Galaxy Collision
Written by Fraser Cain
Colliding Galaxies. Image credit: Hubble
I don't want to scare you, but our own Milky Way is on a collision course with Andromeda. Some time in the next few billion years, our galaxy is going to crash into Andromeda with catastrophic consequences. Stars will be thrown out of the galaxy, others will be destroyed as they crash into the merging supermassive black holes. The delicate spiral structure of both galaxies will be destroyed as they become a single, giant elliptical galaxy.
How do astronomers know this galaxy collision is going to happen? They've measured the direction and speed of both galaxies, and calculated that it's going to happen. But more importantly, when astronomers look out into the Universe, they see galaxy collisions happening everywhere.
Galaxies are held together by mutual gravity, orbiting a common center of gravity; imagine bees buzzing around a beehive. Sometimes the galaxies get close enough to tear at each other with their gravity. This gravitational interaction distorts the disk of galaxies and pulls off long tidal tails; streams of stars connecting galaxies together. And if the galaxies get even closer, they'll collide.
In a galaxy collision, large galaxies absorb smaller galaxies entirely, tearing them apart and adding their stars to the galaxy. But when the galaxies are similar sizes, like our Milky Way and Andromeda, the close encounter destroys the spiral structure entirely. The two groups of stars eventually become a giant elliptical galaxy with no discernible spiral structure.
Another consequence of galaxy collisions is star formation. When the galaxies collide, it also causes vast clouds of hydrogen gas to collapse. This collapsing gas creates pockets of star formation. A galaxy collision ages a galaxy prematurely, causing much of its gas to convert into star formation. After this period of rampant star formation, galaxies run out of fuel. The youngest hottest stars detonate as supernovae, and all that's left are the older, cooler red stars with much longer lives.
This is why the giant elliptical galaxies, the results of galaxy collisions, have so many old red stars, and very little active star formation.
We have written many articles about galaxies for Universe Today. Here's an article about how a galaxy collision will also separate out all the dark matter from a galaxy.
Want more resources on galaxies? Here's a link to the Messier catalog's section on Galaxies. And here's NASA's World Book on galaxies.
We have also recorded an episode of Astronomy Cast about galaxies – Episode 97: Galaxies.
Filed under: Astronomy
