The Milky Way’s Monster

Published in the September/October 2022 issue of SkyNews Magazine

Astronomers have known since the 1930s that something massive lurks at the centre of our galaxy. But now they have looked past the veil of dust at the Milky Way’s core and seen it.

The image of a supermassive black hole, known as Sagittarius A* — abbreviated to Sgr A* and pronounced “sadge-ay-star” — was revealed at six press conferences around the world simultaneously on May 12.

It shows a donut-shaped ring of superhot plasma orbiting a spherical void 27,000 light-years away. In fact, a press release from the team that captured the image compared the feat to photographing a donut on the Moon. The photo is the ultimate long-exposure shot made possible by more than 300 researchers at 80 institutions around the world.

This group includes Avery Broderick, a researcher at the University of Waterloo and a member of the Perimeter Institute for Theoretical Physics. Broderick is also a founding member of the Event Horizon Telescope (EHT), a network of radio telescopes that captured the image. The long-awaited picture is a triumph of global logistics and scientific collaboration. Broderick and his colleagues say the image is also part of an exciting time for astronomy across the world and in Canada.

“When it comes to generating images and pushing the envelope of what’s possible, Canadian groups have been pushing the envelope of what’s possible,” he said in an interview. “We’re very proud of being able to do that, despite being a much smaller country compared to many others.”

Overcoming Obstacles

The photo does not actually show the black hole itself, since it emits no light. What is pictured is the black hole’s silhouette against a storm of swirling gases encircling a point outside the event horizon.

Seeing such an extraordinary object required extraordinary engineering. Imaging Sagittarius A* began in April 2017, when eight radio telescopes — two each in Hawaii and Chile, and one each in Antarctica, Arizona, Mexico, and Spain — began scanning the region of space over several days. When they work together as the EHT, they become the highest-resolution imaging instrument in the history of astronomy. It’s a network Broderick and his colleagues like to call an “Earth-sized telescope.”

The Sagittarius A* project collected roughly five petabytes of data — too large to send over the internet. Instead, the information was stored on hundreds of hard drives and shipped to supercomputers at MIT’s Haystack Observatory and the Max Planck Institute for Radio Astronomy in Germany.

“For us, the fastest internet connection is still FedEx,” Broderick said.

This system was how the EHT produced the first image of a black hole, M87*, in 2019. That black hole was easier to study despite being 54 million light-years away. M87* is 1,500 times larger than Sagittarius A*. While both rings move close to the speed of light, their different sizes mean M87* moves slower and Sagittarius A* flickers.

Sagittarius A* is also shrouded behind walls of matter and debris in the densest part of our galaxy. It took years for supercomputers to sift through background noise like diffraction or dust. As a comparison, a golfer can see a building at the other side of the course but miss the ball hidden beneath the grass at their feet.

“It’s incredibly difficult to resolve,” Broderick said. “Bigger dishes can see smaller things. If you want to see smaller things like black holes... you really need an Earth-sized telescope, and that’s what the EHT fundamentally means.”

The Human Component

The black hole was first detected in 1934 when astronomer Karl Jansky found a radio signal coming from the constellation Sagittarius. The radio source was identified by astronomers Bruce Balick and Robert L. Brown in 1974, who concluded it was likely a black hole. During the next few decades, astronomers began eliminating other possible candidates for what the signal could be — such as superdense clusters of stars.

Starting in the 1990s, astronomers began gathering proof that the mysterious object at the galaxy’s centre could only be a black hole. Teams led by astronomers Andrea Ghez and Reinhard Genzel calculated that whatever was at the Milky Way’s core was more than four million times heavier than our Sun, confined within an area the size of Mercury’s orbit.

This conclusion was based on observations of the orbits and velocities of nearby stars. The only possible object that could exert such gravitational force, they argued, was a black hole. Their achievement earned them the Nobel Prize in Physics in 2020.

Broderick believes the general public has a solid grasp of what black holes are, even if popular culture still clings to some misconceptions — such as the idea that they are cosmic wormholes to alternate realities or other regions of space. But he says there’s a Lovecraftian element of cosmic horror that makes black holes fascinating to both scientists and non-scientists. After all, these are unseen objects deep in the void where gravity warps the very fabric of space and time.

“It’s extraordinary, slightly ominous. You kind of hope monsters don’t exist, but they do,” said Broderick.

“The key feature is a point of no return, this perfect prison ... There’s something philosophically powerful about this.

You can know what’s inside but you’ll never be able to tell anyone outside. You can take the trip, but that knowledge stays there. I think that’s a concept people viscerally understand.”

The Next Phase

The Event Horizon Telescope has begun a new chapter in the study of Sagittarius A* and black hole research. Astronomers can now use it to test theories of gravity and astrophysics — and there are still plenty of questions left to answer.

How did the stars orbiting Sagittarius A* form in such extreme conditions? Or did they form at all? How is the gas funnelling into the black hole reaching the galactic centre? Those are some of the questions that Avery Broderick continues to think about.

“The more observations we have, the better we can understand and see through the astrophysical drama down to the gravitational stage. We can begin to ask deep, precise questions about how gravity works in these most extreme environments and extreme objects,” he said. “Now that we know we can image the black hole, what other things can we extract from it?”

Among those seeking answers is Daryl Haggard, an astronomer at McGill University. Haggard and PhD candidate Hope Boyce ran parallel observations of the black hole at the same time the Event Horizon Telescope was collecting its own data. They used instruments that detect different wavelengths of light, assembling X-ray data from NASA’s Chandra X-ray Observatory, the Nuclear Spectroscopic Telescope Array (NuSTAR), and the Neil Gehrels Swift Observatory.

“I was not sure there would be a ring at first ... for a while I was not sure if we would have something so spectacular,” Boyce said.

If a black hole is a hurricane, Haggard says, her team studies its environment and weather systems. Observations of both black holes — in 2017, 2018, 2021, and earlier this year — will keep astronomers busy for years to come.

“We have a colossal amount of data just waiting for us to be doing more sophisticated studies,” said Haggard. “We’re on the cusp of being able to start to give more of that context.”