How Big Is a Black Hole?
Black holes range enormously in size—from tiny stellar black holes just a few miles across to supermassive black holes larger than our entire solar system.
Measuring Black Holes
Two Key Measurements
1. Mass: How much stuff is in the black hole
2. Event horizon: The "size" we can conceptualize (Schwarzschild radius)
The more massive a black hole, the larger its event horizon.
Types by Size
Stellar Black Holes
Formation: Collapsed massive stars
Mass: 3-50 solar masses
Event horizon: 6-90 miles (10-150 km) diameter
- A 10 solar mass black hole has an event horizon about 37 miles across
- About the size of a city like Los Angeles
How many: Estimated 100 million in our galaxy
Intermediate Black Holes
Mass: 100-100,000 solar masses
Event horizon: 185-185,000 miles diameter
Status: Hard to detect, fewer confirmed. May form from stellar black hole mergers.
Size comparison: Larger than Earth, smaller than the Sun
Supermassive Black Holes
Location: Centers of galaxies (including ours)
Mass: Millions to billions of solar masses
Event horizon: Millions to billions of miles
Examples:
| Black Hole | Mass | Event Horizon |
|------------|------|---------------|
| Sagittarius A* (Milky Way) | 4 million suns | 14.6 million miles |
| M87* (first imaged) | 6.5 billion suns | ~24 billion miles |
| TON 618 (largest known) | 66 billion suns | ~200 billion miles |
- Sagittarius A*: Fits inside Mercury's orbit
- M87*: Larger than our entire solar system
- TON 618: Absolutely colossal—our solar system would be tiny inside it
Visualizing the Scale
Sagittarius A* (Our Galaxy's Black Hole)
- Mass: 4 million Suns
- Event horizon: 14.6 million miles diameter
- If placed at the Sun's position: Would extend halfway to Mercury
- Only 26,000 light-years away
M87* (First Photographed Black Hole)
- Mass: 6.5 billion Suns
- Event horizon: ~24 billion miles diameter
- Size: Larger than Pluto's orbit
- 55 million light-years away
The Schwarzschild Radius Formula
Event horizon size = (2 × G × M) / c²
- G = gravitational constant
- M = mass
- c = speed of light
Simplified: Event horizon radius ≈ 3 km per solar mass
- 1 solar mass → 3 km radius (6 km diameter)
- 10 solar masses → 30 km radius
- 4 million solar masses → 12 million km radius
Common Misconceptions
"Black holes suck everything in"
No. They have strong gravity, but only affect things that get very close. If our Sun became a black hole (impossible), Earth's orbit wouldn't change.
"Black holes are just big spheres"
The event horizon isn't a surface—it's a boundary. Black holes that rotate (most of them) actually have more complex shapes.
"Bigger is more dangerous"
Ironically, supermassive black holes are gentler to approach. Tidal forces at the event horizon are weaker due to the larger size.
The Density Paradox
- TON 618: Average density less than air
- This is because density = mass/volume, and volume grows faster (³) than mass
Stellar black holes have unimaginably high density—squeezing 10+ solar masses into a city-sized sphere.
Summary
| Type | Mass | Event Horizon | Example |
|------|------|---------------|---------|
| Stellar | 3-50 suns | City-sized | Cygnus X-1 |
| Intermediate | 100-100K suns | Earth to Sun | M82 X-1 |
| Supermassive | Millions-billions | Solar system+ | Sagittarius A* |