NARRATOR: Black holes sound wildly complicated.
After all, there are all sorts of bizarre
things going on: intense gravity, the warping
of the fabric of space, the distortion of
time itself.
But when it comes to describing black holes,
it comes down to just two numbers: the mass
of the black hole and its spin.
That’s right.
Everything you physically need to describe
a black hole is found in just these two numbers.
So, if it’s so simple, astronomers must
know these two numbers for lots and lots of
black holes, right?
In fact, getting these two numbers turns out
to be incredibly hard.
Dr. Jeffrey McClintock of the Harvard-Smithsonian
Center for Astrophysics has been trying to
tackle this problem.
Just recently, it turns out he and his colleagues
did.
SCIENTIST: Getting the mass is the easy part.
These small black holes we study are orbited
by an ordinary star like the sun.
Using an optical telescope, we measure the
speed of this sun-like star and the time it
takes to completely orbit the black hole.
This is old hat.
This is how astronomers have measured the
masses of stars in ordinary binary systems
and planetary systems for many years.
But when I say that getting the mass is easy,
I should be more careful.
For example, we just spent two years getting
the mass of one special black hole in the
nearby galaxy M33.
So it's really no piece of cake.
Anyway, we found that the mass of this X-ray
source, called M33 X-7, is about 16 times
more massive than our sun, with a margin of
error of one and a half times the mass of
the sun.
This is the most accurate mass that has been
measured for any black hole.
NARRATOR: But this wasn’t the end of the
story with M33 X-7.
McClintock and his colleagues set out to find
that other elusive number: the spin.
SCIENTIST: Measuring spin is really hard,
because you have to understand what’s going
on in the Alice-in-Wonderland world close
to a black hole.
Let's start the story out near the ordinary
star.
The black hole’s gravity strips gas from
that star, and the gas falls toward the black
hole, forming a swirling disk of orbiting
matter.
Very near the black hole, this gas gets heated
to millions of degrees by the colossal force
of the gravity, and it shines brightly in
X-rays, which we easily observe using Chandra.
Of course, eventually, all this hot gas is
destined to disappear forever once it falls
through the event horizon, which is located
25 km from the dead center of the black hole.
But far away from this dreaded event horizon,
a point is reached where the force of gravity
becomes so immense that the super-hot gas
can’t any longer maintain itself in a stable
orbit around the black hole.
At this point, the disk abruptly ends and
the gas in orbit there suddenly plunges inward,
reaching the event horizon in less than one-thousandth
of a second.
NARRATOR: But what happens next?
SCIENTIST: This leaves a large dark hole in
the center of the disk that extends down to
the event horizon.
The radius of this dark hole depends only
on the two numbers in question, namely, the
black hole's mass and how fast it is spinning.
For a black hole that is not spinning at all
and that weighs 16 sun masses (like our M33
X-7), the radius of this dark region is 75
km, which is 3 times the radius of the event
horizon.
The faster the black hole spins, the smaller
this radius becomes.
For a black hole with the maximum spin allowed
by relativity theory, the radius becomes equal
to the radius of the event horizon.
By studying the spectrum of the X-rays from
M33 X-7, we’ve been able to accurately measure
the inner radius of the hot disk to be 45
km, and this tells us that the spin of M33
X-7 is about three quarters of its theoretical
maximum value.
Near the event horizon, the black hole's spin
drags everything around with it, an apple,
an astronaut, even space itself, at the dizzying
rate of 750 revolutions per second.
NARRATOR: So what does this all mean?
Jeff McClintock puts the result into perspective.
SCIENTIST: As you said at the beginning, only
two numbers are needed to describe a black
hole.
A black hole’s as simple as an electron
and far simpler to describe than, say a grain
of sand.
It is absolutely amazing to me that Chandra
has allowed us to obtain a complete description
of an object the size of an asteroid that
is situated at a distance of about 3 million
light years.
NARRATOR: So there you have it.
Two numbers to describe one of the Universe’s
most mysterious objects.
Getting to know these values is not just for
fun.
When they are plugged into theoretical models,
they can help astronomers better understand
things like how black holes are born, how
gravity behaves under extreme conditions,
and how black holes make powerful relativistic
jets, and more.
Who would have guessed that just two numbers
could do all 
of that?
