~3–5 Billion

Collision with Andromeda

Our Milky Way galaxy is indeed an island universe in the sense that it is an organized, isolated collection of perhaps 400 billion separate stars, plus gravitationally bound gas, dust, and dark matter. But our galaxy is also part of a larger collection of gravitationally bound neighbor galaxies dubbed the Local Group by astronomer Edwin Hubble. The Local Group consists of more than 30 galaxies, including the Milky Way and its satellite galaxies, such as the Large and Small Magellanic Clouds and the Sagittarius Dwarf Elliptical Galaxy, the Andromeda galaxy and its satellite galaxies, and others. Astronomers estimate that the Local Group spans about 10 million light-years across and has a combined mass exceeding 1 trillion solar masses.

The gravitational center of the Local Group is located somewhere between the Milky Way and Andromeda galaxies, which dominate the mass of the group. Astronomers have discovered that these two large spiral galaxies are moving toward each other, and that in the distant future, perhaps 3–5 billion years from now, they could actually collide, with the nature of the collision depending on the details of their velocities and their distributions of dark matter.

Collision is probably not the best word to describe the interactions of galaxies, however. Because they are actually made up of mostly empty space, it is unlikely that many stars would actually physically collide with each other. Rather, as the two galaxies essentially pass through each other, the gravitational and tidal forces between their associated stars and satellite galaxies will likely rip apart their beautiful spiral structures and could cause them to eventually merge into one larger irregular or elliptical supergalaxy.

The Local Group is in turn part of an even larger collection of galaxies in our part of the universe called the Virgo Supercluster, which consists of more than 100 interacting galaxy clusters like the Local Group, spanning more than 110 million light-years. The Virgo Supercluster is, in turn, just one of perhaps millions of similar superclusters that form the largest-scale structures in the observable universe—the Walls of Galaxies, often referred to as the cosmic web.

The spectacular Andromeda galaxy, also known as Messier 31, is the closest spiral galaxy to the Milky Way. Even though it is more than 2 million light-years away, it is part of our “local group” of galaxies. Andromeda and the Milky Way appear to be heading toward a collision in the far future.