When staring up at the nighttime stars, it's easy to forget that they, too, will die. In comparison
to the brief dust of a single human life, with a very visible beginning, middle, and end,
bound to an Earth defined by interwoven ecological cycles, the stars do indeed seem eternal.
But let's put that in perspective: It's taken 13.8 billion years since
the Big Bang to reach the point where we can even make such an observation.
The Milky Way galaxy is a 100,000-light-year-wide spiral galaxy with a supermassive black hole
at its center. Our sun is merely one of 100 billion stars in its expanse,
and our single galaxy is but one of two trillion within the observable universe.
And the observable universe? It's a mere 1/250th of the entirety of the cosmos,
a 23-trillion-light-year-wide reality over 15 million times greater in volume
than anything we will ever see or visit,
because the universe is expanding at a rate faster than the speed of light.
The relatively short-term fate of the Milky Way, though, isn't bound to the death of its stars,
but the collision between us and our neighbor, the Andromeda galaxy.
"There's no point in acting all surprised about it. The plans and demolition orders have been
on display at your local planning office in Alpha Centauri for 50 of your Earth years."
Before we dive into the fate of our Milky Way galaxy, we've got to take a little foray into
the life cycle of stars themselves. Like humans, like that totally adorable little
Corgi that lives down the street, like the seemingly ancient redwood forests, or, in fact,
just like any other terrestrial life form, stars are born, pass through phases, and then die.
Sorry.
Nothing can escape the inevitable: Not even entire galaxies, or the universe itself.
Our own sun is a good reference point. In a 13.8-billion-year-old universe,
the sun is a modestly-sized, modestly-hot yellow dwarf star about 4.5 billion years old.
Its core is 15 million degrees Celsius — or 27 million degrees Fahrenheit — and
its gravitational pull extends beyond Pluto and for up to two light years.
Sure, that's impressive, but it faces the same end as every single star in the Milky Way and beyond:
go quietly into that good night in a shrunken,
white-dwarfed fuel-burning fizzle, or explode in a supernova and scatter its remnants everywhere.
But here's the thing: There will eventually be no new suns, anywhere.
This is the only era of the universe — the second era of five — when stars can be born.
And this era, the Stelliferous Era, is nearing the end. We're about 13.8 billion years in,
we've got about 200 million years to go. After that, stars will only die,
and occasionally form new specks of light in the sky when they collide with each other.
So how long does a star live? It depends on the mass.
Very large stars can go supernova in only a few million years — a relative blip in the cosmic
night. Smaller stars like our sun, however, can run on their fusion tanks up to 10 billion years.
The Milky Way is full of a variety of such stars that will all continue to age.
About 5 billion years from now, our sun will start to transition to its red giant phase.
By about 8 billion years in the future, it'll consume the inner planets: Mercury, Venus,
and yes, Earth. Our solar system's habitable zone will be Jupiter and Saturn and their moons.
Before then — and about 2 billion years from now — the Andromeda Galaxy, a spiral galaxy about
2.5 million light years away, will start to grow larger and larger in our night sky. Eventually,
it'll clash with our own galaxy and swirl into a new nebula of gases and intermingled star systems
to form the fused, new galaxy. That's when the Milky Way, technically, will cease to exist.
While it's possible that collisions might happen, it's unlikely because
of the amount of emptiness in space. So, that's some good news in all of this.
It's only after that, whoever is left behind will see our sun set for the final time,
shrinking into a white dwarf and eventually a black dwarf: the end of a star's life cycle and
fizzled-out companion to a black hole, which forms when a sun goes supernova.
Beyond that? All stars will wink out one by one. Black holes will consume everything,
including other black holes, and grow bigger and bigger until they, too,
eventually expel all the energy they've captured and leave in their place absolutely nothing at
all. The entire universe will be locked, permanently, in a state of absolute zero.
There will be no more heat because there will be no more energy difference between anything.
Our chilled, starless cosmos will look like a dark dream of peace fulfilled.
So, to answer the question: Will the Milky Way galaxy live forever?
Of course not. This can be a terrifying thought,
sure, but it can also be a very good reason to gaze up into the sky and appreciate the one,
brief epoch when such wonder is possible. And in the meantime… don't sweat the small stuff.
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