Local Group

Lore & Background

The term "The Local Group" was introduced by Edwin Hubble in Chapter VI of his 1936 book The Realm of the Nebulae. There, he described it as "a typical small group of nebulae which is isolated in the general field" and delineated, by decreasing luminosity, its members to be M31, Milky Way, M33, Large Magellanic Cloud, Small Magellanic Cloud, M32, NGC 205, NGC 6822, NGC 185, IC 1613 and NGC 147. He also identified IC 10 as a possible part of the Local Group. The Local Group has a total mass of the order of 2×10^12 solar masses and a total diameter of 5.11 megaparsecs based on density matching and the potential surface of its parent structure, Local Sheet. It is itself a part of the Local Volume and the larger Virgo Supercluster, which is a part of the even greater Laniakea Supercluster along with the Pisces–Cetus Supercluster Complex. The exact number of galaxies in the Local Group is unknown, as the Milky Way obscures some; however, a current total of 134 members is known within 1 megaparsec from the center, most of which are dwarf galaxies. The two largest members, the Andromeda and the Milky Way galaxies, are both spiral galaxies with masses of about 10^12 solar masses each. Each has its own system of satellite galaxies. The Triangulum Galaxy (M33) is the third-largest member, with a mass of approximately 5×10^10 M☉, and is the third spiral galaxy. It is unclear whether the Triangulum Galaxy is a companion of the Andromeda Galaxy; the two galaxies are 750,000 light years apart, and experienced a close passage 2–4 billion years ago which triggered star formation across Andromeda's disk.

Reader's Guide

The Local Group is significant as the galaxy group that contains Earth's Milky Way, providing the immediate cosmic environment for studying galaxy formation, interaction, and evolution. Its dumbbell structure, with the Milky Way and Andromeda as the two dominant lobes moving toward each other at 123 km/s, offers a natural laboratory for understanding galactic mergers. The eventual coalescence of Andromeda and the Milky Way, likely over tens of billions of years into a single elliptical galaxy, represents the predominant future event in the group. The group's membership, including 134 known galaxies within 1 megaparsec from the center, mostly dwarf galaxies, helps astronomers trace dark matter distribution and the history of galactic accretion. The uncertainty over whether the Antlia-Sextans Group is gravitationally bound to the Local Group highlights the dynamic nature of defining group boundaries. The Local Group's place within the Local Volume, Virgo Supercluster, and Laniakea Supercluster illustrates the hierarchical structure of the universe. Its legacy includes being the first galaxy group identified by Edwin Hubble, establishing the concept of galaxy groups as isolated collections in the general field.

Did You Know?

• The term 'The Local Group' was introduced by Edwin Hubble in Chapter VI of his 1936 book The Realm of the Nebulae.
• The two collections of galaxies in the Local Group are separated by about 800 kiloparsecs (3×10^6 ly) and are moving toward one another with a velocity of 123 km/s.
• The Local Group has a total mass of the order of 2×10^12 solar masses and a total diameter of 5.11 megaparsecs.
• The Triangulum Galaxy (M33) and Andromeda Galaxy are 750,000 light years apart and experienced a close passage 2–4 billion years ago which triggered star formation across Andromeda's disk.
• The exact number of galaxies in the Local Group is unknown, as the Milky Way obscures some; however, a current total of 134 members is known within 1 megaparsec from the center.

The Nested Architecture of Local

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Hubble's Bold Assumption

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Composition and Cosmic Neighbors

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From van de Hulst to Modern Cosmography

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