Local Group

The Local Group is the specific collection of galaxies that contains our home, the Milky Way.

This cosmic neighborhood is not a single uniform cluster. Instead, it forms a distinct "dumbbell" shape. This structure consists of two main lobes. One lobe contains the Milky Way and its many satellite galaxies. The other lobe contains the Andromeda Galaxy and its own set of satellites. These two large collections are separated by a massive distance. Despite this distance, they are currently moving toward each other at a velocity of 110 kilometers per second.

Gravity is the primary mechanism that governs the movement and structure of the Local Group. The total mass of the group is estimated to be around 2 trillion solar masses. This mass is largely accounted for by the two largest members: the Milky Way and Andromeda. Estimates for the mass of each galaxy are often cited around 100 billion solar masses. However, different scientific studies provide slightly different figures. Peñarrubia et al. (2014) provided one estimate, while Karachentsev and Kashibadze (2006) suggested a somewhat lower value. The center of the group is located approximately 0.5 megaparsecs from the Milky Way. This center is situated slightly closer to the Andromeda Galaxy by about 0.1 megaparsecs. This positioning suggests that Andromeda may be more massive than the Milky Way.

The members of the Local Group vary greatly in type and size. The two largest members, Andromeda and the Milky Way, are both spiral galaxies. Andromeda is the largest, with a diameter of 152,000 light-years. The Milky Way is the second largest, with a diameter of 87,400 light-years. The Triangulum Galaxy (M33) is the third-largest member. It is a spiral galaxy with a mass of approximately 100 billion solar masses. Unlike the others, Triangulum is an unbarred spiral galaxy. It may be a satellite of Andromeda. In fact, the two galaxies are 750,000 light-years apart. They experienced a close passage 2 to 4 billion years ago. This event triggered star formation across the disk of Andromeda.

Beyond these large spirals, the group contains many other galaxy types. There are elliptical galaxies, such as M32 and M110, which orbit Andromeda. The Large Magellanic Cloud is a Magellanic spiral galaxy and a satellite of the Milky Way. It is the fourth largest known member of the group. The Small Magellanic Cloud is the fifth largest known member. Many other members are dwarf galaxies, which are much smaller and less bright. Some are dwarf spheroidal galaxies, while others are irregular galaxies. For example, IC 10 is the only known starburst galaxy in the Local Group. This means it is experiencing a period of intense star formation.

Our understanding of this group has grown through historical scientific discovery. The term "Local Group" was first introduced by the astronomer Edwin Hubble. He used this name in his 1936 book, *The Realm of the Nebulae*. Hubble described the group as a typical small collection of nebulae that is isolated in the general field. He listed several specific members by their decreasing luminosity. These included M31, the Milky Way, M33, and the Magellanic Clouds. He also identified IC 10 as a potential member. Today, we know of at least 134 members within 1 megaparsec of the group's center. However, the exact total number remains unknown because the Milky Way obscures some galaxies.

There are interesting complexities regarding which galaxies truly belong to our group. Some galaxies, like the members of the Antlia-Sextans Group, may not be gravitationally bound to us. They likely lie outside the Local Group's zero-velocity surface. This surface defines the boundary where the group's gravity can hold objects. However, this boundary might change over time. As the Milky Way and Andromeda coalesce, the total mass and density of the group will increase. This increase could widen the radius of the zero-velocity surface, potentially pulling in more galaxies.

The Local Group is not an isolated system in the universe. It is a small part of a much larger hierarchical structure. It is a component of the Local Volume. The Local Volume is part of the Virgo Supercluster. This supercluster is itself a part of the Laniakea Supercluster. The Laniakea Supercluster is one of the largest known structures in space. It includes the Pisces–Cetus Supercluster Complex. Understanding the Local Group helps astronomers map the vast, connected layers of the entire cosmos.