Local Group

The Local Group is the galaxy group containing the Milky Way, Earth’s home galaxy. Its structure resembles a dumbbell: one lobe holds the Milky Way and its satellite galaxies, while the other holds the Andromeda Galaxy and its satellites. These two lobes are roughly 800 kiloparsecs (3 million light-years) apart and are moving toward each other at 123 kilometers per second. The group’s center lies about 450 kiloparsecs from the Milky Way, placing it slightly closer to Andromeda by about 300 kiloparsecs; Andromeda may also be more massive than the Milky Way.

The Local Group’s total mass is around 2×10¹² solar masses, and its diameter is about 5.11 megaparsecs (17 million light-years), based on density matching and the potential surface of its parent structure, the Local Sheet. It belongs to the Local Volume, the Virgo Supercluster, and the even larger Laniakea Supercluster, which is part of the Pisces–Cetus Supercluster Complex. The exact number of galaxies in the group is unknown because the Milky Way obscures some, but 134 members are currently known within 1 megaparsec of the center, most of them dwarf galaxies.

The two largest members, Andromeda and the Milky Way, are spiral galaxies, each with a mass around 10¹² solar masses. Each has its own system of satellite galaxies. Andromeda’s satellites include Messier 32, Messier 110, NGC 147, NGC 185, Andromeda I through XXII (with gaps), and several ultra-faint dwarf spheroidals. The Milky Way’s satellites include the Sagittarius Dwarf Galaxy, the Large and Small Magellanic Clouds, the disputed Canis Major Dwarf Galaxy, and many others such as the Ursa Minor, Draco, Carina, Sextans, Sculptor, and Fornax Dwarf Galaxies, plus Leo I and II, Ursa Major I and II, and additional ultra-faint dwarf spheroidals.

The Triangulum Galaxy (M33) is the third-largest member, with a mass of about 5×10¹⁰ solar masses, and is the third spiral galaxy. Whether it is a companion of Andromeda is unclear; the two are 750,000 light-years apart and had a close passage 2–4 billion years ago that triggered star formation across Andromeda’s disk. The Pisces Dwarf Galaxy lies equidistant from Andromeda and Triangulum, so it may be a satellite of either.

Other members likely gravitationally separate from these large subgroups include IC 10, IC 1613, the Phoenix Dwarf, Leo A, the Tucana Dwarf, the Cetus Dwarf, the Pegasus Dwarf Irregular, Wolf–Lundmark–Melotte, the Aquarius Dwarf, and the Sagittarius Dwarf Irregular. The membership of NGC 3109, along with its companions Sextans A, the Antlia Dwarf, Sextans B, Leo P, Antlia B, and possibly Leo A, is uncertain due to their extreme distances from the group’s center. The Antlia-Sextans Group likely lies outside the Local Group’s zero-velocity surface, making it a separate galaxy group rather than a subgroup—though this independence may vanish as the Milky Way and Andromeda merge, increasing the group’s mass and possibly widening its zero-velocity surface.

The term “Local Group” was introduced by Edwin Hubble in Chapter VI of his 1936 book *The Realm of the Nebulae*. He described it as “a typical small group of nebulae which is isolated in the general field” and listed its members by decreasing luminosity: M31, the Milky Way, M33, the Large Magellanic Cloud, the Small Magellanic Cloud, M32, NGC 205, NGC 6822, NGC 185, IC 1613, and NGC 147. He also identified IC 10 as a possible member.

The group contains several streams of gas and stars, including the Magellanic Stream (gas stripped from the Magellanic Clouds by their interaction with the Milky Way), the Monoceros Ring (a stellar ring around the Milky Way possibly torn from the Canis Major Dwarf), the Virgo Stream (formed from a dwarf galaxy), and the Helmi Stream.

Over tens of billions of years, the galaxies of the Local Group are expected to merge under their own gravity into a single elliptical galaxy, with the collision of Andromeda and the Milky Way being the main event. Debate continues over whether the merged galaxy will become elliptical immediately after the collision or only after an intermediate period of retaining a spiral structure.