Free Astronomy Magazine July-August 2024

25 JULY-AUGUST 2024 ASTRO PUBLISHING S ize comparison of the two black holes imaged by the Event Horizon Telescope (EHT) Collaboration: M87*, at the heart of the galaxy Messier 87, and Sagit- tarius A* (Sgr A*), at the centre of the Milky Way. The image shows the scale of Sgr A* in comparison with both M87* and other elements of the Solar System such as the orbits of Pluto and Mercury. Also displayed is the Sun’s diameter and the current location of the Voyager 1 space probe, the furthest spacecraft from Earth. M87*, which lies 55 million light-years away, is one of the largest black holes known. While Sgr A*, 27,000 light-years away, has a mass roughly four million times the Sun’s mass, M87* is more than 1000 times more massive. Be- cause of their relative distances from Earth, both black holes appear the same size in the sky. [EHT collaboration (acknowledgment: Lia Medeiros, xkcd)] erico II, Italy, said, “With a sample of two black holes — with very differ- ent masses and very different host galaxies — it’s important to deter- mine what they agree and disagree on. Since both are pointing us to- ward strong magnetic fields, it sug- gests that this may be a universal and perhaps fundamental feature of these kinds of systems. One of the similarities between these two black holes might be a jet, but while we’ve imaged a very obvious one in M87*, we’ve yet to find one in Sgr A*.” To observe Sgr A*, the collaboration linked eight telescopes around the world to create a virtual Earth-sized telescope, the EHT. The Atacama Large Millimeter/submillimeter Ar- ray (ALMA), in which ESO is a part- ner, and the ESO-hosted Atacama Pathfinder Experiment (APEX), both in northern Chile, were part of the network that made the observa- tions, conducted in 2017. “As the largest and most powerful of the telescopes in the EHT, ALMA played a key role in making this image possible,” says ESO’s María Díaz Trigo, European ALMA Pro- gramme Scientist. “ALMA is now planning an ‘extreme makeover’, the Wideband Sensitivity Upgrade, which will make ALMA even more sensitive and keep it a fundamental player in future EHT observations of Sgr A* and other black holes.” The EHT has conducted several ob- servations since 2017 and is sched- uled to observe Sgr A* again in April 2024. Each year, the images improve as the EHT incorporates new tele- scopes, larger bandwidth, and new observing frequencies. Planned ex- pansions for the next decade will enable high-fidelity movies of Sgr A*, may reveal a hidden jet, and could allow astronomers to observe similar polarisation features in other black holes. Meanwhile, extending the EHT into space would provide sharper images of black holes than ever before. !