Name: 回転するブラックホール (Spinning Black Holes)
Uploaded: 2021-01-14T10:25:48.000Z
Duration: 10 min 14 s
Description: VoiceTubeの動画で発音を聞きながら英語表現を覚えよう！学べる英語：

that's the All Sky Automated Survey for Super Novae

the signal came  from the center of a galaxy around 290 million light-years away

and what we now belive happened was a star came too close to a supermassive black hole

with a mass millions of times that of our Sun and it was eaten

and yes, this is the actual termenology astrophysicists use to describe it

occurring maybe once every 10,000 to 100,000 years in a galaxy

they're called Tidal Disruption Events, or Tidal Disruption Flares

as the star approached the side closest to the black hole experienced a much gravitational pull than the other side,

matter spiraling into the black hole formed an accretion disk

that's accelerating and heating up emitting visible light

Now what's remarkable about this event is that it transformed a dormant

one that wasn't really feeding into one that we can observe

thanks to the matter falling in from that star

check out these artists renditions of the same event

"Well how do we know that's what really happened?

what if the scientists are just making this up to get more grant funding

well I'll explain how we know this is actually what went down

scientists trained three X-ray telescopes to observe this part of the sky for years after the event

and what they found was a strong and regular pulse of X-rays

brightening and dimming every 131 seconds

and it shows up in the data from all three telescopes

but the pulse maintained this rhythm and didn't get weaker

in fact as time went on the relative strength of the pulse got stronger

modulating the X-ray signal by around 40%

and what was causing these periodic flashes of X-rays

and what could it tell us about the black hole

because black holes are some of the simplest objects in the Universe

by that I just mean that they are characterized by only two attributes

but since black holes should essentially be neutral mass and spin

far away from a black hole, you can even use Newtonian physics.

By measuring the gravitational effects of the black hole on other bodies，

you can estimate the mass of of the black hole

this has been done and black holes been found with masses ranging from

just few times our Sun, stellar-mass black holes,

up to billions of solar masses -- supermassive black holes

it's generally accepted that there is a supermassive black hole at the centers of most galaxies, including our own.

since black holes form from collapsing stars and all known starts rotate,

I mean, what are the chances that a bunch of matter just collapses into a point perfectly with no rotation?

And then additional matter falling into the black hole contributes its angular momentum.

So like a figure skater pulling their arms into a... point object,

You can imagine black holes get spinning pretty fast.

But spin is harder to measure because unlike mass, it only affects objects relatively close to the black hole.

But there is a way to do it. Actually, 3 ways.

To understand all of them, you have to understand isco.

In Newtonian physics, around a compact mass,

you can place an object in a circular object at any radius and it will be stable.

This is not the case according to general relativity.

Here, there is an innermost stable circular orbit,

Closer than this, and no orbits are stable: they all fall into the black hole.

So when you're looking at a black hole that is feeding, the innermost edge of the accretion disk is at r-isco.

What's useful for our purposes is that r-isco depends on the spin of the black hole.

The faster it's spinning, the smaller r-isco becomes.

Assuming it's spinning in the same direction as the matter in the accretion disk.

The rotation enables particles to orbit closer to the black hole,

than they'd be able to for a non-spinning black hole.

So you can kind of think of it as though the spin is supporting the particles against the relentless pull of gravity.

Now spin is normally discussed in terms of a dimensionless parameter

that ranges from 0, no spin, to 1, maximum spin.

Though I guess you could also have spins down to -1 if the black hole is spinning in the opposite direction from the accretion disk.

Now, as spin increases, r-isco decreases.

by a factor of 6, shrinking down to the size of the event horizon.

And this sets what many scientists think is the maximum spin a black hole can have.

Because if the minimum stable orbit were the size of the event horizon,

Then light could escape from the black hole, allowing us to see into the singularity.

This is called a naked singularity, and it makes a lot of scientists uncomfortable.

As yet, there isn't a strong theoretical reason why a black hole can't exceed this maximum spin,

It's just that we haven't seen one, and the thought of an exposed singularity just kind of... feels wrong.

Most suspect the maximum real-world spin parameter is around 0.998.

So how can you use r-isco to measure the spin of a black hole?

Well first, let's think of how we measure the size of anything far away from us in deep space, like the radius of a star

Most stars are so far away that they're simply point objects in our telescopes.

Well, first, look at the spectrum of their light.

By seeing how red-shifted absorption lines are, you can determine how far away the star is.

The spectrum also tells you the temperature of the star --

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回転するブラックホール (Spinning Black Holes)

approach

rhythm

reveal

matter