We now have an image of the Milky Way’s central black hole — here’s how it happened

Last week, the planet-wide Event Horizon Telescope revealed a fresh view of the supermassive black hole in the center of our galaxy. The historic first image of Sagittarius A* (Sgr A*) showcased its shape and activity in submillimeter waves, based on 3.5 petabytes of data from several telescopes.

Imaging a black hole like this is extremely challenging, requiring astronomers to pinpoint a small target in the sky while dealing with amounts of data so vast that observatory personnel need to ship hard drives to other facilities for analysis. So how did the EHT get the job done?

Event Horizon telescope technical details

In 2022, EHT a scattering of 11 radio telescope facilities located around the world using a technique called very long baseline interferometry. The goal is to have these various observatories working together to create a single virtual mirror powerful enough to image a distant black hole.

The EHT Collaboration undertook the just-released Sgr A* campaign in 2017, with fewer observatories, but it took a while to process the data. A March 2022 campaign using all 11 telescopes observed a variety of targets, including Sgr A*, but the results are still processing.

“We record the radio signals captured in each of these telescopes at the same time, and then we computationally form a mirror by bringing the data together to a central location and combining all the data,” Lindy Blackburn, a member of the EHT collaboration and astrophysicist at the Center for Astrophysics | Harvard & Smithsonian, tells Inverse.

The data must be precisely timestamped, which astronomers do using atomic timing. Each participating telescope must send out a microwave laser (or maser) beam at hydrogen gas, which, as the most basic element, is abundant in the sky. Because hydrogen atoms have a known frequency, astronomers can chart the wobble to calculate the time the laser was fired. Masers are quite stable, only losing a single second every 100 million years.

Blackburn clarified it’s not impossible to have the observatories working together simultaneously. However, it’s easier to ship the hard drives to the MIT Haystack Observatory and the Max Planck Institute for Radio Astronomy because of the volume of data taken from remote observatories.

“Then when we bring them [the datasets] together, we're sort of freezing the light at these telescopes,” Blackburn says. “We bring it together, and then we play the data back digitally, on the same hard disks, and then we combine them in software.”

What are the challenges of imaging a black hole?

While operating simultaneously at several observatories during a global pandemic is enough of a challenge, the technical problems of trying to image a black hole are almost as massive as the target itself.

“We're pushing the extremes of what can be done from the ground, as far as radio frequencies,” Blackburn says.

The observing frequency is around one millimeter, he says. That, unfortunately, shares a similar frequency to water vapor, which can be abundant in Earth’s atmosphere. If there is too much water vapor, EHT’s observations will experience interference.

“A big challenge is just running when the weather is good enough at all of our sites to be able to actually see the source and take data,” Blackburn says. “So there's a large coordination effort to try to find the night when the weather's pretty decent, and we have a good shot at running the campaign.”

But once the telescopes have good weather, their equivalent resolution is several times better than what NASA’s sharp-eyed James Webb Space Telescope can see from deep space. The challenge, however, is far-off black holes are very small sources. For instance, Sgr A* is around the size of the distance radius of Mercury’s orbit from the Sun, viewed from 25,000 light years away.

“It’s the sharpest image ever made in this industry,” Blackburn says of the Sgr A* photos. “We do hope to sharpen the image a little bit in the future. We're going to move to higher frequencies next year.”

Further in the future, perhaps within the next two decades, Blackburn says there are visions to extend the EHT further by adding more observatories and distance. Some people have even thought about putting a network of radio telescopes in space to get a better view, although such a vision may be further in the future.

The ultimate goal, Blackburn says, is to “get longer baselines, and that bigger virtual mirror, until we can correspondingly see sharper and sharper images.”

Why is the Sgr A* image important?

Sgr A* is a highly variable black hole, changing frequencies every half-hour or so, and is relatively quiet in terms of activity. EHT got around these issues by performing “snapshot imaging” of the target, or having all the observatories instantaneously take an image simultaneously. Blackburn says if the EHT collaboration could double the number of dishes on the ground in future campaigns, this would allow them to track the dynamics of Sgr A* even more closely.

Blackburn says both EHT and the Laser Interferometer Gravitational-Wave Observatory (LIGO), which tracks gravitational waves from big cosmic events like black hole collisions, have been fundamental in charting the features of black holes.

“So far, we haven't seen anything that has been contrary to what's expected from general relativity,” he says, citing Einstein’s work on how space and time behave. He says the implications of better understanding black holes extend to cosmology to mapping out galactic evolution, as most large galaxies have supermassive black holes like Sgr A*.

Blackburn says his team is focused on tasks like verifying the simulations, which aim to chart the accretion of dust and gas around black holes as the matter spirals into the center. “The EHT is a very great way to try to make sure that our hydrodynamical simulations, which are done on supercomputers, [are verified to] see to what extent those are accurate, believable, and extendable.”