You’d think it would be hard to overlook an object with a mass four million times greater than the sun. But when that object is a supermassive black hole like Sagittarius A*, the giant object astronomers have long believed sits at the center of our galaxy, it is, by definition, impossible to see. Because black holes collapsed objects have such a strong gravitational pull that light cannot escape, it is impossible to see them.
All the same, this morning, at a National Press Club even in Washington, D.C., representatives of a team of 300 astronomers from 80 institutes around the world, released the first ever image of Sagittarius A*—or, more specifically, of the super-heated matter swirling around it and destined to be swallowed by it.
The Event Horizon Telescope (EHT) captured the image. This necklace of radio telescopes is located all around the world and works as one giant observatory. The black hole captured by the EHT, said University of Arizona astronomer Feryal Ozel during the unveiling, is “the gentle giant in the center of our galaxy.”
That “gentle” descriptor is true only to a point. Black holes, which are extremely violent objects that shred stars and take other material with them, can be destructive and even swallowed whole. Indeed, the EHT gets its name from the so-called event horizon surrounding a black hole—or, the point at which the gravity of the object grows so great that anything near it falls inside, never to be seen again. As Sheperd Dideleman, director of EHT, put it, black holes are capable of swallowing so many objects that their size can dwarf the object they contain. Post in an interview before the image was released: “Imagine sucking an elephant through a straw.”
Astronomers believe supermassive black holes like Sagittarius A* sit at the center of nearly all galaxies, providing a sort of gravitational and structural pivot point around which the galaxy slowly rotates. At the moment, it is unclear whether supermassive Black holes first appear and galaxies follow them around or if they form later and merge with the center of the galaxies. Either way, what keeps us safe from Sagittarius A*’s ravenous gravitational appetite is simple distance: the supermassive object sits a comfortable 27,000 light years from Earth.
Sagittarius A* gets its name from the spot in the constellation Sagittarius from which, in 1933, engineer Karl Jansky first detected a powerful radio signal streaming toward Earth. Astronomers started to believe that Jansky’s discovery of energy could have been caused by a black hole, as the science of dark holes developed over the years.
Supermassive black holes are, as their name suggests, vastly larger than ordinary black holes, which are the remains of smaller collapsed stars—those with a mass about 20 times that of our sun. These black holes are thought to number in the millions. This supermassive type is far less frequent.
Sagittarius A* is only the second supermassive black hole ever to be imaged. Messier 87 is the original supermassive dark hole that was imaged by EHT in 2019. It’s a huge beast. Located 53 million light years from Earth, it is estimated to be about 1,500 larger than Sagittarius A*.
That size differential is important to scientists since it’s long been assumed that supermassive black holes exist in a whole range of different masses. The belief raised questions about the consistency of the physics between the different sizes. This gives us the chance to see how they compare.
“Despite being 1,500 times smaller than M87*, the new images of Sagittarius A* look remarkably similar to those of M87*,” Avery Broderick, a professor at the University of Waterloo, who is a part of the EHT team, said in a statement. “Nobody knew for sure if enormous black holes and relatively smaller ones would share much in common, but now we have two unique black holes to compare, so we can better understand how black holes of different sizes eat, how they grow, and how they shape the galaxy around them.”
EHT will scan more of the sky to find supermassive black hole, which in turn will lead to a deeper understanding. Albert Einstein was the first to propose the gravity theories that led to the creation of black holes in the beginning of the 20th Century. It is for his scientific heirs in the 21st century to take the objects’ pictures—and unravel their secrets.
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