Black Holes: From Stellar Collapse to the Event Horizon

## Black Holes

When a star more massive than about 20-25 solar masses collapses, even neutron degeneracy pressure cannot halt the collapse. The result is a black hole — an object so dense that its escape velocity exceeds the speed of light.

### Formation

Stellar black holes form from the most massive stars. During the supernova, if the remnant core exceeds about 2-3 solar masses (the Tolman-Oppenheimer-Volkoff limit), it collapses past the neutron star stage into a singularity.

### Anatomy of a Black Hole

**Singularity**: The central point of theoretically infinite density, where known physics breaks down.

**Event Horizon**: The boundary beyond which nothing can escape. For a non-rotating black hole, its radius (the Schwarzschild radius) is:

r = 2GM/c²

For a 10-solar-mass black hole, this is about 30 km.

**Ergosphere**: For rotating black holes (Kerr black holes), a region outside the event horizon where spacetime is dragged along with the rotation. Energy can be extracted from this region via the Penrose process.

**Accretion Disk**: Material falling toward a black hole forms a superheated disk, reaching millions of degrees and emitting powerful X-rays.

### The First Image

In 2019, the Event Horizon Telescope (EHT) captured the first direct image of a black hole — the 6.5-billion-solar-mass supermassive black hole at the center of galaxy M87. The image showed the predicted 'shadow' surrounded by a bright ring of superheated gas.

### Types of Black Holes

| Type | Mass | Formation |
|---|---|---|
| Stellar | 5-100 M☉ | Core-collapse supernova |
| Intermediate | 100-100,000 M☉ | Mergers, unclear |
| Supermassive | 10⁶-10¹⁰ M☉ | Galaxy centers, early universe |