Southern Hemisphere stargazers have it good. From anywhere on Earth, on a very dark night, the band of the Milky Way can be seen to stretch across the sky in a sideways view through the disk of our spiral galaxy. From the southern hemisphere we can also see the part of the band where it widens into a bright bulge of stars, veiled by lanes of dust, surrounding the supermassive black hole at the very core of the galaxy.
Also from the south, due to the orientation of the Earth and the Solar System, we can see the Large and Small Magellanic Clouds, dwarf satellite galaxies caught in the Milky Way’s gravity. One thing we can’t see, though, is the Andromeda Galaxy.
Which is too bad, since Andromeda, with its trillion stars and central black hole as massive as 100 million suns, is hurtling toward us at 110 km a second.
Galactic collisions are commonplace in the cosmos. Our best theories for how galaxies grow include a healthy dose of cannibalism, at least for the larger ones. Here in the Milky Way, astronomers (known in this context as ‘galactic archaeologists’) have found long streams of stars tracing arcs and loops around the sky, illuminating the remains of smaller objects unravelled by galactic gravity as they fell towards us long ago.
The physics of how galaxies rip each other apart is the same as that which would be responsible for your grisly demise if you fell into a black hole, and it’s why Mars’s moon Phobos will one day be reduced to a ring of pebbles encircling the red planet.
It comes down to the tidal force: the uneven gravitational pull that happens when one end of an object is closer to the source of gravity than the other.
The name ‘tidal’ is no coincidence, as it is also responsible for why the oceans on Earth respond to the Moon.
The main effect of a tidal force is to stretch and squeeze an object, elongating it along the direction pointing toward (and away from) the source of gravity and squeezing it in the perpendicular direction. High tide on one side of the Earth corresponds with high tide on the opposite side, with low tide in the regions in between. Similarly, if you fell into a black hole feet first, you would get much taller from the tidal stretching, but also thinner, in a process vividly termed ‘spaghettification’.
When the Andromeda galaxy hits, in about four billion years, it will be the biggest light show our galaxy has ever seen.
The tidal forces Phobos experiences from Mars are strong enough that the little moon will be broken apart in a few tens of millions of years.
In galaxy collisions, tidal forces can create long streamers of stars stretching out across the cosmos. When small galaxies fall into larger ones (which may one day be the fate of our Magellanic clouds), the stellar debris creates thin faint arcs, tracing their final orbit.
When large galaxies come together, these streams can be flung out in tails thousands of light-years long.
The collisions can be dramatic in other ways as well, as galactic gas coming together can cause a burst of new star formation and feed central black holes. Over time the cores of the galaxies spiral together and the stars wash back and forth, blurring out the original structures to coalesce into an elliptical blob.
We see galaxy mergers all over the sky, and especially in clusters of galaxies, where immense masses gather together into one structure. We also know, however, that mergers happen less often than they used to.
As the universe expands, the distance between galaxies not already tied together by gravity is getting larger, so they bump into each other less often. Over time, that will mean fewer stars, and a darker, lonelier cosmos.
Meanwhile, we have Andromeda. When it hits, in about four billion years, it will be the biggest light show our galaxy has ever seen. While stars will be flung about in dramatic fashion, our Solar System as a whole will probably be OK. The distances between stars are so vast that, even in a galactic collision, individual stars almost always sail right past each other.
By the time it happens, the Sun will have already neared the end of its life: expanding to its red giant phase, boiling off the oceans and dooming the Earth to annihilation. Perhaps life will have another vantage point to watch from by then. Four billion years is a long time, and the show will definitely be something worth waiting for.