WASHINGTON (WHTM) — At the National Science Foundation in Washington, D.C., scientists from the Event Horizon Telescope project made a long-anticipated announcement. Feryl Özel of the University of Arizona broke the news: “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, Sagitarrius A*.”
Sagittarius A*, or Sgr A* (sadge-ay-star) for short, looks from Earth like a not particularly bright star in the constellation Sagittarius. It is, in fact, our very own black hole, around which all the Milky Way galaxy revolves.
The image was created using the Event Horizon Telescope, an interlinked collection of eight radio telescopes. Positioned across the globe, they function as a virtual super telescope the size of a planet. It generated a lot of data.
“When we observed Sagittarius A*,” said Vincent Fish of MIT Haystack Obesrvatory, “eight telescopes collected around 3 1/2 petabytes of data. That’s equivalent to about 100 million TikTok videos.”
Trying to stream that much information would have broken the internet. The teams had to transport hard drives to centers where the data from the telescopes was combined, calibrated, and converted into images.
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The EHT team compared the data to over five million computer simulations to find out which were the closest matches to the images. The work involved over 300 researchers from 80 institutions.
In 2019, the Event Horizon Telescope also created the first picture of a black hole ever taken, which is located in the M87 galaxy. While the two images look similar, the black holes are very different. The M87 black hole has the mass of 6.5 billion suns. The Milky Way black hole has a mere 4.3 million.
According Michael Johnson of the Center for Astrophysics, Harvard/Smithsonian, “While M87 had one of the biggest black holes in the universe, and launches a jet that pierces its entire galaxy, Sgr A* is giving us a view into much more standard shape of black holes. M87 was exciting because it was extraordinary. Srg A* is exciting because it is common.”
Because it is so much smaller than M87, gas circles it in mere minutes, compared to days at M87. It looks like a pretty violent process. So why are they calling it a gentle giant? Well, for one thing, it’s not eating very well.
“We see that only a trickle of material is making it to the black hole,” says Michael Johnson. “If Sgr A* were a person, it would consume a single grain of rice every million years. And while some black holes can be remarkably efficient at turning gravitational energy into light, Srg A* traps nearly all of this energy. Only one part in 1000 is converted into light. So despite looking so bright on these simulated images, the black hole is ravenous, but inefficient. It’s only putting out a few hundred times as much energy than the sun, despite being four million times as massive.”
Feryl Özel noted that the black hole was not as volatile as many had expected. “And what Srg A* showed us, was that our predictions for how much it would vary was actually too much. So it turned out to be a gentler, more cooperative black hole than we had hoped for, in the past decade of simulating its environment. So we love our black hole.”