(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.