Galactic doomsday is nigh!

And now, the topic you've waited all week for: The collision of our Milky Way (MW) galaxy with the Andromeda Galaxy (AG). For a while, we've known that MW and AG were moving toward each other; that's pretty easy to expect from the gravitational force between them, and the component toward us of AG's velocity has been pretty easy to measure. But until recently we did not know the perpendicular components of AG's velocity, which determines the difference between a collision and a miss.

Once these measurements were obtained, van der Marel et al (http://iopscience.iop.org/0004-637X/753/1/9/pdf/0004-637X_753_1_9.pdf) used two complementary methods to predict the future motion of AG relative to MW based on appropriate initial conditions (just like we discussed on Monday).

Below is a sample of their results for the Milky Way (labeled MW), Andromeda (labeled M31), and the Triangulum galaxy (labeled M33):

Image credit: http://iopscience.iop.org/0004-637X/753/1/9/pdf/0004-637X_753_1_9.pdf

Image credit: http://iopscience.iop.org/0004-637X/753/1/9/pdf/0004-637X_753_1_9.pdf

The red curve on the second graph is perhaps the most important, as it shows the separation between the Milky Way and Andromeda as a function of time (with t = 0 indicating today). As you can see, in about 4 billion years, we're in for a collision!

Here are more dramatic demonstrations: A video of the simulation and images of what it will look like from earth!