In approximately 4 billion years, the Milky Way will merge with which nearby galaxy?
And the answer: Andromeda.    
Photo credit: David (Deddy) Dayag

Also known as M31, the Andromeda galaxy is currently 2.5 million light years away, but is drawing ever closer to the Milky Way through the mutual pull of gravity between the two galaxies. When the collision occurs, it is predicted that our sun will be flung to a new region of the galaxy, but the earth and the solar system will likely remain unharmed.

As we rotate around our axis and revolve around the Sun, another significant planetary body draws closer to our Earth every day: the Andromeda galaxy. At a rate of 70 miles per hour, this neighboring galaxy is pulled by gravity towards our very own Milky Way, a fact which will ultimately result in a collision of the galaxies in around 4 billion years. Fascinatingly, though, despite the significant distance, the two galaxies have already begin to spatially interact.

The Andromeda galaxy, our Milky Way and other galaxies all sit enshrouded in a large envelope called a galactic halo, which consists of gas, dust and stray stars. The halos of galaxies are so faint, in fact, that detecting them is not an easy feat. In recent years, astronomers have been able to measure the size of the halo of the Andromeda galaxy by looking at how much it absorbed light from background quasars. They were surprised to find that the Andromeda galaxy’s halo stretches much, much farther beyond its visible boundaries, and thus the haloes of the two galaxies are just about touching.

As Andromeda approaches, the night sky will slowly fill with the sight of its magnificent constellations. Thankfully, due to the fact that galaxies are mainly composed of the space between stars, it's unlikely that any significant collisions will occur to alter the composition of each individual galaxy.

Learn more about the Andromeda galaxy and its collision course here.

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