“Not even light can escape such hollowing, this huge mass in a small space. Even the Milky Way with its open arms is said to have a black hole at its heart.” -Susan B.A. Somers-Willett
Our Milky Way is home to us all. With its hundreds of billions of stars, massive spiral arms, dust lanes, and orbiting globular clusters, it’s no wonder that nearly everything we see in the night sky is contained within it.
I say nearly everything, of course, because there are a few exceptions. The Andromeda Galaxy, for one, as well as the two Magellanic Clouds, for another. It was less than 100 years ago that we realized that the vast majority of those “faint fuzzies” in the night sky — nebulous structures that came in spiral, elliptical, and irregular shapes — were actually galaxies unto themselves.
They range in size from hundredths to hundreds of times the mass of our own Milky Way, but the one thing that almost all of them have in common with one another is that they’re rapidly receding away from us.
The observation that, in general, the farther away a galaxy is from us, the faster it’s moving away from us (and not other interpretations) led us to conclude that the Universe is expanding. While gravitation is working to slow the expansion and attract all massive objects towards one another, the initial expansion was extremely powerful. As a result, it isn’t everywhere and in all cases that gravitation wins.
Sure, in some locations — like the Virgo Cluster, above — thousands of large galaxies become gravitationally bound to one another. There are many great clusters where this is the case, and they are the major causes of deviations and imperfections in the general expansion of the Universe. In locations where great concentrations of mass are vitally important, these galaxies will achieve some maximum, finite distance between them, and wind up eventually moving back towards one another and merging far into the future.
While this is the fate of many of the galaxies visible in our night sky — to merge with some of the largest clusters in the Universe — that is not the fate of the Milky Way. Not to merge with the Virgo Cluster, nor with any other large group of galaxies. In fact, thanks to the presence and dominance of dark energy, it is a certainty that only the galaxies that are today moving towards us have a chance of someday merging with us; all the others will eventually expand away, towards infinity, into the abyss of deep space.
Want to know who will wind up coming back to us in the future?
There’s everyone presently in our local group, which includes our bigger sister, Andromeda, as well as — in order of decreasing size — M33, the Large Magellanic Cloud, the Small Magellanic Cloud, M32, NGC 205, NGC 6822, NGC 185, IC 1613 and NGC 147. They’re all going to merge with us, and they’re all going to undergo rapid bursts of star formation when one of the other local-group members gets close enough to gravitationally induce star formation.
In other words, everyone in the local group has fireworks in their future. Everyone, that is, except the dwarf galaxy IC 10, which has fireworks right now.
The closest (at just 2.3 million light years distant) known starburst galaxy — a galaxy that’s forming stars so rapidly the entire galaxy has become a star-forming region — and the only starburst galaxy within the local group, this burst of star formation awaits each member of the local group in the future, as we will all eventually merge into a single, giant elliptical.
All of these — plus the other, even smaller members of our local group — will eventually merge together. Even the ones that are moving (slightly) away from us at the moment won’t be for long, as they will come back to us under the intense pull of eveyone’s combined gravity. That’s our fate, but there’s one more possibility: someone who will probably not be joining us, but may, if things work out right.
The giant spiral galaxy, M81, is shown above with its starbursting (and smaller) companion, M82. M81 is the largest galaxy in its local group of 34 small objects, and is well outside our local group at a distance of nearly 12 million light years.
M81 is moving towards us, ever so slightly, at a speed of about 1 km/s. Does that mean it will merge with us, overcoming the Hubble expansion of the Universe, and leading to a great galactic pileup?
Unlikely! While M81 is moving towards us ever-so-slightly, M82 is moving rapidly away from us (about 200 times as quickly as M81 moves towards us), and both M81 and M82 are bound, gravitationally, to one another.
This very likely means that the motion of M81 towards us is ephemeral, and that it will wind up merging with M82, and the combined, post-merger galaxy will continue to expand away from us, into the great and distant void. Still, there’s much more to learn before we can say with absolute certainty that this is the case.
We’ve got no choice but to let go of these and all the other distant galaxies in the Universe. If they come back to us, they’ll be ours forever, but if not, don’t despair; they have their own futures to write!