The size of the photographed black hole event horizon

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/c2, 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 1.9×1010 km. This is quite large, about three times the distance of the planet Pluto from the Sun, which is 5.9×109 km.


  1. 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.)

  2. 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.

  3. Holms says

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

  4. 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.

  5. Rob Grigjanis says

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

    The size of the shadow is fully determined by general relativity. Its outline is the photon orbit radius, magnified because of gravitational lensing. For a nonspinning black hole, the photon orbit is at 1.5 RS and the silhouette size turns out to be Rshadow = √ ̅27/2 RS. For a spinning black hole, the photon orbit lies deeper in the gravitational field of the black hole. However, photons experience a higher degree of lensing than for black holes that don’t spin. As a result, the size of the black hole shadow is independent of the spin and orientation of the black hole to within 4%.

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