In a worldwide press conference this morning, the Event Horizon Telescope Collaboration revealed the first direct images ever taken of Sagittarius A*, the supermassive black hole at the center of the Milky Way. This exciting first comes two years after the same collaboration of scientists released the first image ever taken of a black hole.
Nothing escapes a black hole, not even light. The only way we can see a black hole is by the luminous remnant streaks of whatever hapless stars it’s devouring. But there’s so much obscuring gas, dust and debris between us and the galactic center that only the longest wavelengths of infrared light can make it through to us. Nevertheless, our long-wave radio telescopes were able to capture this first image of Sagittarius A*.
SA* is about 27,000 light years from Earth. It has four million times the mass of our Sun, but it only takes up a tiny spot in the sky: about the same size as a donut on the surface of the Moon. However, radio telescopes around the globe have combined forces to create the Event Horizon Telescope (EHT). The EHT triangulates between radio arrays to create one giant radio telescope, the size of the Earth. This affords unrivaled angular resolution.
Journey to the Center of the Galaxy
Eight different radio telescopes contributed to these observations. The telescopes in the image below are part of the Atacama Large Millimeter/submillimeter Array (ALMA), which is part of the EHT collaboration. From Earth, Sagittarius A* (abbreviated Sgr A*, pronounced “sadge-ay-star”) lies within the ecliptic. Inset, you can see where Sgr A* sits in the Milky Way, along the backbone of night.
Scientists from the EHT Collaboration also created a fly-through video that starts at ALMA and zooms in the whole way to Sgr A*. The video begins in the visible spectrum, shifting into the infrared as it approaches the black hole’s event horizon. Be warned: The effect can be a bit dizzying.
Videos like this are made from hundreds of thousands of different snapshots of the night sky. Astronomers then composite the images into a 3D representation of what a traveler from Earth would see as they traveled toward Sgr A*.
Imaging a Black Hole
To capture these images of Sgr A*, EHT scientists used a technique called very-long-baseline interferometry, or VLBI. VLBI compares the timestamps of readings it receives, and uses the minuscule differences in timing to make precise measurements of distant objects in the deep sky. It’s a bit like parallax, but in a temporal sense; we’re trying to find the distance between objects so distant they don’t move in the sky. So, instead of tracking their movement, we use radio telescopes to track the times when those distant signals arrive.
This process generates terabytes of data every day. The EHT cleans and calibrates the data, and then stores it on helium-filled hard drives, which they fly to the MIT Haystack Observatory, or to the Max Planck Institute for Radio Astronomy. There, they do their analysis using supercomputers called “correlators,” which cluster and average all the readings.
The black hole itself presents a real challenge to astronomers trying to get a clear snapshot. The scientists needed a long exposure, of “perhaps eight to ten hours,” to complete their portrait of Sgr A*. But accreting gas and dust are whipping around the black hole at near-lightspeed, in an orbit that takes mere minutes to complete.
EHT scientists believe that Sgr A* is currently ingesting two or more itinerant stars. But the accretion rate is uneven. This may account for the fact that we see “blobs” of light, instead of an even, symmetrical halo. When they combined their thousands of frames into video, the blobs of light resolved into streaks of incandescent star-stuff.
Spirit in the Sky
The EHT network itself continues to expand. Eight observatories participated in the making of this first image of Sagittarius A*. However, as of March 2022, three more observatories have joined the effort. Technological upgrades are ongoing.
Meanwhile: When the EHT Collaboration released these first images of Sagittarius A*, they also open-sourced their entire project. Michael Janssen, an EHT spokesperson, explained that the data the team used to construct these images is “fully public, on multiple levels.” Janssen added that the EHT Collaboration released their raw data, along with their algorithms and their cleaned-up data set, “so anyone can reproduce what we did, from scratch.”
This achievement follows the EHT Collaboration’s 2019 release of the first image ever taken of a black hole, called M87* for where it sits in its own galaxy, Messier 87. M87* is a thousand times the size of Sgr A*, and a thousand times more massive. But the two black holes bear a striking resemblance. EHT scientists mean to find out why.
“Now we can study the differences between these two supermassive black holes to gain valuable new clues about how this important process works,” said EHT scientist Keiichi Asada. “We have images for two black holes — one at the large end and one at the small end of supermassive black holes in the Universe — so we can go a lot further in testing how gravity behaves in these extreme environments than ever before.”
The EHT team’s results are published today in a special issue of The Astrophysical Journal Letters.