Blak Holes, From NASA

Most people think of a black hole

as a voracious whirlpool in space, sucking

down everything around it. But that’s not

really true! A black hole is a place where gravity

has gotten so strong that the escape velocity is faster

than light. But what does that mean, exactly?

For the Earth, that (gravity) velocity is about 11

kilometers per second (7 miles/second). But

an object’s escape velocity depends on its gravity:

more gravity means a higher escape velocity, because

the gravity will “hold onto” things more strongly. The Sun

has far more gravity than the Earth, so its escape velocity is

much higher—more than 600 km/s (380 miles/s).

That’s 3000 times faster than a jet plane!

The most common way for a black hole to form is

probably in a supernova, an exploding star. When a star

with about 25 times the mass of the Sun ends its life, it

explodes. The outer part of the star screams outward

at high speed, but the inner part of the star, its core,

collapses down. If there is enough mass, the gravity of

the collapsing core will compress it so much that it can

become a black hole. When it’s all over, the black hole

will have a few times the mass of the Sun. This is called

a “stellar-mass black hole”, what many astronomers

think of as a “regular” black hole.

But there are also monsters, called supermassive black

holes. These lurk in the centers of galaxies, and are

huge: they can be millions or even billions of times the

mass of the Sun! They probably formed at the same

time as their parent galaxies, but exactly how is not

known for sure. Perhaps each one started as a

single huge star which

exploded to create a

black hole, and then

accumulated more

material (including

other black holes).

Astronomers think

there is a supermassive

black hole in the center

of nearly every large

galaxy, including our own Milky Way.

Once you pass the point where the escape velocity is faster than light, you can’t get out. This region is

called the event horizon. That’s because no information

from inside can escape, so any event inside is forever beyond

our horizon.

It’s as if the matter has disappeared from the Universe, but its mass is still

there. At the singularity, space and time as we know them come

to an end.

If black holes are black, how can we find them?

Fortunately, astronomers have

discovered a signpost that points the way to black holes: X-rays.

if a black

hole is “eating” matter from a companion star, that matter gets very hot and emits

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