Black Holes – Wiki

What Is a Black Hole?

A black hole is a region of space where gravity is so strong that nothing, not even light, can escape. It forms when a large mass is squeezed into a very small space. Einstein's Theory of Relativity (general relativity) predicts them, and they have since been observed many times.

How Black Holes Form

Stellar black holes: the core of a very massive star (very roughly more than 20 times the Sun's mass) collapses at the end of its life, usually in a supernova. See Types of Stars in the Universe.
Mergers: two neutron stars, or two black holes, merge.
Supermassive black holes: how they formed is still being researched. Possibilities include early "seed" black holes that grew by swallowing gas and merging, or huge gas clouds that collapsed directly in the young universe.

Structure

Part	Description
Singularity	The centre, where general relativity predicts infinite density. This probably means the theory breaks down there
Event horizon	The "point of no return". Inside it, even light cannot escape
Photon sphere	At 1.5 × the Schwarzschild radius (for a non-rotating black hole). Light can circle the black hole here
ISCO (innermost stable circular orbit)	At 3 × the Schwarzschild radius (non-rotating). Closer than this, matter cannot orbit stably and falls in
Accretion disk	Gas spiralling inward. It heats up to millions of degrees and shines brightly in X-rays
Jets	Beams of particles shot out from the poles at nearly the speed of light, in some black holes

The Schwarzschild Radius

This is the radius of the event horizon of a non-rotating black hole:

r_s = 2GM / c²

If compressed into a black hole	Schwarzschild radius
Earth	about 9 mm (the size of a marble)
The Sun	about 3 km
Sagittarius A* (4.3 million Suns)	about 12.7 million km

Note: If the Sun were replaced by a black hole of the same mass, Earth's orbit would not change. Black holes don't "suck"; at a safe distance their gravity is the same as that of any other object with the same mass.

Types of Black Holes

Type	Mass (Sun = 1)	Example
Stellar	about 3 – 100+	Cygnus X-1 (about 21 solar masses), the first widely accepted black hole
Intermediate	100 – 100,000	Rare and hard to confirm. Candidates exist, for example in the star cluster Omega Centauri
Supermassive	millions – billions	Sagittarius A* at the centre of the Milky Way (about 4.3 million), M87* (about 6.5 billion)

Almost every large galaxy has a supermassive black hole at its centre.

Falling In

Spaghettification: near a small black hole, gravity pulls much harder on your feet than on your head. You would be stretched like a noodle. At a supermassive black hole these tidal forces are weaker, so you could cross the event horizon unharmed.
Time dilation: someone watching from far away sees a falling object slow down more and more and turn redder and dimmer, without ever seeing it cross the horizon. The falling object itself crosses in a finite amount of its own time.

How We Observe Them

Black holes themselves cannot be seen, but their effects can:

Method	Example
Stars orbiting an "empty" spot	Stars orbiting Sagittarius A* (Nobel Prize 2020 for Andrea Ghez and Reinhard Genzel)
X-rays from an accretion disk	Cygnus X-1
Gravitational waves from mergers	LIGO's first detection, GW150914: about 36 + 29 Suns merging
Imaging the "shadow"	Event Horizon Telescope: M87* (2019) and Sagittarius A* (2022)
Gravitational lensing	Light from objects behind a black hole is bent

Hawking Radiation

In 1974, Stephen Hawking showed that because of quantum effects, black holes should slowly radiate energy and lose mass:

T = ħc³ / (8πGMk_B)

The heavier the black hole, the colder it is. A black hole with the Sun's mass would have a temperature of only about 60 billionths of a kelvin. That is far colder than the cosmic microwave background, so for now it takes in more energy than it gives off. It would take about 10⁶⁷ years, far longer than the current age of the universe, to evaporate completely. Hawking radiation has not been observed directly yet.