Cosmic Space

See our galaxy’s black hole — and hear what’s next

After years of observation and weeks of rumor-mill rumblings, astronomers today unveiled their first image of the supermassive black hole at the center of our own Milky Way galaxy, Sagittarius A*.

Technically, the picture from the Event Horizon Telescope project doesn’t show light from the black hole itself. After all, a black hole is a gravitational singularity so dense that nothing, not even light, can escape its grip. Rather, the picture shows the “shadow” of a black hole, surrounded by the superheated, glowing gas that surrounds it.

And technically, the picture may not match what folks might see with their own eyes up close. Rather, the readings come from eight observatories around the world that combined their observations in radio wavelengths.

Nevertheless, the new view of Sagittarius A*, or Sgr A* for short (pronounced “sadge-ay-star”), serves to confirm in graphic terms what astronomers have long suspected: that our galaxy, like many others, has a supermassive black hole at its heart.

Today’s revelations follow up on the Event Horizon Telescope’s first-ever black hole image, which was released in 2019 and showed the supermassive black hole at the center of M87, an elliptical galaxy about 55 million light-years away.

Sgr A* is much closer — a mere 27,000 light-years from Earth, in the constellation Sagittarius. But there’s nothing to fear from this black hole: It’s relatively quiescent, in contrast to the galaxy-gobbling behemoths that are standard science-fiction fare.

Our galaxy’s black hole is thought to hold the mass of 4 million suns within an area that’s roughly as big around as Mercury’s orbit. Checking those dimensions against the image data serves as a test of relativity theory. Spoiler alert: Albert Einstein was right … again.

“We were stunned by how well the size of the ring agreed with predictions from Einstein’s theory of general relativity,” EHT project scientist Geoffrey Bower said in a news release. “These unprecedented observations have greatly improved our understanding of what happens at the very center of our galaxy and offer new insights on how these giant black holes interact with their surroundings.”

The EHT’s findings about Sgr A* are the subject of a special issue of The Astrophysical Journal Letters — and to whet your appetite for all that reading material, here are three videos that summarize the past, present and future of black hole imaging:

Sgr A* and M87’s black hole led the list of targets for the Event Horizon Telescope collaboration, which has more than 300 scientists on the team. Strangely enough, both black holes have roughly the same apparent angular size, but it was much harder for the EHT team to get a fix on the gassy disk surrounding our own galaxy’s black hole.

“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*,” said Chi-kwan (CK) Chan, an astronomer at the Steward Observatory and the University of Arizona’s Data Science Institute. “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.”

Astronomers had to use complex algorithms to process several quadrillion bytes of data sent in from far-flung radio observatories in order to come up with the images. The challenge has been compared to getting a picture of a doughnut sitting on the moon, or imaging a grain of salt in New York using a camera in Los Angeles. That explains why these pictures of black holes look so fuzzy.

So what’s next? It’s theoretically possible to sharpen up the pictures, but that will require more telescopes, more data and more processing power. Kari Haworth, chief technology officer for the Harvard-Smithsonian Center for Astrophysics, said the next-generation EHT project aims to quadruple the pace of data analysis.

“It’s a mind-boggling amount of data,” Haworth said in a video.

Haworth said the next-generation plan also calls for doubling the number of radio telescopes, and using satellite networks (presumably including SpaceX’s Starlink constellation) to speed up data transfer from each of the sites.

“One of the problems in the past was, the South Pole takes six to nine months to get data out … because you have to go in at the right time,” she explained. “If we could transfer that data off, then the scientists could get the data a lot more quickly, and we could get our results a lot more quickly.”

The eventual goal is to move beyond mere snapshots of black holes.

“You want to see if you can start making movies,” said the EHT’s founding director, Shep Doeleman of the Center for Astrophysics. “Can we capture the dynamics of the gas orbiting the black hole? The lensing of the light around the black hole is one way to test Einstein’s theory, but looking at the motion of matter as it orbits the black hole — that’s an entirely new dimension of the problem.”

By Alan Boyle

Mastermind of Cosmic Log, contributor to GeekWire and Universe Today, author of "The Case for Pluto: How a Little Planet Made a Big Difference," past president of the Council for the Advancement of Science Writing.

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