Nebulas to nebulas, dust to dust.
What happens when a galaxy goes bust?
Hello aspiring galactic overlords and their bumbling henchmen, Trace here.
This is DNews.
Galaxies are collections of millions and billions of stars, the Milky Way probably has 100 to
400 billion or so (we don’t really know).
Andromeda, our closest neighbor galaxy, probably has 400 billion stars (but could have a trillion!).
The thing is, Andromeda is likely dead.
Probably has been for a few billion years.
Just like stars, planets, you, me, and our hopes and dreams -- galaxies can die, or ‘quench’
as astronomers call it.
In fact, half the galaxies in the universe are dead, and thanks to a recent study, we
are starting to figure out exactly how they died.
Gasses like hydrogen are used by galaxies to make new stars; as the gasses fuse, the
hydrogen is fused into helium, and then the helium into more elements like carbon, oxygen,
iron, and so on.
As long as stars are being created, a galaxy is deemed “alive,” but when galaxies stop
making new stars…
they’re considered “dead.”
The Sloan Digital Sky Survey (which has been routinely observing the sky since 2000) has
found nearly 50 million galaxies so far, and through all this data scientists recently
found one way galaxies can die…
Galaxy Strangulation.
Basically, the hydrogen gas that the galaxy needs to make stars is slowly cut off, and,
four billion years later, according to this recent paper in Nature, GALACTIC DEATH.
There are a number of things that can strangle galaxies.
If the victim galaxy ventures near a massive body like a black hole, another galaxy, or
galactic cluster, gravity can pulls gas from the interloper, suppressing star formation
-- and starting it on a path to death.
This is called Galaxy Harassment.
Or, if that weren’t bad enough, there’s Ram Pressure Stripping, when victim galaxy
gets too close to a hot x-ray-emitting cluster, and the “wind” from the hot cluster “blows”
the star-forming gas out of the victim!
Space is pretty insane.
They finally figured out galactic strangulation by looking at all those metals the galaxies
make while alive, and determined: that by looking at the shape and color of a galaxy
you can see it’s life cycle, and how it died.
Galaxies have color and shape, collectively called morphology.
They can be lots of shapes, but their colors are a bit simpler to grasp.
New, young stars make galaxies blue; they’re burning their fuel fast and loose, being reckless
and staying out late.
Inside those galaxies are patches of red and pink -- called HII (H-two) regions.
These are the star-forming regions, and they’re mostly visible in ultraviolet (which is one
of many reasons why we need lots of telescopes to see all the things going on in the night
sky).
As they age, galaxies lose the HII regions, the young stars within them swell and cool…
and the galaxy turns red... damn, too real.
As they get old, they tend to collapse onto themselves forming elliptical little sad reddish
blobs.
On the shift from young blue galaxies to old red ones, they are thought to move through
what astronomers call the “green valley,” -- a joke about the color of light in-between
blue and red.
Without new stars coming up, the amount of metal we can see increases.
Supernovas fuse more heavy elements, and the now-dead galaxy cools and cools… eventually
become an infrared corpse galaxy.
InfraRIP.
So, I hate to be the one to tell you this, but the Milky Way?
We just might be in the green valley.
We’re still making stars, but according to a 2011 paper in the Astrophysical Journal,
we’re definitely in the mid-life stretch.
Don’t worry, y’all.
For now, our good old Milky Way is just fine.
For now...
If astronomers are right, we’ve got billions of years of galactic life left.
And just because it’s not forming new stars, doesn’t mean a galaxy is entirely dead.
There’s still a chance it can save itself.
Sometimes, “dead” galaxies, become zombie galaxies…
Like our friend Andromeda.
Andromeda is dead, but still on the move, and it’s heading this way.
Andromeda should collide with the Milky Way in about 4 billion years, forming a new, massive
galaxy!
(FYI: our solar system will see this happen.
Our sun will have expanded and roasted earth, but it will still be here.)
In true Báthorian fashion, old galaxies can sometimes consume younger, star-forming galaxies
to extend their lives!
Maybe the merged Andromedy will be able to reinvigorate its formation furnaces, and kick
some more life out of our galactic tires...
Even if we don’t, we’ll eventually merge with more and more galaxies forming a massive
elliptical galaxy, filled with metal, and burning near infrared.
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We talked a lot about colors and ages of galaxies but what about their shapes?
How do they get those shapes?
Find out in this video.
Let us know down in the comments if you have any other science questions you want us to
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