How a black hole forms
The short answer
A heavy star is held up by heat from its core pushing against gravity. When the fuel runs out, gravity wins: the core collapses and the outer layers are blown away. The smaller a mass is squeezed, the faster you must go to escape its surface. For the Sun that is about 618 km/s today; squeezed to about 6 km across, it would take the speed of light, so nothing gets out. That boundary is the event horizon.
Chapters and transcript
- 0:00
A star is a tug of war.
A heavy star holds itself up.
- 0:09
Gravity pulls in. Core heat pushes out.
Gravity pulls the star inwards. Heat from nuclear fusion in its core pushes outwards, and the two stay in balance.
- 0:20
Its fuel runs out. The push stops.
When the core runs out of fuel, nothing pushes back against gravity.
- 0:28
In a far heavier star, gravity wins.
In stars far heavier than the Sun, the core is crushed by its own gravity.
- 0:33
The core collapses. The outer layers blow away.
The core collapses and the outer layers are blown off into space.
- 0:42
Why can't anything get out? Take the Sun.
To see why nothing escapes a black hole, take the Sun, 1,391,400 km across.
- 0:49
To leave its surface takes about 618 km/s.
To escape the Sun's surface, something must leave at about 618 km/s. The film computes this from the Sun's mass (GM) and radius in NASA's Sun Fact Sheet.
- 0:56
Squeeze it smaller: the pull at its surface grows.
Squeeze the same mass into a smaller ball and its surface is closer to the centre, where the pull is stronger. The picture is not to scale; the numbers are.
- 1:08
Escaping takes more and more speed.
The speed needed to escape is the square root of 2GM divided by the radius, so it rises as the Sun shrinks.
- 1:18
Its light turns red and dim.
Light leaving the surface loses energy as it climbs out, so it reaches us redder and dimmer.
- 1:26
About 6 km across: not even light gets out.
At a radius of 2GM/c², about 2.95 km for the Sun, the escape speed reaches the speed of light, 299,792 km/s. Nothing gets out: this is the event horizon.
- 1:36
Light that passes close is bent.
These light paths are computed from the equation for light around a black hole in Einstein's theory. Light passing nearby is bent.
- 1:44
Closer still, it falls in.
Light that comes closer than a certain distance falls in and never comes out.
- 1:51
Some swings all the way around.
Light aimed just outside that distance swings around the black hole before getting away.
- 1:58
The Sun is too light to collapse like this.
The Sun will never be squeezed this small on its own; only far heavier stars collapse into black holes.
- 2:05
A black hole is not a hole. It is mass squeezed small.
A black hole is mass packed into so small a space that not even light can escape it.
Sources
Every figure in the film comes from these. The animation is illustrative.
- NASA Sun Fact Sheet: the Sun's mass, GM, radius and escape velocity
- CODATA 2022 (NIST): speed of light in vacuum, 299 792 458 m/s
- Schwarzschild, K. (1916). Über das Gravitationsfeld eines Massenpunktes nach der Einsteinschen Theorie. Sitzungsber. Preuss. Akad. Wiss. Berlin, 189–196