So, Ted, the big stars. They explode when
they get old, but that's not the end of the story, is it?
Did you say they become black holes? They sound really cool. Could you tell me more?
Yes, that's right, Little Bear. They can become black holes if they are big stars.
If not, they become neutron stars, but i'll tell you about those another time.
Let's talk about black holes for now.
So when the star explodes it leaves behind it's core
the very middle of the star which falls in on itself due to its own gravity.
Gravity is the force that attracts things together.
So the core gets all squashed together
due to this force.
This means that there's a large mass, the stuff the star is made of, all squashed into a much smaller space.
The pull of gravity becomes so strong, that nothing can escape from it, not even light.
This is why we call it a black hole.
But if it's black how can you see it?
Oh, well, they are actually one of the brightest sources of gamma rays.
So, in a way, they aren't black
at all. They're very bright.
We can normally tell where there is a black hole not because of the gamma rays but
because we can see lots of planets and
stars orbit around what seems like empty black space.
This means that there must be something big there that is attracting all the stars and planets so it must be a black hole.
Black holes have a very strong gravitational pull so they attract lots of space stuff from all around them.
They eat lots of this up and
grow bigger and bigger.
While they're gobbling up all this stuff, gamma rays are released.
But will our Sun turn into a black hole and eat us all up?
Oh no, Little Bear, don't worry our Sun is too small a star to turn into a black hole.
Now there is also such a thing as a supermassive black hole.
These are gigantic black holes!
Supermassive black  holes are more massive so have more stuff in them than a million suns put together.
Scientists think that there may be a supermassive black hole in the middle of every galaxy.
In fact, at the middle of our galaxy, the Milky Way, there is a supermassive black hole.
The stars in our galaxy, including the Sun, orbit around this supermassive black hole.
Some galaxies have supermassive black holes in them that eat so much space stuff, and even other stars
that they get indigestion and spew out powerful jets!
These jets are perfect for speeding up
little particles to really high speeds
and also making gamma rays.
The middle of the galaxies that make these jets are called active galactic nuclei or AGN for short.
We want to learn more about black holes and what
goes on around them like how they make
gamma rays and how they speed up particles.
Wow, it's really hard to
imagine something so huge!
So black holes are still a bit of a mystery?
Yes, plenty still to learn about and discover.
Maybe you could make the next big discovery, Little Bear.
That would be the best! I want to know all about what's going on in space.
