The <a href="https://en.wikipedia.org/wiki/Event_Horizon_Telescope" class="extiw" title="w:Event Horizon Telescope">Event Horizon Telescope</a> (EHT) — a planet-scale array of eight ground-based radio telescopes forged through international collaboration — was designed to capture images of a <a href="https://en.wikipedia.org/wiki/black_hole" class="extiw" title="w:black hole">black hole</a>. In coordinated press conferences across the globe, EHT researchers revealed that they succeeded, unveiling the first direct visual evidence of the supermassive black hole in the centre of <a href="https://en.wikipedia.org/wiki/Messier_87" class="extiw" title="w:Messier 87">Messier 87</a> and its shadow.<br>In this image of M87* taken on 11 April 2017 (a representative example of the images collected in a global 2017 EHT campaign), the shadow of a black hole is the closest we can come to an image of the black hole itself, a completely dark object from which light cannot escape. The black hole’s boundary — the event horizon from which the EHT takes its name — is around 2.5 times smaller than the shadow it casts and measures just under 40 billion km across. While this may sound large, this ring is only about 40 microarcseconds across — equivalent to measuring the length of a credit card on the surface of the Moon.<br>Although the telescopes making up the EHT are not physically connected, they are able to synchronize their recorded data with atomic clocks — hydrogen masers — which precisely time their observations. These observations were collected at a wavelength of 1.3 mm in the 2017 campaign. Each telescope of the EHT produced enormous amounts of data – roughly 350 terabytes per day – which was stored on high-performance helium-filled hard drives. These data were flown to highly specialised supercomputers — known as correlators — at the Max Planck Institute for Radio Astronomy and MIT Haystack Observatory to be combined. They were then painstakingly converted into an image using novel computational tools developed by the collaboration.<br>This image is the average of three different imaging methods after convolving each with a circular Gaussian kernel to give matched resolutions. The image is shown in units of brightness temperature, <span class="mwe-math-element"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle {T}_{\rm {b}}=S{\lambda }^{2}/2{k}_{\rm {B}}{\rm {\Omega }}}"><semantics><mrow class="MJX-TeXAtom-ORD"><mstyle displaystyle="true" scriptlevel="0"><msub><mrow class="MJX-TeXAtom-ORD"><mi>T</mi></mrow><mrow class="MJX-TeXAtom-ORD"><mrow class="MJX-TeXAtom-ORD"><mi mathvariant="normal">b</mi></mrow></mrow></msub><mo>=</mo><mi>S</mi><msup><mrow class="MJX-TeXAtom-ORD"><mi>λ<!-- λ --></mi></mrow><mrow class="MJX-TeXAtom-ORD"><mn>2</mn></mrow></msup><mrow class="MJX-TeXAtom-ORD"><mo>/</mo></mrow><mn>2</mn><msub><mrow class="MJX-TeXAtom-ORD"><mi>k</mi></mrow><mrow class="MJX-TeXAtom-ORD"><mrow class="MJX-TeXAtom-ORD"><mi mathvariant="normal">B</mi></mrow></mrow></msub><mrow class="MJX-TeXAtom-ORD"><mrow class="MJX-TeXAtom-ORD"><mi mathvariant="normal">Ω<!-- Ω --></mi></mrow></mrow></mstyle></mrow><annotation encoding="application/x-tex">{\displaystyle {T}_{\rm {b}}=S{\lambda }^{2}/2{k}_{\rm {B}}{\rm {\Omega }}}</annotation></semantics></math></span><img src="https://wikimedia.org/api/rest_v1/media/math/render/svg/068e6ad38b6ee20805c1fd176a68343c8bd005f9" class="mwe-math-fallback-image-inline" aria-hidden="true" style="vertical-align: -0.838ex; width:16.121ex; height:3.176ex;" alt="{\displaystyle {T}_{\rm {b}}=S{\lambda }^{2}/2{k}_{\rm {B}}{\rm {\Omega }}}"></span>, where S is the flux density, λ is the observing wavelength, <span class="mwe-math-element"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle {k}_{\rm {B}}}"><semantics><mrow class="MJX-TeXAtom-ORD"><mstyle displaystyle="true" scriptlevel="0"><msub><mrow class="MJX-TeXAtom-ORD"><mi>k</mi></mrow><mrow class="MJX-TeXAtom-ORD"><mrow class="MJX-TeXAtom-ORD"><mi mathvariant="normal">B</mi></mrow></mrow></msub></mstyle></mrow><annotation encoding="application/x-tex">{\displaystyle {k}_{\rm {B}}}</annotation></semantics></math></span><img src="https://wikimedia.org/api/rest_v1/media/math/render/svg/fc7fadc2641881cbfa453ef6b7ce7db5e7f15e7b" class="mwe-math-fallback-image-inline" aria-hidden="true" style="vertical-align: -0.671ex; width:2.607ex; height:2.509ex;" alt="{\displaystyle {k}_{\rm {B}}}"></span> is the Boltzmann constant, and Ω is the solid angle of the resolution element.
<bdi><a href="https://en.wikipedia.org/wiki/en:Event_Horizon_Telescope" class="extiw" title="w:en:Event Horizon Telescope"><span title="project to create a global interferometric telescope array">Event Horizon Telescope</span></a>
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