Black Hole Event Horizon Calculator

Because this calculator reports the ergosphere's EQUATORIAL boundary, and the Kerr metric shows that boundary sits at exactly 2GM/c^2 regardless of spin -- the same formula as the non-rotating Schwarzschild Radius. The ergosphere narrows toward the event horizon only near the poles, a variation this calculator does not compute; the equatorial bulge stays fixed no matter how fast the hole spins, which is a genuine feature of the Kerr metric rather than a rounding coincidence.

It shrinks the Event Horizon Radius specifically, not the hole's mass or total gravity. A maximally spinning Kerr hole's horizon sits at half the radius of a non-spinning hole of the same mass (GM/c^2 instead of 2GM/c^2), because frame-dragging lets the horizon retreat closer to the singularity while still trapping light. Black Hole Mass, and therefore Hawking Temperature and Hawking Lifetime, are unaffected by Spin Parameter in this model.

Hawking Temperature is inversely proportional to Black Hole Mass, and stellar and supermassive black holes are enormously massive. Even a modest solar-mass black hole computes to a temperature far below a microkelvin -- colder than the 2.7 K cosmic microwave background that bathes all of space -- meaning a real solar-mass black hole absorbs far more radiation than it emits and is still growing, not evaporating, today.

Spin Parameter is the dimensionless Kerr spin a* = Jc/(GM^2). At exactly 1.0 the event horizon and ergosphere coincide and the hole becomes an extremal black hole, a mathematical limiting case current physics doesn't expect any real object to reach. Astrophysical black holes measured via X-ray reflection spectroscopy are typically inferred to have a* well under 1 -- so 0.99 already represents a near-maximal, observationally realistic spin without hitting the singular extremal case.

Hawking Lifetime is computed in seconds from the standard evaporation-time formula and then divided into years purely for readability -- the raw number of seconds in even a solar-mass black hole's lifetime is otherwise unreadable. The conversion doesn't change the underlying physics, which depends on Black Hole Mass through a cube relationship: doubling Black Hole Mass multiplies Hawking Lifetime by eight, not two.