The End of Stars
We currently live in what cosmologists call the luminiferous era—the epoch of our universe that is full of stars, light and warmth. Every year, a major galaxy like our Milky Way manufactures a handful of new stars, with each new generation carrying the torch from the one before.
But when it comes to star formation, our universe is already past its prime. Star formation peaked almost ten billion years ago and has been declining ever since. The reason for this strange dwindling of light is the fact that we live in an expanding universe. Our cosmos grows larger with every passing day. But the amount of matter within the universe remains fixed, and so all that matter slowly gets spread out over more and more volume.
To make a star, you need to compress matter down into relatively small volumes, so as the universe ages, there are simply fewer and fewer opportunities to make that happen.
It’s difficult to speak of the far future of the universe with any level of precision, but we can make rough estimates. Our cosmos is currently 13.77 billion years old, and galaxies throughout the universe will continue making new stars for many years to come. But eventually—roughly one trillion years from now—the last star will be born.
That star will likely be a small red dwarf, barely a fraction of our sun’s mass. Red dwarf stars live fantastically long lives, gently sipping on hydrogen to power a slow but steady fusion reaction. But eventually, all stars, including the red dwarfs, will come to an end. In roughly 100 trillion years, the last light will go out.
The Lonely Cosmos
As the luminiferous era slowly unwinds, the universe itself will change character. With the current age of the cosmos, our observable bubble—defined by the most distant objects that we can see—is roughly 90 billion light years across. The volume bounded by that diameter contains about two trillion galaxies.
Not only is our universe expanding, but that expansion is also accelerating. Discovered in the late 1990s and known as dark energy, the accelerated expansion will eventually close the curtains on our view of the wider universe.
✅ Here’s the reason why all stars will slip from view one day:
In an expanding universe, the more distant a galaxy is from us, the faster it appears to recede away. Indeed, the most distant galaxies from us are already receding away from us faster than the speed of light. This is not a violation of the familiar speed-of-light rule from special relativity, because the galaxies themselves aren’t moving; instead, the space between us is expanding. With more space, there’s more to expand, and so the recession appears faster for greater distances.
We can still see those galaxies, however, because they emitted their light long ago, when they were much closer to us. The light they are emitting right now will never, ever reach us. And because the expansion of the universe is accelerating, this boundary of sight inches ever closer to us. As time goes on, the distance beyond which galaxies recede from view gets nearer and nearer.
One by one, the inexorable cosmic expansion will rip galaxies away from us, pulling them so quickly that their light will never reach us again. Anything not already gravitationally bound to us will not survive the acceleration-driven onslaught. Only the Local Group, consisting of the Milky Way, Andromeda, and Triangulum galaxies, along with dozens of satellite dwarf galaxies, will remain nearby.
It won’t be pretty. Our three galaxies will eventually merge into one mega-galaxy, completely and utterly isolated from everything else in the entire universe. In other words, our universe will be … just us.
Achingly slowly, even that mega-galaxy, a universe unto itself, will dissolve. Chance interactions will scatter individual stars—or what’s left of them—into random orbits, sending them careening into the giant black hole at the center of the galaxy or flying off into the void, never to be seen again. After 1020–1030 years, no complex system will remain, with all macroscopic objects left as islands, lost and adrift in a sea of infinite blackness.
The Degenerates
In that extreme, unfathomable future, our slowly dissolving galaxy will not appear as it does today. The stars will be long gone before our galaxy unbinds itself. Instead, the death of the last star marks the beginning of the degenerate era, the epoch of our universe that will occupy quintillions of years (each quintillion is a billion billion years).
Individual planets will survive into this epoch, although they will lose all internal sources of heat. So too will asteroids, comets, and other assorted bits of space debris. The largest stars will die and give way to neutron stars and black holes. Stars like our own sun will become white dwarfs. Red dwarfs will lose their ability to continue fusion, turning into black dwarfs—a strange kind of non-radiating stellar object that does not yet exist in our comparatively young universe.
Random quantum interactions will eventually, slowly, dissolve these macroscopic objects as well. As each leftover dead star or wandering lonely planet finds itself alone in the cosmos, individual atoms will leave them, bit by bit. After approximately 1065 years, macroscopic objects will cease to exist in the universe.
Black holes will be the last holdouts, but they too will succumb to the darkness. An exotic (having unusual properties) quantum process known as Hawking radiation forces all black holes to slowly radiate away energy and particles. The process is the very definition of inefficient—a typical black hole emits roughly one particle every single year—but at these timescales even the slowest process eventually comes to completion. With each emission of radiation, the black holes lose mass.
After 10100 years, they too will wash away.
Approaching Oblivion
In the ultimate far future of the universe, after all the stars have left the stage—along with their degenerate leftovers and black holes—nothing more than individual particles will dominate the universe.
We don’t know yet if the proton is stable over these lifetimes. If it is, then protons will survive as the largest objects in the universe, until they too must eventually decay after 10200 years.
If dark energy continues to dominate the universe and the expansion of the cosmos continues, we now encounter what’s known as the heat death of the universe. The present epoch of our cosmos features vast energy and heat differences, but the iron laws of thermodynamics dictate that eventually those differences will vanish.
The universe—what’s left of it—will reach thermal equilibrium, with no significant heat differences remaining. And that temperature will continue to drop, slowly approaching, but never quite reaching, absolute zero. With that death of heat comes the death of any form of life, no matter how exotic and alien.
And then … well, who knows. The earliest moments of the Big Bang are a mystery to us because the conditions there are so extreme, beyond our current understanding of physics. The same goes for the tremendously distant future. All of our knowledge of physics is based on the experiments and observations we’ve been able to make in our present-day universe. We simply have no complete basis to reckon with processes operating over multi-quintillion-year (and that’s just the start) timescales. It’s simply too weird.
Who knows what strange quantum tricks the universe may get up to in its cold future. A new Big Bang could suddenly spring from the vacuum, birthing a new universe from the ashes of the old. The chances of that are almost impossibly small, but when compared against the enormous timescales of the future existence of the universe, even the most incredible odds become near guarantees.
Or something completely unexpected could happen, something that we don’t even have the language for right now, because it’s not part of the way that the universe operates today. I suppose we’ll just have to wait and find out.
Paul M. Sutter is a science educator and a theoretical cosmologist at the Institute for Advanced Computational Science at Stony Brook University and the author of How to Die in Space: A Journey Through Dangerous Astrophysical Phenomena and Your Place in the Universe: Understanding Our Big, Messy Existence. Sutter is also the host of various science programs, and he’s on social media. Check out his Ask a Spaceman podcast and his YouTube page.
