The closest spiral galaxy to the Milky Way Galaxy. The name of the Andromeda Galaxy (Fig. 1) derives from its appearance in the constellation of Andromeda. The galaxy is also known as M31 and NGC 224, based on its entries in two widely used catalogs. See also: Andromeda; Astronomical catalogs; Constellation; Messier Catalog; Milky Way Galaxy

One of the large self-gravitating aggregates of stars, gas, dust, planets, rocks, and dark matter that occurs by the hundreds of billions in the universe, collectively containing most of the cosmos' visible matter. Everywhere we look beyond our own galaxy into the broader cosmos, the objects we see are, likewise, galaxies (Fig. 1). These “islands in the void” often occur in groups or clusters containing from a few to as many as thousands of individual galaxies, with the clusters ranging from a few hundred thousand to tens of millions of light-years in expanse. See also: Light-year; Matter (physics); Star

How the diverse array of galaxies that are now observed originated and evolved into their present form. Deep telescope observations of the cosmos have found evidence for the first galaxies arising within several hundred million years after the big bang. Since that time more than 13 billion years ago, galaxies have emerged throughout the universe, numbering in the hundreds of billions and often appearing in grand agglomerations known as clusters and in long, vast threads known as filaments. Most galaxies also have evolved to fall into three, broad categories of shape: spiral (with curving internal structures of stars), elliptical (rounded), and irregular (no defined shape) (Fig. 1). Researchers studying galaxy formation and evolution “work backwards” in explaining how galaxies assumed these shapes, predominantly occur in clusters, and have changed as a population over cosmic history. See also: Big bang theory; Cosmology; Galaxy; Universe

The small cluster of galaxies that contains the Milky Way Galaxy. The Local Group (see illustration) contains at least 80 galaxies, with two spiral galaxies, the Andromeda Galaxy and the Milky Way Galaxy being the largest, dominant members in terms of mass, followed by a third, smaller spiral galaxy, the Triangulum Galaxy. Most of the members of the Local Group are dwarf galaxies that are associated with either the Milky Way or Andromeda. The overall shape of the Local Group is that of a dumbbell, with the Milky Way and Andromeda and their respective dwarf galaxy collections each forming a lobe. A distance of about 2.5 million light-years separates the two lobes of the Local Group, centered on the Milky Way and Andromeda. See also: Andromeda Galaxy; Astronomy; Galaxy; Light-year; Milky Way Galaxy

Two small, irregular galaxies that are close companions of the Milky Way Galaxy. Both are nearby galaxies that are located in the southern sky, not far from the south celestial pole (Figs. 1 and 2). When viewed without a telescope, they resemble small sections of the Milky Way that might have drifted away from the main arc of that soft, glowing belt of light. On a very dark, clear night the Magellanic Clouds can also be mistaken for small atmospheric clouds because of their diffuse appearance. See also: Milky Way Galaxy

An astronomical object that appears starlike on ground-based images but possesses many other characteristics, such as a large redshift, that prove it is not a star. Quasars are the most luminous members of a larger family of objects referred to as active galactic nuclei (AGNs). Found at the centers of galaxies, quasars often outshine their galactic host by a factor of 100–1000, making them among the most luminous and energetic sources in the universe. Because of their high luminosity, quasars can be detected at great distances, and are therefore also among the most distant objects ever observed. Astronomers widely believe that the accretion of hot gas into supermassive black holes at galactic centers provides the energy that powers quasars (Fig. 1). Quasars are useful probes of cosmology, the formation of black holes, and the evolution of galaxies in the universe. See also: Black hole; Cosmology; Galaxy; Supermassive black hole; Universe

The nearest large cluster of galaxies and one of the best studied. It dominates the distribution of bright galaxies in the sky, forming a conspicuous clump in the distribution of easily visible elliptical and spiral galaxies. The clump lies mostly in the constellation of Virgo but also extends into neighboring constellations, especially Coma Berenices. The well-known spirals M58, M61, and M100, for example, are members of the Virgo Cluster. See also: Constellation; Universe; Virgo