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I recently read an article that said that the event horizon telescope took a picture of a black hole (Sagitarius A*) and it will be presented on the 10th of April. I was wondering how this picture is taken and what a picture of a black hole even means?

The only explanation I could think of was that the telescope took pictures of the stars surrounding the black hole and then a sudden absence of stars pointed that a black hole is present.

Can anyone explain how black hole photos are taken or how does event horizon telescope work?

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The project is using a network of radio telescopes to measure radio waves emitted by ionized matter in the accretion disk around the black hole, and by ionized matter in relativistic jets that are being ejected along the rotation axis.

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  • $\begingroup$ We already had radio telescopes then what makes event horizon telescope any special? $\endgroup$
    – Manvendra Somvanshi
    Apr 5, 2019 at 19:59
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    $\begingroup$ In order to be able to resolve something this small so far away, the signals from many telescopes across the Earth need to be combined to have as large an effective baseline as possible. The black hole has about 4 million solar masses, so its event horizon has a radius of about $10^{10}$ meters. It is about 8000 parsecs from us, which is more than $10^{20}$ meters. So its angular size in the sky is only about $10^{-10}$ radians. $\endgroup$
    – G. Smith
    Apr 5, 2019 at 20:37
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    $\begingroup$ Even supermassive black holes are not particularly large. The radius of this one at the center of our galaxy is about 13 times smaller than the Earth's orbital radius. So it's like trying to resolve a small solar system 26,000 light-years away. Not easy! $\endgroup$
    – G. Smith
    Apr 5, 2019 at 20:39

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