Orbits Near a Black Hole

The innermost stable orbit

An interactive lesson with 7 steps, about 6 minutes: you predict, drag and solve, with instant feedback on every answer. It runs in your browser and needs JavaScript.

What you’ll learn

• Near a black hole, GR adds an extra attraction that removes stable orbits close in.
• Innermost stable circular orbit: $3r_s$; photon sphere: $1.5r_s$.
• Accretion converts about 6% (or more) of rest mass into radiation: far more than fusion.

Steps

1. Not quite Newton
2. The last stable orbit
3. How close?
4. Light in orbit
5. Light skimming a black hole
6. Hundreds of laps a second
7. The most efficient engine