Local Group is a small concentration of galaxies that includes our own galaxy, the Milky Way , in addition to many nearby galaxies. Astronomers have identified about 50 galaxies in the Local Group. These galaxies occupy a roughly spherical region of space around 10 million light-years in diameter. One light-year is the distance light travels in a vacuum in a year—about 5.88 trillion miles (9.46 trillion kilometers).
The Andromeda Galaxy and the Milky Way rank as the largest members of the Local Group, shining brighter and having a greater mass (amount of matter) than the remaining galaxies combined. The Andromeda and Milky Way galaxies are spiral galaxies, which feature “arms” of stars that coil outward from a distinct center. The Local Group also includes a smaller spiral galaxy called the Triangulum Galaxy . Much smaller dwarf galaxies make up the remainder of the Local Group. Some of these dwarf galaxies have an irregular shape with no distinct pattern of stars. Many of the rest are elliptical galaxies with a rounded shape and a bright, well-defined center. A few dwarf galaxies in the Local Group combine irregular and elliptical features. These galaxies may be in the process of changing from one type into the other.
The gravitational pulls of the Milky Way and the Andromeda Galaxy exert a powerful influence over the Local Group. Many of the smaller galaxies revolve around the Milky Way or the Andromeda Galaxy, much as a satellite orbits a planet. These satellite galaxies can experience strong tidal forces, stresses resulting from the uneven pull of gravity of a massive object. Tidal forces created by a large galaxy can shred apart nearby satellite galaxies and trigger violent periods of star formation in more distant ones. Satellite galaxies destroyed by tidal forces contribute stars, gas, and other matter to the larger galaxy. In this way, the Milky Way and the Andromeda Galaxy grow larger by consuming smaller galaxies.
Gravitational attraction also binds the Milky Way and the Andromeda Galaxy to each other. Today, the two galaxies are pulling closer together. In a few billion years, they will probably collide and begin to form a larger galaxy, most likely an elliptical one.
The Milky Way and four other Local Group galaxies can be seen with the naked eye or through binoculars. The Andromeda and Triangulum galaxies appear in the northern sky. The Magellanic Clouds , two irregular dwarf galaxies, appear in the southern sky.
