All language subtitles for Black Holes 101 _ National Geographic_HD.smi

(mysterious music)

- [Woman] Black holes are among

the most fascinating
objects in our universe,

and also the most mysterious.

A black hole is a region in space

where the force of gravity is so strong,

not even light, the fastest known entity

in our universe, can escape.

The boundary of a black hole
is called the event horizon,

a point of no return, beyond
which we truly cannot see.

When something crosses the event horizon,

it collapses into the
black hole's singularity,

an infinitely small,
infinitely dense point

where space, time, and the laws
of physics no longer apply.

Scientists have theorized
several different types

of black holes, with stellar
and supermassive black holes

being the most common.

Stellar black holes
form when massive stars

die and collapse.

They're roughly 10 to 20
times the mass of our sun,

and scattered throughout the universe.

There could be millions of
these stellar black holes

in the Milky Way alone.

Supermassive black holes
are giants by comparison,

measuring millions,
even billions of times,

more massive than our sun.

Scientists can only guess how they form,

but we do know they exist at the center

of just about every large
galaxy, including our own.

Sagittarius A, the supermassive black hole

at the center of the Milky Way,

has a mass of roughly four million suns,

and has a diameter about the distance

between the earth and our sun.

Because black holes are invisible,

the only way for scientists to detect

and study them is to observe their effect

on nearby matter.

This includes accretion disks,

a disk of particles that
form when gases and dust

fall toward a black hole, and quasars,

jets of particles that blast out

of supermassive black holes.

Black holes remained largely
unknown until the 20th century.

In 1916, using Einstein's
general theory of relativity,

a German physicist named
Karl Schwartzschild

calculated that any mass
can become a black hole

if it were compressed tightly enough.

But it wasn't until 1971
when theory became reality.

Astronomers studying
the constellation Cygnus

discovered the first black hole.

An untold number of black holes

are scattered throughout the universe,

constantly warping space and time,

altering entire galaxies,
and endlessly inspiring

both scientists and our
collective imagination.