Black Hole Size Worksheet
Goal: Calculate the size of a black hole’s event horizon from its mass, and describe how the horizon grows with mass.
What to do
- Press the “Sun’s mass” preset. Record the horizon radius and diameter.
- Press “Stellar mass” and record the readouts. Then type 20 into Black hole mass and record them again.
- Press “Sagittarius A*” and “M87*” and record the readouts.
Record your results
| Setting | Horizon radius | Horizon diameter |
|---|---|---|
| Sun’s mass (1 Sun) | ||
| Stellar mass (10 Suns) | ||
| 20 Suns | ||
| Sagittarius A* (4.3 million Suns) | ||
| M87* (6.5 billion Suns) |
Questions
How big would the event horizon be if the Sun became a black hole without changing its mass? Compare it with the Sun’s real radius of about 696,000 km.
Compare 10 and 20 solar masses. What happens to the horizon radius when the mass doubles?
Use rₛ ≈ 2.95 km × (mass in Suns) to estimate the horizon of Sagittarius A* (4.3 million Suns), then check with the lab.
Earth’s mass is about 0.000003 Suns, too small to enter in the lab. Calculate the size of an Earth-mass black hole.
If the Sun were replaced by a black hole with the same mass, would Earth be pulled in? Explain.
The lab shows a glowing ring around the black hole. Is that ring the event horizon? Explain.
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