The Once And Future Milky Way
Ever wonder about the ultimate fate of our home galaxy, the Milky Way? I’ve been wondering about that lately and decided to find out what is likely to happen to the place we call home down through the depths of time.
Right now we live in a barred spiral galaxy at least 100,000 light years across containing 200 to 400 billion stars. I was surprised that the number of potential stars had such a huge range. Of course, determining such a huge number accurately could be difficult for a host of reasons not the least of which includes all the dust that obscures much of our galaxy. It’s not just stars that are hard to see though. Much of our galaxy’s mass is invisible to us as well if you consider that 50 to 90% of it may be locked up in undetectable dark matter.
Our galaxy has existed in this state for quite some time. At it’s birth an astounding 13+ billion years ago, it was quite different though in shape and size. It didn’t form a recognizable disk until 10-12 billion years ago.
As old as our galaxy is, it’s far from withering and dying; in fact our galaxy’s immediate future is all about growth. When galaxies grow though they don’t seem to primarily do it by making other stars. You may be surprised to learn that the Milky Way’s net growth by this method is rather paltry, only around 7 new stars per year. Instead, our growth over the eons is driven primarily by cannibalizing or eating other galaxies. There’s plenty of evidence that we’ve done this in the past. We’re perhaps even doing that right now. Some astronomers believe that the Milky Way is currently munching on a dwarf galaxy called Canis Major which is the closest galaxy to us (I guess by definition it must since we’re eating it). After that, the next items on the menu could very well be the large and small Magellanic clouds which are irregular dwarf galaxies around 170,000 light years away.
All of this is a just a warm-up though.Barreling towards us at 111 km per second is the beautiful Andromeda galaxy. That’s fast enough to travel to the moon in a zippy 60 minutes. Still, at 2.5 million light years away, it’s gonna take a while…4 billion years before the primary hit. This isn’t a fast process however. It’ll take at least 2 more billion years before the merger is complete. At that point my friends, the Milky Way will essentially be no more. We won’t be a barred spiral any longer; more likely we’ll be an elliptical galaxy. And don’t forget, since Andromeda is bigger than we are it is us who will have been eaten. Still, in our hubris, astronomers think we should give this new galaxy a hybrid name like MilkoMeda. This is understandable though since a hefty chunk of Milkomeda will still be erstwhile Milky Way stars.
The merging is far from over though. Milkomeda will still be gravitationally bound to our local group of galaxies which contains 30 -50 galaxies. In 150 billion years these galaxies will start seriously merging all together. This will take a mind-bending trillion years to finish. Eventually all of them will merge into one monster galaxy that no one has given a name to yet as far as I can tell. Let’s call it the Local Monster Galaxy or LMG.
Soon after the LMG forms an important milestone will have been reached. Mergers are good not only for acquiring new stars but creating them as well since much of the gas between the stars will have crashed together causing rapid star formation. That means that very little gas will be left after LMG forms. Since no other galaxies will be left to merge with, the beginning of the end will have truly arrived for the galaxy formerly known as the Milky Way. It’s just a waiting game now.
And it’s a long wait indeed,
It will take about 110 trillion years (give or take a week or so) for all the stars in the LMG to flicker out. Once this happens, the LMG will be just a shell of its former glory. The only things visible to our robot descendent’s eyes now are are dwarf stars, neutron stars, and black holes. Since this is all degenerate matter, this is known as the Degenerate Era.
The next milestone occurs 100 quintillion years in the future and its a doozy. At around this time, orbital decay will cause the remains of the earth to collide into the dwarf remnant of the sun. This assumes of course that the earth wasn’t swallowed by the sun far earlier in its red giant phase.
Even these hardy and sparse remnants of the LMG won’t last forever. In 10 to the 40th power years (10 duodecillion), protons and neutrons themselves will likely have decayed in a puff of physics and existential boredom.
When this happens, the only thing left of the Local Monster Galaxy is a supermassive black hole containing perhaps 200 million solar masses. This has been called the Black Hole Era.
Even this mighty and final denizen of our galaxy, not to mention the entire visible universe, will not last forever. It could take 10 to the 68th power years (100,000 Vigintillion years) but even this lone survivor of the Milky Way will eventually evaporate in an explosion of hawking radiation signaling the end of the Black Hole era and ushering in the aptly named Dark Era.
So please enjoy yourselves while you still have time.