Astronomers unveil first image of the Milky Way’s central black hole


The Event Horizon Telescope collaboration unveiled the first image of the super massive black hole at the heart of the Milky Way, showing a ring of blazing, super-heated material just outside the shadow of its event horizon, the gravitational point of no return for anything falling in. Credit: Event Horizon Telescope Collaboration

Three years after capturing the first image of a supermassive black hole in a galaxy 55 million light years away, astronomers have managed to “photograph” the gaping maw of the smaller but much closer black hole quietly lurking at the core of the Milky Way, researchers announced Thursday.

“We are peering into a new environment, the curved spacetime near a supermassive black hole,” said Michael Johnson, a researcher at the Harvard-Smithsonian Center for Astrophysics. “And it is teeming with activity, always burbling with turbulent energy and occasionally erupting into bright flares of emission.”

The 2019 target was a mind boggling black hole at the core of M-87, a giant elliptical galaxy in the constellation Virgo, a hole with the mass of 6.5 billion suns. Its enormous gravity pulls surrounding material into a disc, accelerating it to nearly the speed of light and heating it to extreme temperatures, resulting in torrents of radiation that can be seen from Earth.

The black hole at the center of the Milky Way, known as Sagittarius A*, or Sgr A* for short (pronounced Sag A-star), is much closer, about 26,000 light years from Earth, but it is much smaller. The 6.5 billion solar masses making up the black hole in M-87 would fill the entire solar system. The 4 million solar masses of Sgr A* would fit inside the orbit of Mercury.

Now, after years of careful data collection using eight radio telescopes electronically combined and synchronized with atomic clocks to form a virtual dish the size of planet Earth, collaborators with the Event Horizon Telescope project unveiled the long-sought-after image of Sgr A*.

It was a feat roughly equivalent to a photographing a single grain of salt in New York City using a camera in Los Angeles.

Sgr A* has been the focus of “intense astronomical studies for decades,” said Feryal Özel, a theoretical astrophysicist at the University of Arizona and a leader of the EHT team. “Observations of stars orbiting around it revealed the presence of an object that is very massive, 4 million times the mass of our sun, but also very faint.

“Until now, we didn’t have the direct picture confirming that Sgr A* was indeed a black hole,” she said. “Today, the Event Horizon Telescope is delighted to share with you the first direct image of the gentle giant in the center of our galaxy.”

The image, based on multiple observations using a variety of algorithms to tease out subtle details, “shows a bright ring surrounding the darkness, the telltale sign of the shadow of the black hole,” Özel said.

“Light escaping from the hot gas swirling around the black hole appears to us as the bright ring. Light that is too close to the black hole, close enough to be swallowed by it, eventually crosses its horizon and leaves behind just the dark void in the center.”

By definition, black holes cannot be directly observed because nothing, not even light, can escape the crushing inward force of their titanic gravity.

But their presence can be indirectly detected by observing the effects of that gravity on the trajectories of nearby stars and by the radiation emitted across the electromagnetic spectrum by material heated to extreme temperatures as it’s sucked into a rapidly rotating “accretion disk” and then into the hole itself.

The motions of stars in the dust-shrouded core of the Milky Way near Sgr A* have been closely monitored for the past two decades, allowing astronomers to calculate the mass of the invisible body warping their trajectories.

The 2020 Nobel Prize went to three researchers whose pioneering observations and analysis all but confirmed the presence of a supermassive black hole. The Event Horizon Telescope captured the first actual image of the massive object.

That image shows Sgr A*’s dark central core — the shadow of its “event horizon” — surrounded by a lopsided ring of light emitted by particles racing around the hole at nearly the speed of light.

The event horizon is the invisible boundary between a black hole and the rest of the universe, a zone where nothing, not even light, can escape the hole’s gravitational clutches. Gas, dust, wayward stars and the light they emit, anything crossing that invisible line vanishes from the known universe.

