Astronomers capture 1st image of Milky Way’s huge black hole

At the center of the Milky Way is a massive, mysterious presence that is having a powerful impact on the stars around it – and on the imaginations of astronomers.

Now scientists have the first-ever photo of the massive force at the center of our galaxy: Sagittarius A*, a supermassive black hole with the mass of 4 million suns.

The image revealed on Thursday was captured by a network of eight radio observatories in six locations around the world. Together they form the practical equivalent of an Earth-sized telescope designed to see some of the most mysterious and bewildering objects in the universe.

Photographing a black hole is a unique feat because its defining feature is that nothing within its gravitational domain can escape — including light.

But astronomers can see the annular boundary known as the event horizon, and beyond it the golden, razor-thin ring of superheated gas and bent light that surrounds the edge of the black hole’s point of no return.

“What’s better than seeing the black hole at the center of our own Milky Way galaxy?” said Katie Bouman, a Caltech professor of computational imaging and a member of the international telescope team.

The results were published in the Astrophysical Journal Letters on Thursday.

Black holes are the densest objects in the universe. When a giant star explodes in a final, dramatic supernova, its collapse creates a tiny blob of matter so dense that its gravitational pull distorts the fabric of space and time around it.

Scientists have long suspected that supermassive black holes lie at the center of every galaxy, including our own. Yet despite their colossal size, they are an elusive presence in the universe, observable only through their impact on the objects around them.

Capturing an image of an object from which no light can escape is the monumental challenge faced by the Event Horizon Telescope consortium in 2009. The effort involves the collaborative work of more than 300 scientists and engineers at 80 institutions worldwide.

It took a decade to create the first photo of a black hole at the center of the galaxy Messier 87, some 55 million light-years away (the black hole is also known as M87*). Its event horizon is nearly 25 billion miles across, with a mass of about 6.5 billion suns.

Although Sagittarius A* — pronounced “Sagittarius A-star” and known as Sgr A* for short — is only 27,000 light-years from Earth, it has less than 0.1% the mass of M87*. If it hadn’t been conveniently located in our own galaxy, it would have been nearly impossible to photograph. Bouman compared it to standing in Los Angeles and photographing a grain of salt in New York.

“It’s a gentler, more cooperative black hole than we had hoped,” said Feryal Ozel, a University of Arizona astronomer and founding member of the telescope consortium. “We love our black hole.”

In fact, the images provide the strongest evidence yet for Einstein’s general theory of relativity. At Sgr A* in particular, the size and shape of the ring surrounding the event horizon is remarkably consistent with what scientists have predicted based on Einstein’s theory.

“They are so different in so many ways, and yet the same theory of gravity actually explains the shape of both images,” Bouman said. “And that’s a great result. It’s actually very exciting that they look a lot alike.”

The supermassive black holes at the center of the galaxy Messier 87, left, and the Milky Way.

The supermassive black hole on the left is at the center of the galaxy Messier 87. The one on the right is at the center of our Milky Way.

(EHT collaboration)

A popular classroom model of a black hole offers a useful way to visualize this cosmic phenomenon. Think of the fabric of space-time as a sheet of plastic pulled tight, and the Earth as a tennis ball falling in its midst. The ball creates a slight curvature in the film, just like our relatively humble planet creates space-time. However, a steel ball bends the foil much further. If the ball is heavy enough, the foil will sag so much that all other objects will inevitably roll down on the heaviest. That’s what black holes do with time and space.

“Black holes aren’t the big cosmic vacuum cleaners that Hollywood likes to make them out to be,” Bouman said.

The smaller and less efficient Sgr A* is likely a better representative of the typical black hole in the Universe than the ultramassive M87*, Bouman said.

UCLA astronomer Andrea Ghez was awarded the 2020 Nobel Prize for the discovery of Sgr A*. The image produced by the EHT was “remarkably similar” to the supermassive black hole she and her colleagues suspected was at the center of this galaxy.

“There’s a prediction that you should see this concentration of light around the black hole, just outside the event horizon, and that you can actually see this is remarkable,” Ghez said.

Photographing a black hole with a single telescope would have required a lens
13 million meters wide – in other words, an Earth-sized telescope.

The South Pole Telescope is located at the National Science Foundation's Amundsen-Scott South Pole Station in Antarctica.

The South Pole Telescope at the National Science Foundation’s Amundsen-Scott South Pole Station in Antarctica is the most extreme location of the eight telescopes in the Event Horizon Telescope Array.

(Junhan Kim / University of Arizona)

Instead of this logistical impossibility, the Event Horizon Telescope collects data on eight radio observatories in Greenland, Antarctica and six locations in between, synchronized with atomic clocks. As the Earth spins, observatories view their target from a variety of angles.

Sgr A*’s glamor shot was distilled from 5 petabytes of data, the equivalent of 100 million TikToks, said EHT member Vincent Fish of the MIT Haystack Observatory. The published image is an average of several images pulled from this data.

The EHT collaboration used ray tracing to create a variety of possible images of Sagittarius A*.

The EHT collaboration created a series of images of Sagittarius A* and then computed them into a single image.

(Ben Prather / EHT Theory Working Group; Chi-Kwan Chan)

Just two decades ago, “I would have thought we would never see images like this. That would be too difficult,” said Daniel Stern, an astrophysicist who studies black holes at NASA’s Jet Propulsion Laboratory in La Cañada Flintridge.

“It looked better than I expected,” he said. “This is consistent with decades-old theories of how we imagined black holes.”

Because this black hole is so much smaller, the ring around it appears much busier. Gases that take weeks to orbit M87* can orbit Sgr A* in just a few minutes. Given the rapid changes in emissions, it’s possible the telescope will be able to capture moving images of activity around the event horizon for years to come, Bouman said — potentially in multiple dimensions.

“What if we could actually map where the gas is in three dimensions around the black hole over time?” said Buman. “That’s one thing I’m really looking forward to.”

https://www.latimes.com/science/story/2022-05-12/astronomers-capture-first-image-milky-way-galaxy-black-hole Astronomers capture 1st image of Milky Way’s huge black hole

Russell Falcon

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