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Astro Bob: Doughnut of the deeps! First peeps of Milky Way's black hole

Astronomers release the first photos of the monster, gyrating black hole at the heart of the galaxy.

Milky Way black hole
This is the first image of the supermassive black hole at the center of our galaxy called Sagittarius A*. It was captured by the Event Horizon Telescope, an array of eight radio observatories across the planet linked together to form a single “Earth-sized” virtual telescope. Taken in the light of radio waves, it shows glowing gas orbiting around the central black hole four million times more massive than the sun.
Contributed / EHT Collarboration
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On Thursday, May 12, astronomers unveiled the first image of the supermassive black hole at the center of our own galaxy. The image was the work of 300 researchers from 80 different institutes who worked for more than five years with the Event Horizon Telescope , a worldwide network of eight radio telescopes linked together to create a single "virtual" instrument the size of the Earth. The photo reminds me of looking down a well — a very deep well where imagination has no limit.

Comparison of the sizes of two black holes: M87* and Sagittarius
The galaxy M87 in Virgo was the first galaxy to have its central black hole, known as M87*, photographed back in 2019. These photos compare the sizes of M87* and SgrA* using the sun, the orbits of Mercury and Pluto and the location of the Voyager 1 space probe for scale. M87* lies 55 million light-years away and has one of the largest black holes known — 1,000 times more massive than Sgr A*. The actual diameter of the Milky Way's black hole is 14.6 million miles (23.5 million km), but when you add the glowing halo of hot gas, it would fill the orbit of Mercury, a span of about 80 million miles.
Contributed / EHT collaboration, NSF

We've long known of the Milky Way's bulky black hole because astronomers have tracked stars orbiting a massive, invisible object at the galaxy's center. Based on their orbital tracks, something with a mass four million times greater than the sun holds them in place, suggesting that the object — called Sagittarius A* or simply "sadge-ay-star" — could only be a black hole.

Today's photos provide the first direct visual evidence of the object. It was no small achievement: seeing this unholy doughnut is equivalent to spotting an actual doughnut on the moon.

3032288+Black hole rotating ESO_FEA.jpg
In this model, a ring of hot material — gas or a ripped-apart planet or star — is heated as it swirls around a black hole. The material radiates light before disappearing forever over the edge of the hole, called the event horizon. Event horizon is also the name of the telescope array used to image the Milky Way's central black hole.

Remarkably, it looks very much like the model of a black hole astronomers have derived based on observation and good, old physics, with a central "shadow" surrounded by a glowing ring of light. The light represents radiation released from material heated to trillions of degrees as it spirals down the hole on a one-way ticket to oblivion. Thanks, Einstein! His Theory of General Relatively predicted the existence of these bizarre objects long before their discovery.

Montage of the Event Horizon Telescope observatories (day)
This montage depicts the radio observatories that form the Event Horizon Telescope network used to image the Milky Way’s central black hole, Sagittarius A*. These include the Atacama Large Millimeter/submillimeter Array, the Atacama Pathfinder Experiment, IRAM 30-meter telescope, James Clark Maxwell Telescope, Large Millimeter Telescope, Submillimeter Array, Submillimeter Telescope and South Pole Telescope. The slightly transparent telescopes in the background represent the three telescopes added to the EHT network after 2018: the Greenland Telescope, the Northern Extended Millimeter Array, and the ARO 12-meter Telescope at Kitt Peak.
Contributed / ESO, M. Kornmesser

The gravitational pull of a black hole is so tenacious that nothing, not even light, can escape its pull, the reason it appears black. Most black holes form in the aftermath of a supernova explosion, when the core of a failing supergiant implodes, compressing itself into an object so dense it literally disappears from sight.

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Single black holes are understandable. But astronomers still don't know exactly how the supermassive variety forms, and we detect them in the cores of almost every galaxy we see. One path might be through collisions of stars in compact star clusters that would lead to the formation of multiple smaller black holes. Over time, these would drift to the center of the galaxy and merge to form a single supermassive black hole.

Milky Way montage
This image shows the Atacama Large Millimeter/submillimeter Array (ALMA) looking up at the Milky Way as well as the first image of Sagittarius A*, the supermassive black hole at our galactic center. Located in the Atacama Desert in Chile, ALMA is the most sensitive of all the observatories in the EHT array.
Contributed / José Francisco Salgado PhD, ESO, EHT Collaboration


Sgr A* sits at the very center of the Milky Way 27,000 light-years away, but it's invisible in regular optical telescopes because it's hidden behind clouds of light-blocking cosmic dust. Telescopes that can see in other "colors" of light such as infrared and radio light render it visible.

Milky Way black hole Sgr A 2022 image variety.jpg
The Event Horizon Telescope Collaboration created a single image (top frame) of the Sgr A* by combining images extracted from many observations (below). The appearance of the black hole changes every five minutes or so because it fluctuates rapidly.
Contributed / EHT Collaboration

The EHT achievement follows the collaboration’s 2019 release of the first image of a black hole, called M87* (M87 star), at the center of the more distant Messier 87 galaxy 54 million light-years away. The Milky Way's black hole was much more difficult to pin down:

“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* were 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 scientist Chi-kwan (‘CK’) Chan.

What's up next? Upgrades and expansion of the telescope network are planned that will allow scientists to share even clearer images as well as movies of black holes in the near future. Movies? I can't wait!

Related Topics: SCIENCE AND NATURE
"Astro" Bob King is a freelance writer and retired photographer for the Duluth News Tribune.
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