A black hole's event horizon, photon sphere and innermost stable circular orbit (ISCO) all scale directly with its mass through the Schwarzschild radius.
r_s = 2GM/c² (event horizon)
r_photon = 1.5 · r_s (photons can orbit here)
r_ISCO = 3 · r_s (closest stable matter orbit, disk inner edge)
T_Hawking = ħc³ / (8πGMk_B) (smaller black holes are hotter)
- Mass — sets every other scale; heavier holes have a bigger horizon but a colder, gentler tidal field.
- Spin — a spinning (Kerr) hole drags the disk's inner edge closer in and speeds up the near-horizon swirl.
- Accretion rate — how much infalling gas is glowing in the disk right now.
- Hawking glow — toggles a stylized visualization of the (astronomically faint, never actually observed) thermal radiation the horizon should emit.
Real-world relevance: the 2019 Event Horizon Telescope image of M87* showed exactly this structure — a dark shadow near the photon sphere ringed by a glowing, Doppler-brightened accretion disk.