The EHT image of Sgr A* is similar in appearance to the historic image of M-87’s huge black hole and closely resembles what astronomers expected based on computer simulations running the equations of Einstein’s general theory of relativity

While Sgr A* is the supermassive black hole in the center of our own galaxy, the supermassive black hole M87 resides more than 55,000,000 lightyears from Earth. Credit: National Science Foundation/Keyi “Onyx” Li


M-87’s black hole “is 1,500 times more massive, making its horizon 1,500 times larger,” Özel said. “But it is also 2,000 times farther away from us. This makes the two images appear very similar to us when we gaze at them in the sky. But the two black holes couldn’t have been (more) different from each other in practically every other way.

“The one in M-87 is accumulating matter at a significantly faster rate than than Sgr A*. But perhaps more importantly, the one in M-87 launches a powerful jet that extends as far as the edge of that galaxy. Our black hole does not. And yet, when we look at the heart of each black hole, we find a bright ring surrounding the black hole’s shadow. It seems that black holes like doughnuts,” she joked.

Johnson said “only a trickle of material is actually making it all the way to the black hole.”

“If Sgr A* were a person, it would consume a single grain of rice every million years,” he said. “And while some black holes can be remarkably efficient at converting gravitational energy into light, Sgr A* traps nearly all of this energy, only one part in 1,000 is converted into light.

“So despite looking so bright on the simulated images, the black hole is ravenous but inefficient. It’s only putting out a few hundred times as much energy as the sun despite being 4 million times as massive. The only reason we can study it at all is because it’s in our own galaxy.”

While M-87 features one of the most massive black holes in the known universe, Sgr A* “is giving us a view into the much more standard state of black holes, quiet and quiescent,” he said. “M-87 was exciting because it was the extraordinary. Sgr A* is exciting because it’s common.”

To “see” Sgr A*, the Event Horizon Telescope team used an array of eight radio telescopes in Hawaii, North, Central and South America, Europe and Antarctica.

Using a technique known as very long baseline interferometry, precisely timed data from each radio telescope can be combined to produce images comparable to what an Earth-size dish would detect. The resulting virtual telescope has the highest resolution of any instrument ever built, capable of detecting a doughnut on the moon.

Some 3.5 petebytes of data were collected, roughly the same amount as a million TikTok videos. Scores of hard drives then were physically shipped to researchers in Europe and the United States for super computer processing and analysis.

“Every once in a while, you just have to pinch yourself and you’re like, this is the black hole at the center of our galaxy!” said Katie Bouman, an assistant professor at Caltech and an EHT team member. “It’s pretty amazing … that we were actually able to do this.”

Stable stars live in a state of “hydrostatic equilibrium,” balancing the inward force of gravity with the outward push of radiation generation by fusion reactions in the core. In the sun, 600 million tons of hydrogen are fused into helium every second to produce the outward radiation pressure needed to offset gravity and maintain stability.

When smaller stars like the sun finally run out of nuclear fuel over billions of years, their cores collapse to a point where quantum forces, not fusion, maintain stability. Such dead, slowly cooling stars are known as white dwarfs.

When more massive stars run out of fuel, core collapse continues past the white dwarf stage.

For collapsing cores with up to three times the mass of the sun, the result is a neutron star, cramming more than twice the mass of Earth’s sun into a body less than 10 miles across. Neutron stars, propped up by a different sort of quantum force, are the densest objects in the visible universe.

For even more massive stars, a different fate awaits. Gravity overcomes all known nuclear forces and core collapse proceeds past the point where it vanishes from the visible universe, leaving behind nothing but an enormously concentrated “gravity well” of deeply distorted space.

Such remnants are known as stellar mass black holes because they are formed by the death of a single star.

A handful of larger intermediate-mass black holes have been found, possible stepping stones to the formation of the supermassive black holes now thought to exist in the cores of all major galaxies. But the details of how such larger holes form is not yet clear.

A major objective of the newly launched James Webb Space Telescope is to help astronomers chart the formation and growth of such black holes in the aftermath of the Big Bang.

“I wish I could tell you that second time is as good as the first one imaging black holes,” Özel said. “But that wouldn’t be true. It is actually better. Now we know that it wasn’t a coincidence, it wasn’t some aspect of the environments that happened to look like the ring that we expected to see.

“We now know that in both cases, what we see is the heart of the black hole, the point of no return. … Spacetime, the fabric of the universe, warps around black holes in exactly the same way, regardless of their mass or what surrounds them.”