Name: 天の川にはブラックホールの円盤があるかもしれない (The Milky Way May Have a Disk of Black Holes)
Uploaded: 2021-01-14T10:28:35.000Z
Duration: 5 min 31 s
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Just about every galaxy has a supermassive black hole at its center,

one millions of times the mass of the Sun.

It's been hard nailing down exactly how many smaller black holes there are out there, though,

especially near galactic centers, because black holes are hard to see.

But a paper published last week in the journal Physical Review Letters

has new insight about where we can look to nail that number down.

They're so dense that even light can't escape from them,

so directly detecting one is nearly impossible.

We can still find them through gravitational effects, though,

and through X-ray light given off by objects that heat up before falling into them.

But it's been a challenge to use those measurements to estimate how many less massive black holes are out there,

Earlier this year, a study of X-ray sources in the Milky Way

suggested that there might be as many as ten or twenty thousand smaller black holes near our galaxy's center.

And a similar study of ring galaxies, the beautiful results of galactic collisions,

found evidence for lots of small black holes, too.

But new simulations have given us a much better idea of where to look for them next.

In last week's paper, a pair of Hungarian physicists made a new computer model of small stars, heavy stars, and black holes

all orbiting near the center of a galaxy like the Milky Way.

They ran their program for millions of simulated years,

enough time for everything to finish jostling for position and get settled.

Once that was all over, they found that low-mass stars ended up with all sorts of orbits,

different sizes, different angles, you name it.

Together, they created a big, bright sphere of stars around the galactic center,

which is pretty much what we see in actual galaxies.

The black holes, though, did something more interesting.

All the pushing and shoving before orbits settled down tended to push them all into one plane,

a lot like how all the planets in our solar system orbit in a single, flat disk.

The same happened to heavier stars, which we've seen in the actual Milky Way.

But we didn't know it would happen with black holes, too.

Now, that insight could help target our searches for black holes near the center of the galaxy,

learning where they are and what they're doing more quickly than we could through general surveys.

Meanwhile, in other black hole news, sort of, scientists are getting one step closer to totally understanding supernovas.

colossal explosions of dying stars that launch heavy elements throughout the universe.

So far, we have a good idea of what happens in the chaos of a supernova,

but there are still some details to work out and some measurements we need to make

before we're completely confident in our models.

The good news is, a paper in last week's Nature Astronomy has helped us check one item off that list:

understanding a small flash before the main explosion.

The paper's authors looked at Type II supernovas.

These start when a massive star begins to run out of nuclear fuel,

and its gravity starts pulling everything in toward its center.

As things get denser, protons and electrons in the star's core combine into neutrons,

creating an incredibly dense neutron star.

Then, when the original star's outer layers collide with the edge of that inner neutron star,

they violently bounce off, creating an explosion that can outshine an entire galaxy.

Before that main explosion, though, we've also seen a smaller flash.

We've only observed them a couple of times,

but we think they should happen before pretty much every Type II supernova.

We've just never been able to confirm that,

because we haven't had instruments sensitive enough and haven't looked long enough to catch them all.

Using a telescope in Chile equipped with one of the highest-resolution cameras in the world,

these researchers watched that process unfold on 26 different Type II supernovas.

And they found a flash of light before most of the main explosions.

That confirmed those flashes are a regular thing.

We already knew that for thousands of years before a supernova,

some of the star's atmosphere leaks into space,

leaving a bunch of gas hanging around when the star finally does start collapsing.

We think the flashes happen when some of the energy from the formation of the neutron star

sneaks out and hits this gassy layer before the energy of the rebounding gas does.

But we haven't had a good idea of how much gas is actually out there

because we've seen so few of these flashes.

So this paper's authors worked with an existing model of supernovas to figure it out.

They found that, to make the flashes they observed,

the star would need to lose as much gas as you'd find in our entire Sun before becoming a Type II supernova.

Which goes to show you how huge these stars are to begin with.

They're some of the biggest ones in the universe.

But it's only enough to create a tiny flash that we hardly noticed before,

because it then gets drowned out by the supernova itself.

With bigger and better telescopes coming online all the time,

and with models that are constantly being improved by research like this,

we should be seeing more and more of these flashes as time goes on.

And that means we should gain a better and better understanding of what happens in some of the universe's largest explosions.

Thanks for watching this episode of SciShow Space News!

and if you want to learn about the latest news in planets, moons, black holes, and everything else,

you can go to youtube.com/scishowspace to subscribe.