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19 JULY-AUGUST 2022 ASTRO PUBLISHING — around both Sgr A* and M87*. But where gas takes days to weeks to orbit the larger M87*, in the much smaller Sgr A* it completes an orbit in mere minutes. This means the brightness and pat- tern of the gas around Sgr A* were changing rapidly as the EHT Collabora- tion was observing it — a bit like trying to take a clear pic- ture of a puppy quickly chasing its tail.” The researchers had to develop sophisti- cated new tools that accounted for the gas movement around Sgr A*. While M87* was an easier, steadier tar- get, with nearly all images looking the same, that was not the case for Sgr A*. The image of the Sgr A* black hole is an average of the different im- ages the team extracted, finally re- vealing the giant lurking at the centre of our galaxy for the first time. The effort was made possible through the ingenuity of more than 300 researchers from 80 institutes around the world that together make up the EHT Collaboration. In addition to developing complex tools to overcome the challenges of imaging Sgr A*, the team worked rigorously for five years, using su- percomputers to combine and an- alyse their data, all while compiling an unprecedented library of simu- lated black holes to compare with the observations. A global map showing the radio observatories that form the Event Horizon Telescope (EHT) net- work used to image the Milky Way’s central black hole, Sagittarius A*. The telescopes high- lighted in yellow were part of the EHT network during the observations of Sagittarius A* in 2017. These include the Atacama Large Millimeter/submillimeter Array (ALMA), the Atacama Pathfinder EXperiment (APEX), IRAM 30-meter telescope, James Clark Maxwell Telescope (JCMT), Large Mil- limeter Telescope (LMT), Submillimeter Array (SMA), Submillimetere Telescope (SMT) and South Pole Telescope (SPT). Highlighted in blue are the three telescopes added to the EHT Collaboration after 2018: the Greenland Telescope, the NOrthern Extended Millimeter Array (NOEMA) in France, and the UArizona ARO 12-meter Telescope at Kitt Peak. [ESO/M. Kornmesser] Scientists are particularly excited to finally have images of two black holes of very different sizes, which offers the opportunity to under- stand how they compare and con- trast. They have also begun to use the new data to test theories and models of how gas behaves around supermassive black holes. This process is not yet fully under- stood but is thought to play a key role in shaping the formation and evolution of galaxies. “Now we can study the differences between these two supermassive black holes to gain valuable new clues about how this important process works,” said EHT scientist Keiichi Asada from the Institute of Astronomy and Astrophysics, Aca- demia Sinica, Taipei. “We have im- ages for two black holes — one at the large end and one at the small end of supermassive black holes in the Universe — so we can go a lot further in testing how gravity be- haves in these extreme environ- ments than ever before.” Progress on the EHT continues: a ma- jor observation campaign in March 2022 included more tele- scopes than ever before. The ongo- ing expansion of the EHT network and significant technological up- grades will allow scientists to share even more impressive images as well as movies of black holes in the near future. !