Andromeda and Milky Way Might Collide Sooner Than We Think.

Andromeda’s halo is gargantuan. Extending for at least 2 million light years, if we could see in our night sky it would be 100 times the diameter of the Moon or 50 degrees across!
 Credit: NASA
The merger of the Milky Way and Andromeda galaxy won’t happen for another 4 billion years, but the recent discovery of a massive halo of hot gas around Andromeda may mean our galaxies are already touching. University of Notre Dame astrophysicist Nicholas Lehner led a team of scientists using the Hubble Space Telescope to identify an enormous halo of hot, ionized gas at least 2 million light years in diameter surrounding the galaxy.

The Andromeda Galaxy is the largest member of a ragtag collection of some 54 galaxies, including the Milky Way, called the Local Group. With a trillion stars — twice as many as the Milky Way — it shines 25% brighter and can easily be seen with the naked eye from suburban and rural skies.