• Astronomers unveiled the first image of the supermassive black hole, called Sagittarius A* or Sgr A*, lurking at the centre of our own Milky Way galaxy which is four million times more massive than the Sun, terming it a “gentle giant”.

• The finding, published in The Astrophysical Journal Letters, greatly improves our understanding of what happens at the very centre of our galaxy and offers new insights into how these giant black holes interact with their surroundings.

• The image shows a fuzzy glowing ring of red, yellow, and white surrounding a darker center. The picture of Sagittarius A* —  only the second one ever to be imaged — was produced by a global research team called the Event Horizon Telescope (EHT) Collaboration, using observations from a worldwide network of radio telescopes.

• The breakthrough follows the EHT collaboration’s 2019 release of the first image of a black hole, called M87*, at the centre of the more distant Messier 87 galaxy.

• The two black holes look remarkably similar, even though our galaxy’s black hole is more than a thousand times smaller and less massive than M87*.

What is a black hole?

• A black hole is born when a large star collapses in on itself. Far from being a “hole”, they are instead incredibly dense objects with a gravitational pull so strong that nothing, not even light, may escape them. 

• As they suck in matter such as gas, dust and space debris, they form an accretion disk — a churning mass of super-accelerated particles that are among the brightest objects in the Universe — around them.

• Scientists generally believe that there are two types of black holes. The more common stellar black holes — up to 20 times more massive than the Sun — form when the centre of a very big star collapses in on itself.

• Supermassive black holes are at least a million times bigger than the Sun and their origins are uncertain.

How was the image captured?

• Because the black hole is about 27,000 light-years away from Earth, it appears to us to have about the same size in the sky as a donut on the Moon. To image it, the team created the powerful EHT, which linked together eight existing radio observatories across the planet to form a single “Earth-sized” virtual telescope.

• The EHT observed Sgr A* on multiple nights, collecting data for many hours in a row, similar to using a long exposure time on a camera.

• The gas in the vicinity of the black holes moves at the same speed — nearly as fast as light — 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 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.

• The researchers had to develop sophisticated new tools that accounted for the gas movement around Sgr A*. While M87* was an easier, steadier target, 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 images the team extracted, finally revealing the giant lurking at the centre of our galaxy for the first time.

• Although we cannot see the black hole itself, because it is completely dark, glowing gas around it reveals a telltale signature: a dark central region (called a “shadow”) surrounded by a bright ring-like structure. The new view captures light bent by the powerful gravity of the black hole, which is four million times more massive than our Sun.

• 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 supercomputers to combine and analyse their data, all while compiling an unprecedented library of simulated black holes to compare with the observations.

• In addition to other facilities, the EHT network of radio observatories includes the Atacama Large Millimeter/submillimeter Array (ALMA) and the Atacama Pathfinder EXperiment (APEX) in the Atacama Desert in Chile.

• Scientists are particularly excited to finally have images of two black holes of very different sizes, which offers the opportunity to understand how they compare and contrast. 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 understood but is thought to play a key role in shaping the formation and evolution of galaxies.

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