The Andromeda Galaxy Is on a Collision Course With the Milky Way
In about four billion years, we're going to see a collision of quite literally galactic proportions: the Milky Way and the Andromeda galaxy, the two largest galaxies in our part of the universe, are going to crash into each other.
It's going to be one huge clash, but like a cosmic monster truck rally, only one will emerge victorious—the other will be absorbed. With each galaxy containing hundreds of billions of stars, it's tough to judge which one is more massive, but new research by scientists from the University of Sydney and the University of Western Australia may have proven that the two spiral galaxies are on equal footing.
The problem with estimating a galaxy's mass is due in part to the sheer size of the subject: both galaxies are spiral-shaped, but the Milky Way is about 100,000 light years across, while Andromeda is estimated to be about 220,000 light years in diameter. The Milky Way's total mass is about 800 billion times that of our sun, but estimating the mass of Andromeda is much more complicated than just counting stars, adding planets, throwing in gas, ice, and dust, and then looking at the sum of it all. No, that frustrating, mysterious substance known as dark matter gets in the way of that.
It's estimated that observable matter makes up only about "a couple of tens of percent" of any given galaxy—the rest is dark energy and dark matter. In fact, all observable matter, from stars to asteroids to galaxies, makes up only about 5 percent of the universe. This means that the Andromeda galaxy could be packing a lot of invisible mass in addition to being about twice the diameter of the Milky Way.
However, this doesn't appear to be the case—according to research by Professor Geraint Lewis and post-doctoral student Prajwal Kafle, who calculated the escape velocity of Andromeda's stars to figure out the gravitational pull (and therefore the mass) of the galaxy's dark matter, it turns out that the Milky Way and its galactic rival are about the same in terms of mass.
When the two inevitably collide in four billion years, it'll be a nail-biter.