Obtaining the first-ever photograph of a black hole was an impressive feat. We tend to think of black holes as being tiny and they are. A black hole represents a singularity in space-time where gravitational field becomes so large that there is extreme curvature of space. But the ‘event horizon’ of a black hole, the region inside from which no light can escape, need not be tiny. It is the event horizon that gives rise to the dark region seen in the photograph and the size of the event horizon for any mass *M* is given by the Schwarzschild radius that is equal to *2GM/c ^{2}*, where

*G*is the gravitational constant and

*c*is the speed of light.

The black hole at the center of the galaxy M87 that was photographed has a mass 6.5 billion times the mass of the Sun and thus its Schwarzschild radius is about 1.9×10^{10} km. This is quite large, about three times the distance of the planet Pluto from the Sun, which is 5.9×10^{9} km.

Mobius says

A nice ballpark figure for the Schwarzschild radius is 2 miles (or 3 kilometers) per solar mass of the black hole (which comes very close to the figure given above.)

Rob Grigjanis says

It’s dodgy to talk about the Schwarzschild radial coordinate r as a physical distance, unless r is much greater than the Schwarzschild radius. So, for an observer near the event horizon, a small change in r corresponds to a (possibly much) greater change in actual radial distance.

It’s better to talk about surface area, because the physical surface area of a sphere at radial coordinate r is 4πr², by construction.

Holms says

Very large, and yet tiny -- in the order of one thousandth of the distance between the sun and Proxima Centauri.

Jörg says

Randall Munroe has a graphic: “M87 Black Hole Size Comparison”.

Rob Grigjanis says

My GR is a bit rusty, but I think the size comparisons are misleading. The shadow in the picture does not show the extent of the event horizon; it shows the extent of the photon sphere, which has a radial coordinate of 1.5 times the Schwarzschild radius. Further, the shadow is magnified by gravitational lensing, so it appears larger than it actually is.

Rob Grigjanis says

OK, I found an article by two members of the EHT team.