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Hey Crazies.
We've been hearing about supermassive black holes a lot in the news lately.
We just took a picture of one at the center of our own galaxy.
They aren't kidding when they say these things are supermassive either.
The one I just showed you has a mass of 4 million Suns
and that's actually on the low end for supermassive black holes.
Some of their masses are in the billions of Suns.
The problem is, none of them should exist.
[Intro Music]
What do you mean "they don't exist"?
I didn't say "they don't exist." I said "they shouldn't exist."
But more on that later.
First, we need to understand exactly what "supermassive" means.
Black holes come in two classes based on their mass.
Stellar-mass black holes are exactly as they sound.
They have a mass comparable to a star.
Measuring that in solar masses, multiples of the Sun's mass,
they range anywhere from about 3 to about 200 solar masses.
Then... there's a big gap.
We don't find black holes again on this chart until we get into the millions of solar mass.
That's the beginning of what we call "supermassive black holes."
They're not just massive. They're supermassive.
Anyway, we tend to find these supermassive black holes at the centers of galaxies.
Our own Milky Way galaxy has one weighing in at about 4 million solar masses.
We've known it was there for a while because of what it's doing to the nearby stars.
Those stars are also what helped us calculate its mass.
The recent picture we took is only confirming what we already knew.
But don't get the wrong impression.
This bad boy might have a mass of 4 million Suns,
but it would fit inside Mercury's orbit.
At most, it can only fit 17 suns across its diameter.
This thing is smaller than most active giant stars,
but contains the mass of millions.
And that's just the upper bound on its mass.
The fact that it's spinning makes it even smaller.
Be careful not to confuse mass with volume.
Then again, as far as supermassive black holes go, ours is kind of a runt.
We also took a picture of the one at the center of M87.
That one weighs in at about 6.5 billion solar masses.
That's over 1600 times the mass of our own black hole.
And that isn't even the most massive one we found.
[Transition Music]
The universe isn't old enough for any stellar-mass black holes to have become supermassive.
But, again, more on that later.
First, we need to understand how black holes usually form.
Inside an active star, there's a very delicate balance at work called "hydrostatic equilibrium."
The inward collapse of gravity is held at bay by the outward radiation pressure from the core.
At the end of a star's life, the nuclear fuel runs out and that radiation pressure stops.
If the star is massive enough, the core collapses into a black hole
and the immense energy released during the collapse blows the rest away in a supernova.
That's an explosion that can outshine an entire galaxy.
So only the core becomes a black hole? Yep.
How much of the star's mass is that? Only about 10 or 15%,
which limits the mass a black hole can have when it forms.
That outward radiation pressure from a star's core doesn't just overcome gravity.
It also pushes particles out of the star surface into space.
We call it a "stellar wind."
The more massive a star is, the hotter its core,
which means more radiation pressure,
which means there are more particles being emitted.
If the star's mass is high enough, that stellar wind is so intense that the mass of the star noticeably decreases.
It's called the Eddington Limit.
If a star tries to form with a higher mass, the mass of that star will decrease down to that limit.
We estimate it's about 130 solar masses.
And, if the core is only at most 15% of that mass,
the black hole it eventually makes will only be about 20 solar masses.
Wait, wait, wait. Didn't you say that stellar-mass black holes can get up to 200 solar masses?
Yeah. That's where black hole collisions become important.
This 20 solar mass limit is only for the formation of a stellar-mass black hole.
But stars often form in binary systems and sometimes even trinary systems.
If both stars in a binary system are in the upper-mass bracket, then they're both going to become black holes.
And if we give them enough time, they'll emit gravitational waves and merge with each other.
Given even more time, those larger merged black holes can find each other and merge again.
Based on our estimates,
stars have been around long enough for that process to reach a maximum of about 200 solar masses.
This estimate is consistent with data from our gravitational wave observatories.
The vast majority are actually under 100.
Black holes with higher masses are outliers.
Even with mergers though, there hasn't been enough time to make a black hole larger than 200 solar masses,
let alone anything supermassive.
But more on... that right now.
[Transition Music]
Most of the stars around today are third generation stars.
That means it's the third time that material has been used to make a star.
First generation stars were certainly some of the most massive that have ever lived,
and they lived very short lives.
But they still only made stellar-mass black holes when they ended.
We're going to need them to start way bigger and way earlier to reach supermassiveness.
(Off Camera) That's not even a word!
I do what I want!
To make a supermassive black hole, we need to start with a star with tens of thousands of solar masses
and we need that star to exist during a period of the universe when stars shouldn't have been possible.
(Firm skeptical look from clone) The early universe was a very different place.
Back before stars, back before galaxies, the universe was pretty uniform.
But it wasn't perfectly uniform.
These slight, non-uniformities are what ultimately became superclusters full of galaxies.
Let's say for the sake of argument though, that something else formed there first.
Say the material collapsed so fast in one particular spot
that it skipped over nuclear fusion and immediately became a tiny black hole.
Now, if a tiny black hole formed today, it would almost immediately evaporate.
Fast fast!
But, back in the early universe, it wouldn't have had a chance.
It was surrounded by mostly uniform material,
material that would fall in causing the black hole to grow.
This continuous collapse would release a lot of energy,
but the surrounding material has so much gravity that a balance could be reached.
It could reach hydrostatic equilibrium kind of like a star.
It would be a quasi-star,
though a better name for it would be "black hole star."
Rather than an outward pressure caused by nuclear fusion,
it would be caused by material falling into a black hole.
The core of this quasi-star would be a black hole.
What about the Eddington limit? It doesn't apply.
That limit assumed there was mostly empty space for particles to escape to.
But, in the early universe, this quasi-star wouldn't be surrounded by empty space.
It would have been surrounded by mostly uniform material,
only slightly less dense than the star itself.
Any particles emitted in a stellar wind would just get replaced.
Over time, though, the surrounding material would spread out and cool off.
The outer layers of the quasi-star would gradually dissipate or fall into the black hole.
By then, the black hole will have grown to at least 1000 solar masses,
but probably more likely in the tens of thousands.
These intermediate-mass black holes could then accumulate material over time
and occasionally find each other and merge,
ultimately becoming the mammoth black holes we see at the centers of galaxies today.
Do we know this for sure? Well, no.
Do black holes exist? Absolutely.
Do supermassive black holes exist? Yes.
But black holes are very simple objects. They only have a few properties:
mass, angular momentum, and maybe charge. That's it.
It doesn't give us much to use.
Plus, these properties are notoriously difficult to measure.
Whether directly or indirectly, we need to be able to see orbiting objects or in-falling material.
And, even then, we're making some minor assumptions about orientation.
For example, we know there are black holes more massive than the one in M87.
We just don't know exactly how much more massive they are.
Given the age of the universe, we're pretty sure the absolute upper limit is around 50 billion solar masses.
But a few black holes seem to be larger.
Then again, that's probably because we haven't gotten a good look at them yet.
Quasi-stars are just our best guess for their origin, based on current physics.
We haven't even had confirmed observations of first-generation actual stars,
let alone these possible quasi-stars.
But these are both things we'll be looking for with the James Webb Space Telescope.
Astrophysics is hard, ok?! We're doing our best.
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So what do you think of quasi-stars?
Let us know in the comments.
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And until next time, remember, it's okay to be a little crazy.
I got a bunch of feedback on how to do thermal camera experiments better, and I really appreciate it.
I might use some of the suggestions to make a proper follow-up video,
one that's more than just me playing around with a new toy.
Also, everyone can stop complaining. I have a proper workbench now.
Anyway, thanks for watching.