Ask Us: What happens when galaxies collide?

Two galaxies collide because of mutual gravitational attraction, leading to the formation of a single larger galaxy. Image: Hubble/NASA.

When mutual gravitational attraction brings two nearby galaxies together (a relatively common event), their gases collide violently.

As gas and dust smash together at speeds of millions of kilometres an hour, they combine the raw materials for new stars in a spectacularly hot and bright display. Over millions of years, new stars and even new galaxies can form this way. Highly energetic collisions could also be the cause of rare cosmic objects called quasars, which, despite being much smaller, can be hundreds of times as bright as giant galaxies, allowing them to be observed from great distances.

Whereas some galaxies formed on their own, many others, including our galaxy, the Milky Way, probably developed from the collisions and mergers of smaller ones. And in turn, collisions between two spiral galaxies, like ours, are thought to form still larger elliptical galaxies.

Andromeda and the Milky Way are among the largest galaxies in our galactic cluster, known as the Local Group. Andromeda, which is located about two million light-years away from our galaxy, is on track to collide with it in three to four billion years. When this occurs, many new stars will form and others will go supernova. Nearby exploding stars will probably render Earth uninhabitable. That is, if a larger, hotter sun — something our star will become — hasn’t already done so.

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