Black hole and a donut on the Moon

This collage was composed by the staff of the European Southern Observatory (ESO). On the left we can see the image of the shadow of Sagittarius A* — a supermassive black hole located in the heart of the Milky Way. This picture was provided by the network of telescopes called the Event Horizon Telescope (EHT). On the right we see a picture of a donut on the surface of the moon. But what is the connection between these two images?

Чорна діра та пончик
Collage showing the shadow of the black hole Sagittarius A *, and a donut on the surface of the moon. Source: EHT Collaboration, ESO / M. Kornmesser, NASA, Lu Am

The fact is that the shadow of Sagittarius A * occupies an area in the sky the size of only 52 angular microseconds. And this corresponds exactly to the size of a donut lying on the moon visible from the Earth.

Of course, even the most advanced telescopes are not able to see such a microscopic area. Therefore, EHT participants resorted to the method of Very-long-baseline interferometry. They combined a network of radio observatories located on different continents to create a virtual telescope the size of a globe. Then, using sophisticated algorithms and programs, the scientists compiled a single image from the obtained signals.

Recall that black holes cannot be observed directly. They do not emit any radiation. However, astronomers are able to record the fluxes of radio waves emitted by the surrounding accretion disks, which are heated to enormous temperatures. The eclipse in the center of this area corresponds exactly to the silhouette of a black hole.

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