How big is Local Group? | Scale of the Universe

The Local Group is the small group of galaxies that includes the Milky Way and the Andromeda Galaxy. Measured to the boundary where its gravity stops nearby galaxies from moving away, it is about 6 to 7 million light-years across. A light-year is the distance light travels in one year, about 9.46 trillion kilometers.

Size

The universe is expanding, so most galaxies are moving away from the Local Group. Galaxies close to the group are slowed by its gravity. At one distance from the group's center the two effects cancel, and a galaxy there neither moves away nor falls in. Astronomers call this boundary the zero-velocity surface and use it as the edge of the group.

A 2009 study of 30 nearby galaxies put this boundary 0.96 megaparsecs from the center. A megaparsec is a million parsecs, about 3.26 million light-years. A 2012 study of the group's outermost members found 1.06 megaparsecs. Together they give a diameter of 6.3–6.9 million light-years.

The group has no sharp edge, so descriptions of its size vary. NASA's educational site describes its galaxies as spread over nearly 10 million light-years, a wider region than the one measured from the motions of galaxies.

Size comparisons

The Milky Way is about 100,000 light-years across, so about 65 Milky Ways side by side would span the Local Group.
The Andromeda Galaxy is about 2.5 million light-years from the Milky Way, a distance the explorer shows near the Local Group. The group is about two and a half times as wide as that distance.
If the Milky Way were shrunk to the size of a basketball, Andromeda would be about 6 meters (20 feet) away, and the whole Local Group would be 15 to 17 meters (49 to 54 feet) across.
The Virgo Cluster, the nearest large cluster of galaxies, has about 330 times the mass of the Local Group.
Light takes 6 to 7 million years to cross the Local Group from one side to the other.

What the group contains

The Milky Way and Andromeda are the most massive members, and the group's center lies between them. The Triangulum Galaxy is the third largest. NASA counts more than 50 other galaxies in the group as of 2026. Most of them are dwarf galaxies, small galaxies with up to a few billion stars.

A 2012 catalog listed 73 galaxies that were definite or very likely members, not counting the Milky Way and Andromeda. Its author judged that the count of members far from the two big galaxies was probably still very incomplete. From the way the group slows the galaxies around it, its total mass is about 1.9 trillion times the mass of the Sun.

Its place in the universe

Edwin Hubble was the first to recognize the Local Group, in the years when astronomers first measured the distances to other galaxies. Most galaxies are moving away from the Milky Way, but Andromeda is moving toward it, pulled by the gravity between the two galaxies.

The Local Group lies near one edge of the Virgo Supercluster, a much larger gathering of galaxy groups centered on the Virgo Cluster. The Local Group is also part of the Laniakea Supercluster, which is larger still. The Local Group lies on Laniakea's outer edges.