The bright side of the disk is beamed toward you — relativistic Doppler beaming
Presets
Mass10 M☉
Schwarzschild radius Rs—
Photon sphere (1.5 Rs)—
Innermost stable orbit (3 Rs)—
Accretion diskOn, orbiting from 3 Rs
Doppler beamingApproaching side brighter
Size ladder (log scale): the event horizon (Rs), photon sphere (1.5 Rs) and ISCO (3 Rs) plotted in km against fixed real-world sizes — Mt Everest, a large city, the Moon and the Earth. Move the mass and the three coloured markers slide along the scale while the grey landmarks stay put, so you can read straight off how far the horizon has grown past them.