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At a worldwide press conference this morning, the Event Horizon Telescope Collaboration unveiled the first direct images ever taken of Sagittarius A *, the supermassive black hole at the center of the Milky Way. This exciting first comes two years after the same collaboration of scientists released the first image of a black hole ever made.

Nothing escapes a black hole, not even light. The only way to see a black hole is through the glowing remnants of the unfortunate stars it swallows. But there is so much hazy gas, dust, and debris between us and the galactic center that only the longest wavelengths of infrared light can reach us. However, our long-wave radio telescopes were able to capture this first image of Sagittarius A *.

This is the first image of Sagittarius A *, the black hole in the center of our galaxy. Credit: EHT Collaboration

SA * is about 27,000 light-years from Earth. It is four million times the mass of our Sun, but occupies only a small place in the sky: about the same size as a donut on the surface of the moon. However, radio telescopes around the world have joined forces to create the Event Horizon Telescope (EHT). EHT triangulated between radio systems to create a giant, Earth-sized radio telescope. This ensures unsurpassed angular resolution.

Journey to the center of the galaxy

Eight different radio telescopes contributed to these observations. The telescopes in the image below are part of the Atacama Large Millimeter / Submilimeter Array (ALMA), which is part of the EHT collaboration. From Earth, Sagittarius A * (abbreviated Sgr A *, pronounced “saj-ai-star”) is located in the ecliptic. Inside you can see where Sgr A * sits in the Milky Way, along the spine of the night.

    This image shows the large millimeter / submillimeter massif Atacama (ALMA) facing up the Milky Way, as well as the location of Sagittarius A *, the supermassive black hole at our galactic center.  Marked in the field is the image of Sagittarius A *, made by Event Horizon Telescope (EHT) Collaboration.  Located in the Atacama Desert in Chile, ALMA is the most sensitive of all the observatories in the EHT massif, and ESO is a co-owner of ALMA on behalf of its European Member States.

The foreground telescopes are part of the large millimeter / submillimeter Atacama array. Credit: EHT Collaboration

EHT Collaboration scientists have also created a flight video that starts with ALMA and zooms in to Sgr A *. The video begins in the visible spectrum, moving to the infrared as it approaches the event horizon of the black hole. Be warned: The effect can be a little dizzying.

Videos like this have been taken from hundreds of thousands of different photos of the night sky. Astronomers then combine the images into a 3D representation of what a Earth traveler will see while traveling to Sgr A *.

Image of a black hole

To capture these images of Sgr A *, EHT scientists used a technique called very long baseline interferometry, or VLBI. VLBI compares the timestamps of the readings and uses minor time differences to make accurate measurements of distant objects in the deep sky. It’s a bit like parallax, but in a temporal sense; we try to find the distance between objects so far away that they do not move in the sky. So instead of tracking their movement, we use radio telescopes to track the time when these distant signals arrive.

This process generates terabytes of data every day. EHT cleans and calibrates the data and then stores it on helium-filled hard drives that lead to the MIT Haystack Observatory or the Max Planck Institute for Radio Astronomy. There, they do their analysis using supercomputers called “correlators,” which group and average all readings.

Credit: EHT cooperation

The black hole itself is a real challenge for astronomers trying to get a clear picture. The scientists needed a long exposure, “maybe eight to ten hours,” to complete their portrait of Sgr A *. But the accumulating gas and dust revolve around the black hole at an almost orbital speed of light that takes only minutes to complete.

EHT scientists believe that Sgr A * is currently absorbing two or more traveling stars. But the rate of accumulation is uneven. This may explain the fact that we see “spots” of light instead of a uniform, symmetrical halo. When they combined their thousands of shots into video, the light spots turned into streaks of glowing stellar things.

Spirit in heaven

The EHT network itself continues to expand. Eight observatories participated in the creation of this first image of Sagittarius A *. However, since March 2022, three more observatories have joined the effort. Technological improvements continue.

Meanwhile: When EHT Collaboration released these first Sagittarius A * images, they also released their entire open source project. Michael Jansen, a spokesman for EHT, explained that the data the team used to construct these images was “completely public, on multiple levels”. Jansen added that the EHT Collaboration released its raw data, along with their algorithms and a clean data set, “so everyone can reproduce what we’ve done from scratch.”

This achievement follows the release of the EHT Collaboration in 2019 on the first image ever taken of a black hole called M87 * for the place where it is located in its own galaxy, Messier 87. M87 * is a thousand times larger than Sgr A * and a thousand times more mass. But the two black holes have a striking resemblance. EHT scientists want to know why.

Credit: EHT Cooperation (recognition: Lia Medeiros, xkcd)

“We can now study the differences between these two supermassive black holes to gain valuable new clues as to how this important process works,” said EHT scientist Keiichi Asada. “We have images of two black holes – one at the big end and one at the small end of the supermassive black holes in the universe – so we can go much further in testing how gravity behaves in these extreme environments than ever before.

The results of the EHT team are published today in a special issue of The Astrophysical Journal Letters.

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