxies is surrounded by many smallerdwarf galaxies, sometimes called theircoronas. In addition, about six smaller galaxies are spread more widely and are not clearly tied toeither the Milky Way or Andromeda.Preview Mode
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