It's a first for our galaxy-we now have a picture of the black hole that exists at the center of the Milky Way. The revelatory image is of Sagittarius A*, or Sgr A*, a black hole with a mass four million times that of the sun and located about 27,000 light-years away.
"The four-million solar-mass 'monster' at the center of our galaxy is indeed a black hole," says Alan Marscher, a BU College of Arts & Sciences professor of astronomy who is one of over 300 contributors to the breakthrough. The image was produced by a global research team called Event Horizon Telescope (EHT) Collaboration that used observations from a worldwide network of radio telescopes. The findings were published in a special issue of the Astrophysical Journal Letters.
Marscher and Svetlana Jorstad, a BU senior research scientist and a contributor to the paper, were part of the team that worked on the first attempts at imaging Sgr A*, and were both heavily involved in the first-ever images captured of a black hole in Messier 87 (M87), a galaxy about 55 million light-years from Earth. Both sets of images give us a glimpse of one of the most elusive cosmic objects in the universe-once considered undetectable against the dark backdrop of space.
Black holes have so much mass that their gravitational forces absorb all light particles. The Sgr A* picture shows a ring of orange-yellow light surrounding a blob of darkness, which is called a "shadow." The ring of light is caused by particles heating up as they fall into the black hole, creating gas so hot it gives off a bright light. Astronomers have known about Sgr A* since its discovery in 1974, but the image now confirms its existence.
We finally have the first look at our Milky Way black hole, Sagittarius A*. It's the dawn of a new era of black hole physics. Credit: EHT Collaboration. #OurBlackHole #SgrABlackHole
Link: https://t.co/Ax7ECRVg8A pic.twitter.com/LRWizSYOy9
- Event Horizon 'Scope (@ehtelescope) May 12, 2022
Marscher and Jorstad say capturing an image of our own neighborhood black hole was more challenging than that in M87. The team of scientists had to develop new tools that accounted for gas movement around Sgr A*, according to the EHT Collaboration statement, while the black hole in M87 (known as M87*) was an easier, steadier target. Even though Sgr A* is closer to Earth, our line of sight is obscured by plasma that scatters radio waves coming from the region around the black hole, a challenge less pronounced for M87*, says Jorstad. Sgr A* is also relatively small compared to M87*, meaning the bright clouds of plasma and gas circling it complete their orbit in mere minutes, so the brightness and pattern constantly change.
A supermassive black hole lies at the center of all the major galaxies in the universe, and many of the smaller ones, Marscher says. Astrophysicists have been mathematically certain, based on Albert Einstein's theory of general relativity, that the presence of these cosmic magnets-exerting extreme gravitational pull from the center of galaxies-explains the swirl and trajectory of stars, planetary bodies, and space dust.
The EHT Collaboration analyzed data collected from multiple locations, essentially creating a planet-size telescope. The initial analysis that Jorstad and Marscher were involved with was then used by other researchers for a systematic image survey, which looked at the effects of the black hole's changing parameters like size, shape, and varying brightness features. The survey eventually resulted in the picture we see.
"The fact that these two black holes of vastly different [solar] masses-6.5 billion for M87* and 4 million for Sgr A*-look the same is actually a really exciting finding," says Jorstad. "It is another confirmation of Einstein's theory of general relativity that predicts that all black holes look and behave the same, despite their mass."
Another wild feature of black holes is that some of them release powerful jets of particles as they devour them. It's known that M87* ejects a jet that extends thousands of light-years, and while scientists still don't know if Sgr A* launches a jet, theoretical models predict it should. Neither image of the black holes captured the jets, but in the future that could be possible, says Jorstad. Plans for the expanding EHT could even give scientists the opportunity to make movies of black holes, allowing us to see them gobbling up clumps of matter-or even stars.
