Hi, I'm Brian Koberlein.  I'm an astrophysicist,
and this is the Universe Unleashed.
 So what is a black hole, and how do we know they exist?
To get an idea of a black hole, what you can do
is to think about what happens when you step on a scale on the Earth.
You have the Earth, and you are standing on the Earth.
When you step on a scale, you are measuring
how strongly you are gravitationally attracted to the Earth.
That number depends upon what your mass is,
and it also depend upon the mass of the Earth.
But it also depends upon how close the Earth's mass is to you.
So imagine you have the Earth getting smaller.
All the mass of the Earth is getting more and more condensed.
All of that mass, if you are standing on the surface,
is getting closer to you, and your weight will go up.
If you keep squeezing the Earth more and more,
then eventually you will get to the point
where the gravitational force of the Earth is so strong
that nothing can escape from it.
That's known as a black hole.
Basically all the mass of the Earth would be compressed
down to a very small region.
In order to make a black hole out of the Earth,
you'd have to compress the Earth down to the size of a marble.
That's not going to happen,
but we do know that black holes exist in space,
and one of the ways is by actually looking at the center of our galaxy.
If you look in the center of our galaxy,
what we can see are stars,
and these stars are moving in elliptical orbits.
By looking at how these stars move,
we can actually see how much mass there is in the center.
Right in the center there is so much mass in such a small area
that we know it's a black hole.
In fact we've calculated the mass of that black hole,
and it comes out to be about 4 million times the mass of the Sun.
It's known as a supermassive black hole,
and as far as we can tell, most galaxies have a supermassive black hole
in their centers, including ours.
So the next time you think about it, 
you can think of the supermassive black hole in the center of our galaxy,
and know that they actually exist.
