Crash of the Titans
Astronomers have known for decades that our Milky Way Galaxy and the neighboring Andromeda Galaxy are approaching each other. What we haven't known is just how close the encounter will be. Recent Hubble measurements have been able to pin down the trajectory with a smashing conclusion: in about four billion years the two galaxies will crash together in a nearly head-on collision. Further, the spiral galaxies will have completely merged and transformed to create a single elliptical galaxy around six billion years in the future.
Hubble press release:
NASA's Hubble Shows Milky Way is Destined for Head-on Collision with Andromeda Galaxy
The first image shown in this episode is a drawing of the Milky Way by Robert Hurt of the Spitzer Science Center. Robert is an astronomer working in public outreach, and his diagram is based on the known locations of features within our Milky Way Galaxy. However, there is a lot of our galaxy that is only poorly known or unobserved. Hence, the drawing requires a blending of science and art — something that Robert does particularly well.
Edwin Hubble is also famous for showing that the Great Nebula in Andromeda (as it was then called), is really the Andromeda Galaxy. In the span of twenty years, astronomical knowledge went from a contained universe comprised of just one galaxy to myriad galaxies in an expanding universe. Hubble helped create quite the change in our point of view and is a fitting namesake for a telescope that has helped create similar changes.
Edwin Hubble's measurement of the distance to Andromeda was based on being able to resolve a few of the brighter stars in that galaxy and thus being able to monitor their changes in brightness. I've always thought it somewhat poetic that the Hubble Telescope took that to a new level and can resolve ordinary stars in Andromeda. The Stellar Deep Field in Andromeda is one of those observations that truly amazed me, as I would not have thought it possible when I started in astronomy.
The predictions of one computer simulation does not provide a complete result. Astronomers ran a number of simulations with small variations in the parameters of the encounter as allowed by the uncertainty in the measurements. While the direct hit collision is consistent with the results, the most likely impact has the centers of each galaxy passing through the outer parts of each other's disks. Such a collision is visually similar, but just a bit slower to complete. Also, in about 10% of the computer simulations, the Triangulum Galaxy became involved in the collision between the Milky Way and Andromeda. In those scenarios, all three galaxies would merge to form a single galaxy.