This BBC article contains this "schematic description" of the EHT Sgr A* image:

enter image description here

The source is described only as "EHT Collaboration" and I haven't been able to find the original image.

The image shows a dotted ring labelled Event horizon, however it was made clear when the M87 black hole images were released that the central, dark area was the shadow of the black hole, not the event horizon. ProfRob's answer to this question states that:

When observed from a great distance, the central shadow appears to have a diameter of about 5 times the Schwarzschild radius of the black hole.

So are we looking at the event horizon of Sgr A* in the above image, or is the diagram incorrectly labelled?

  • 1
    $\begingroup$ The diagram is correct to a factor of $\lesssim 5$ since this black hole is rotating with a possibly high spin. I suspect they used a dotted line instead of a solid line to highlight this uncertainty $\endgroup$ Commented May 14, 2022 at 12:21
  • $\begingroup$ The spin rate is interesting, in 2020 the paper mentioned in this article: sci-news.com/astronomy/sagittarius-a-spin-08993.html suggested a slow spin, but the EHT team say " Our results generally favor models with dynamically strong magnetic fields, moderate (prograde) spin..." in their paper: iopscience.iop.org/article/10.3847/2041-8213/ac6674 $\endgroup$ Commented May 15, 2022 at 9:29

1 Answer 1


The event horizon of the Milky Way black hole (if it were non-spinning) has a diameter of 24 million km and is not directly related to the dark area. The black hole shadow should have an apparent diameter of about 63 million km. The fact that this is a non-extremal spinning black hole doesn't change these numbers too much.

The event horizon is marked with a dashed line because it cannot be seen and should not mark the boundary of anything significant in the image. In particular, it should be inside the black hole shadow.

The resolution of the images is not great, maybe 25 million km FWHM on this scale, so the bright illumination immediately around the black hole shadow- the so-called photon ring - will be blurred and overlap with the shadow to some extent and make it appear smaller.

The translation from an angular scale on the image to a linear scale must also have assumed a distance to Sgr A*. This too has some uncertainty.

So yes, it is approximately correctly labelled.

The image below from the first EHT release paper shows the image labelled in angular units. $10\theta_g$ is 5 times the Schwarzschild radius and is the 63 million km figure. It looks about right. The little circle indicates the amount of blurring you would expect in the image.

EHT BH image

  • $\begingroup$ I thought the conclusion was obvious. Perhaps not $\endgroup$
    – ProfRob
    Commented May 14, 2022 at 16:17
  • $\begingroup$ I guess the reason the line is dotted is because the event horizon is not visible in that image, it will always be hidden by the 'shadow'. $\endgroup$ Commented May 15, 2022 at 9:13
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    $\begingroup$ @DaveGremlin Correct; an event horizon, by definition, cannot be seen. The dashed line just indicates where it would be. $\endgroup$
    – ProfRob
    Commented May 15, 2022 at 9:17

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