The Event Horizon Telescope collaboration has released the first picture of our galaxy’s supermassive black hole, Sagittarius A*, and it’s just as predicted
12 May 2022
For the first time, scientists have taken a picture of the black hole at the centre of our galaxy. This is the second image of a black hole ever created.
The image was captured by the Event Horizon Telescope (EHT), a network of observatories around the world operating as a single enormous radio telescope. In 2017, EHT observed two supermassive black holes: the one in the Milky Way, which is called Sagittarius A* or Sgr A*, and the one at the centre of the M87 galaxy known as M87*. The image of M87* was released in 2019, and now, after three more years of painstaking data analysis, the collaboration has finally released its picture of Sgr A*.
“This is what we wanted to deliver on all along,” says EHT researcher Ziri Younsi at University College London. “This is what our black hole looks like.”
Black holes don’t emit any light, so the image shows the black hole’s silhouette against a glowing background of hot plasma swirling around and being pulled into Sagittarius A*.
That process occurs much more quickly with this black hole than with M87*, which is one reason why the new image took so much longer to produce. M87* is one of the largest known black holes in the universe at about 6.5 billion times the mass of the sun, more than 1000 times the mass of Sgr A*. As a result, it takes the plasma around M87* days to weeks to complete an orbit, whereas it takes only minutes for hot plasma to circle Sgr A*.
“This means the brightness and pattern of the gas around Sgr A* was changing rapidly as the EHT collaboration was observing it – a bit like trying to take a clear picture of a puppy quickly chasing its tail,” said EHT researcher Chi-kwan Chan at the University of Arizona in a statement.
Adding to the difficulty was the fact that Earth sits towards the edge of the Milky Way, so the researchers had to deal with light from all the stars, dust and gas between our planet and Sgr A*. To make the final image, they aggregated many snapshots taken over the course of several nights and used a supercomputer to process the data.
“With M87* everyone was just elated, but this was a much harder image to make, so everyone was really cautious this time,” says Younsi. “We approached it quite a bit more conservatively – everyone had their scientist hats on instead of their party hats, really.”
The final image looks remarkably similar to 2019’s picture of M87* despite the different sizes and environments of the black holes. “We have two completely different types of galaxies and two very different black hole masses, but close to the edge of these black holes they look amazingly similar,” said EHT scientist Sera Markoff, at the University of Amsterdam in the Netherlands, in a statement. “This tells us that [Albert Einstein’s] general relativity governs these objects up close, and any differences we see further away must be due to differences in the material that surrounds the black holes.”
What is a black hole?
Black holes are objects with an intense gravitational pull so strong that not even beams of light, the fastest things in the universe, can escape.
The most visible prediction of general relativity is that the ring of light around the black hole ought to be a little lopsided. The gravitational pull of Sgr A* is so strong that it bends the light, making the plasma circling towards us appear brighter than that spinning away towards the black hole’s backside.
When the researchers compared the image of Sgr A* to a library of hundreds of thousands of simulated black holes modelled in scenarios that do not follow general relativity, they found that Sgr A* appears to hew closely to relativistic models. “One of the things which surprises me personally was just how similar these images are to what theory predicts,” says Younsi. “Einstein’s doing well, again, and for people who have all their other theories of what gravity could be it might be a little disappointing.”
As researchers continue to analyse the black hole data and figure out how the two black holes compare to one another, they also have a new set of observations to examine. Three telescopes were added to the EHT network before the most recent observing campaign in March 2022, which means future pictures should be sharper and should illuminate subtle details in the areas around black holes. The team is also working on making a video showing how Sgr A* changes over time.
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