Occasional paper: The edge of the galaxy and the end of everything

So there was a modest puff of articles last month about astronomers discovering “the edge of the galaxy”.  They were referencing this recent paper, which… doesn’t really say that.  But it’s an interesting paper anyway, so let’s discuss.

As we all learned back in school, our Sun is one star in the Milky Way galaxy, which is a flat spiral containing several hundred billion stars.  And spiral galaxies don’t exactly have an “edge”.  We can’t see the details of the Milky Way, because we’re inside it.  But we have excellent photographs of thousands of other spiral galaxies, and they mostly sort of gradually blur off into intergalactic space.



So if you travelled outwards from Earth, away from the center of the Milky Way, there wouldn’t be any sudden cutoff where stars just stopped.  Stars would gradually get further and further apart.  At some point, there wouldn’t be any stars anywhere near you. 

But there wouldn’t be any “edge” as such.  No glowing purple force field —

— or golden pillars or anything.


What the paper actually found was the edge of the star-forming region of the galaxy.  And that’s a different kind of interesting.

So stars are formed from big clouds of interstellar gas.  And galaxies are born with a very large supply of such gas.  Very large, but not infinite.  After some billions of years, the gas gets used up.  After that, the galaxy can’t form any more new stars.  Astronomers call this “quenching”.

Astronomers refer to galaxies that have quenched as “red sequence” galaxies.  Galaxies that are still forming stars are “blue cloud” galaxies.  It turns out that if you plot this graphically, you get a sharply bimodal distribution: you get a lot of star-forming blue galaxies, a lot of quenched red sequence galaxies, and not much in between.  So the relatively sparse region in between got called the “green valley”.

— Why is it sparse?  Because quenching can happen pretty fast.  So you don’t see a lot of galaxies in that in-between period, because it’s a relatively short period in a galaxy’s life.


As it happens, our own Milky Way galaxy seems to be passing through the green valley right now.  The Milky Way is still forming new stars, but the rate of star formation is starting to drop quickly.  Another couple of billion years, and it’ll be mostly done.  Not entirely done, of course — a galaxy is a big place.  Here and there a few gas clouds will remain, occasionally giving birth to new stars.  But it’ll be a long tail of decline, the cosmic equivalent of searching for loose change under the couch cushions.

Which brings us back to the original paper.  Look at a spiral galaxy, and you can see that it gets denser towards the center.



So you’d reasonably assume that star formation would peak and burn out first in the center, because there’s more gas there — more stuff there generally.  And then you’d expect it to peak further out, and then last of all out near the ends of the spiral arms.  Space is emptier out there, so it takes longer for gas clouds to bump into each other and start condensing.  So you might expect to find fewer young stars in the center (raw material gets burned up quickly) and more young stars far out in the arms (star formation takes longer to get started).

As it turns out: half right.  There is indeed a gradient of older-to-younger stars (on average) as you move out from the galactic core.  But then — well before you reach the end of the arms — the gradient goes into reverse.  It makes a U-shaped curve.  The far ends of the spiral arms have a lot of older stars and not a lot of young ones.  The Milky Way is starting to quench, but not all at once.  Our galaxy has an aging heart, and aging extremities, but there’s a big donut-shaped chunk in between that’s still young and vigorous.

Okay, but — why?

Because galaxies churn. 

A galaxy is made up of billions of stars, all orbiting around the galaxy’s center of mass.  Most of the time, those stars are well separated from each other, light years apart.  But every now and then, a couple of stars will pass close enough to influence each other gravitationally.  When that happens, one star will gain a bit of kinetic energy and move “up” into a more distant orbit.  The other star will lose a bit of energy and move “down” towards the center of the galaxy.  This is known as churning, or “dynamical relaxation” if you’re feeling fancy. 


[dynamical relaxation at Eurovision]

So over billions of years, a galaxy will tend to slowly change shape: it will gradually extend outwards (some stars getting bounced up and out) and also get thicker in the center (some stars bouncing down towards the galactic core).

So the reason for the U-shaped curve is that most of the stars in the outer regions of the galaxy didn’t start there.  That region used to be empty intergalactic space, without enough gas for star formation.  But over billions of years, galactic churning kicked stars into higher, more distant orbits that took them into those regions.  The spiral arms slowly stretched outwards into areas of space that had always been pretty empty, without much gas for star formation.  You might occasionally get a young star bounced out there by accident, but no new young stars would form there.  

And there’s a fairly sharp border between the donut-shaped inner bit where stars are still forming, and the outer reaches where they don’t.  We’ve seen that border in other, distant galaxies.  And — this is the point of the paper — now we’ve found it in our own Milky Way.  

Okay, so that’s the “edge of the galaxy” that’s been found.  But what does this have to do with the end of everything?

Well, here’s the thing: the churning never stops. 

Over billions and then trillions of years, the Milky Way will continue to evolve.  Some stars will fall down towards the center; some stars will get pushed further out.  

And this will continue even after all the stars are dead.  The Sun and other stars like it will swell up into red giants, then contract into white dwarfs, then — very very slowly — cool down to dead cinders, black and cold.  Larger stars will become neutron stars or black holes.  Red dwarfs, the smallest stars, will burn for trillions of years, conserving their fuel, but eventually they too will die.  In the distant future of ten trilliion years from now, the Milky Way will be completely dark and cold.

But it will still churn.  The dead stars will still circle the galaxy’s center of mass.  And very eventually, by drunkard’s-walk statistical inevitability, they will either be flung out into interstellar space, to wander forever alone through the endlessly expanding darkness of a dead universe, or they’ll fall down into the Milky Way’s core.  Where they will encounter —



— Sagittarius A-Star, the supermassive black hole at the center of the Milky Way.

Eventually — very, very, very eventually — pretty much all normal matter in the Milky Way will end up a dead dark cinder.  And that cinder will either be flung out into infinite space to fall forever through darkness, or will plunge into the black hole.  The atoms that you’re made of, right now?  Churn, churn, flip a coin: either up and out, or down the hatch, as the universe very slowly grinds to a dead cold halt.

Right.  Have a great weekend, all!