Local Group

The Local Group is a gravitationally bound collection of galaxies that includes the Milky Way, the Andromeda Galaxy, and the Triangulum Galaxy.

Milky Way: The barred spiral galaxy that contains the Solar System. It is the Local Group’s largest member by stellar mass and the home galaxy of the Solar System.
Dark matter halo: A roughly spherical distribution of dark matter surrounding a galaxy or galaxy group. The group’s gravitational mass is dominated by the dark matter halos around its largest galaxies.
Lambda-CDM model: The standard cosmological model, combining cold dark matter with a cosmological constant. It describes how the Local Group assembled within a universe dominated by dark matter and accelerated expansion.
Milky Way–Andromeda collision: The predicted close encounter and eventual merger of the Milky Way and Andromeda galaxies. Their mutual attraction makes this the Local Group’s most consequential anticipated interaction.
Andromeda Galaxy: The large spiral galaxy, also called Messier 31, located about 2.5 million light-years from Earth. It is the Local Group’s other dominant galaxy and is approaching the Milky Way.
Gravitational binding energy: The energy required to separate the parts of a gravitationally bound system to infinite distance. It distinguishes galaxies bound to the group from those merely passing nearby.
Hierarchical structure formation: The process by which small cosmic structures merge and grow into larger ones over time. The Local Group formed as galaxies and dark matter structures accumulated through mergers.
Laniakea Supercluster: The large-scale cosmic basin of attraction that contains the Local Group and the Virgo Cluster. It places the Local Group within a broader flow of galaxies and matter.
Triangulum Galaxy: A spiral galaxy, also called Messier 33, in the constellation Triangulum. It is the Local Group’s third-largest galaxy and may be gravitationally associated with Andromeda.
Cosmic expansion: The increase in distances between large-scale, unbound regions of the universe over time. Local gravitational binding counteracts expansion across the group’s limited extent.

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