A portrait of the black hole at the heart of the Milky Way

Three pandemic-dominated years after sharing a historic image of a black hole , the Event Horizon Telescope (EHT) collaboration unveiled today the portrait of its second subject: Sagittarius A*, the 4-million-solar-mass black hole at the center of our Milky Way. A bright ring of plasma circles at the photon orbit radius. Any inward-moving photon that is located in the region depicted by the image’s shadowy center eventually crosses the point of no return, known as the event horizon. The results were announced at several press conferences around the world, and they appear in a series of papers in Astrophysical Journal Letters.

If you think the blurry, toroidal figure looks familiar, that’s a good thing. Despite being three orders of magnitude closer than the galaxy Messier 87’s (M87’s) central black hole, which the EHT imaged in 2019, Sagittarius A* is also three orders less massive, so the angular sizes of the two black holes are very similar. And according to general relativity, the size of the shadow is determined only by the radius of the event horizon, which in turn is directly proportional to the black hole’s mass (see the Quick Study by Dimitrios Psaltis and Feryal Özel, Physics Today, April 2018, page 70 ). Spacetime “warps around black holes in exactly the same way, regardless of their mass or what surrounds them,” said University of Arizona astrophysicist and EHT founding member Feryal Özel at a press event in Washington, DC.

The images of both black holes are the culmination of analyzing data collected in April 2017 from eight millimeter-wavelength telescopes scattered all over the world. By time-stamping the measurements using atomic clocks installed at each observing site, researchers were able to combine the data using a technique called very-long-baseline interferometry to achieve resolution comparable to that of an Earth-size telescope. After calibrating the data and honing the imaging algorithms, the EHT team was able to turn the 2017 observations of M87* into four images resembling the final product by June 2018 (see “What it took to capture a black hole ,” Physics Today online, 11 April 2019). Obtaining a portrait of Sagittarius A* presented additional challenges—and required a bit of luck.

In many ways, M87* was the perfect first subject, says Dimitrios Psaltis, an astrophysicist at the University of Arizona and a founding member of the EHT collaboration. Though located a sizable 55 million light-years away, the M87 galaxy has an orientation almost perpendicular to our galactic plane, which provides a relatively gas- and dust-free line of sight to the black-hole target. In contrast, capturing the shadow of Sagittarius A* requires looking inward from our vantage point within the disk of the Milky Way, straight into the crowded galactic center about 27 000 light-years away. Fluctuations in the free-electron density of the plasma located along the line of sight introduce phase variations in the microwaves destined for detection by the EHT. The result is a blurring of the source image.

The EHT researchers knew they would have to contend with that blurring, which is a major reason they chose to make observations at 1.3 mm rather than at larger wavelengths, which would scatter even more drastically. To further remove the effects of gas and dust, the team developed a model of the blurring effect and made separate observations with the EHT observatories to quantify various model parameters. In the end, Psaltis says, the impact of blurring was less than the team had feared.

A second complicating factor is Sagittarius A*'s size. If M87* were placed at the center of our solar system, its event horizon would be located somewhere near the Voyager 1 probe in interstellar space; Sagittarius A*'s wouldn’t make it to the orbit of Mercury. That means that the plasma whipping around the Milky Way’s central black hole can complete an orbit within minutes, compared with the days or weeks for the plasma circling M87*.

Part of what allows the EHT telescope network to achieve such high resolution is that it captures its subject from different angles as Earth rotates, much like a computed tomography machine scans from multiple positions. In both cases, the subject (or patient) is assumed to be static. “The algorithms assume the thing you’re taking a picture of is fixed,” Psaltis says. “If it takes 10 hours to take a picture, M87* won’t change much, but for Sagittarius A* the plasma has already circled 50 times.” He and his colleagues were concerned that the black hole’s environment would be so dynamic that a long-exposure observation would never be able to achieve adequate resolution—and no model would be able to cancel out the effects. Fortunately for the EHT team, again the blurring was less than feared. “It turned out to be a gentler, more cooperative black hole than we had hoped for,” Özel said at the press conference.

After taking a break following the April 2019 release of the M87* results, the more-than-300-member team was just ramping up its Sagittarius A* work when the pandemic hit. Face-to-face meetings that had been vital for the imaging and modeling work on M87* were replaced with Zoom meetings among collaborators across the globe. “This is a big reason why it took three years instead of eight months,” Psaltis says.

The new image complements the knowledge scientists have obtained in recent years about the Milky Way’s supermassive black hole. For example, 2020 Nobel laureates Andrea Ghez and Reinhard Genzel lead teams that have tracked the motion of stars orbiting perilously close to Sagittarius A* and have determined the black hole’s mass to a precision of about 1% (see Physics Today, December 2020, page 17 ). The measured angular size of the shadow in today’s image, 51.8 ± 2.3 μas, is consistent with what general relativity predicts for a black hole at that mass and distance. The image also reveals that Sagittarius A* is oriented roughly face-on as viewed from Earth, rather than being aligned with the galactic plane.

Editor’s note, 12 May: The article was updated with quotes from the Washington, DC, press conference and a description of the black hole’s orientation.