Black Hole

A region of spacetime where gravity is so strong that nothing — not matter, not electromagnetic radiation, not light itself — can escape once it has crossed the event horizon. The most extreme gravitational objects in the universe. Formed primarily from the collapse of massive stars or the accumulation of matter at galactic centres.

Types

Stellar black holes (5–100 M☉) — formed when massive stars (>20 M☉) undergo core collapse without a successful supernova explosion; the neutron star formation fails and collapse continues.

Supermassive black holes (millions–billions M☉) — found at the centres of virtually all large galaxies. Formation mechanism still debated: rapid early-universe collapse, successive stellar mergers, or direct collapse from gas clouds. Examples:

• Sagittarius A\* (Sgr A*) — at the centre of the Milky Way; 4 million M☉; first imaged by the Event Horizon Telescope in 2022
• M87\* — 6.5 billion M☉; first black hole ever directly imaged (EHT, 2019)

Intermediate mass black holes (~100–100,000 M☉) — evidence growing; possibly formed in dense star clusters.

Primordial black holes (hypothetical) — possibly formed in the early universe from density fluctuations; candidates for dark matter.

The Event Horizon

The event horizon is not a physical surface but a causal boundary — the point of no return. An infalling observer crosses it without local drama; from outside, however, general relativity predicts the observer appears to slow, redden, and freeze asymptotically at the horizon (from the external perspective, matter never quite crosses).

The Schwarzschild radius — the radius of the event horizon for a non-rotating black hole — is: *r_s = 2GM/c²*

For Earth's mass: ~9 mm. For the Sun: ~3 km. For Sgr A*: ~12 million km (about 17 solar radii).

The No-Hair Theorem

A classical black hole is completely characterised by only three parameters:

1. Mass — determines the size of the event horizon
2. Spin (angular momentum) — rotating black holes (Kerr black holes) drag spacetime around them (frame dragging)
3. Electric charge — astrophysical black holes are essentially neutral

All other information about what formed the black hole is lost to external observers — the "no-hair theorem." This conflicts with quantum mechanics' requirement that information be conserved — the black hole information paradox remains one of the most important unresolved problems in theoretical physics.

Hawking Radiation

Stephen Hawking predicted (1974) that black holes are not perfectly black: quantum effects near the event horizon cause black holes to slowly emit thermal radiation, losing mass over time. For stellar black holes the temperature is negligibly small and the evaporation timescale exceeds the age of the universe by many orders of magnitude. But the principle — that black holes evaporate — is central to the information paradox and to quantum gravity.

Direct Imaging

The Event Horizon Telescope (EHT) — a planet-scale interferometric array — produced the first direct images of black holes:

• M87\* (April 2019): 6.5 billion M☉, 55 million light-years away; showed the characteristic bright ring of photons orbiting the event horizon
• Sgr A\* (May 2022): the Milky Way's own central black hole; harder to image because it changes brightness faster than M87*

Both images show the photon sphere — the region where gravity bends light into orbits around the black hole — as a bright ring surrounding the dark shadow of the event horizon.

Gravitational Waves from Mergers

LIGO's first detection (September 2015, announced February 2016) was the gravitational wave signal from two merging black holes (~36 and ~29 M☉ merging into a ~62 M☉ remnant). The energy radiated in gravitational waves during the ~0.2-second merger exceeded the luminosity of all stars in the observable universe combined.

Black hole mergers are now routinely detected by the LIGO-Virgo-KAGRA network — they are the loudest events in the gravitational wave universe. See the planned gravitational waves note for the full detection picture.

Connection to the Existing Vault

• supernova — stellar black holes form when core collapse supernovae fail to expel the outer layers
• neutron-star — the competing outcome for lower-mass progenitors; the boundary between them is ~15–20 M☉
• milky-way — Sgr A* is the Milky Way's central black hole; its mass was determined by tracking stellar orbits (S2 star in particular)
• stellar-nucleosynthesis — black holes terminate nucleosynthesis rather than contributing to it; they are the stellar graveyard
• cosmic-ray — some of the highest-energy cosmic rays may be accelerated in the jets of supermassive black holes (active galactic nuclei)