On May 12, 2022, astronomers in the Event Horizon Telescope team released an image of a black hole called Sagittarius A* located at the center of the Milky Way galaxy. Christmas Impey, an astronomer at the University of Arizona, explains how the team took this image and why they did it with such a large team.
1. What is Sagittarius A*?
Sagittarius A* is located at the center of our Milky Way galaxy in the direction of the constellation Sagittarius. For decades, astronomers have been measuring the signals of radio waves from an extremely compact source out there. In the 1980s, two teams of astronomers began tracking the movements of stars near this mysterious source of radio waves.
They saw stars orbiting a dark object at speeds up to one-third the speed of light. Their movements indicated a black hole at the center of the Milky Way, 4 million times the mass of the Sun.
Reinhard Genzel and Andrea Ghez shared the Nobel Prize in Physics on this subject.
The size of a black hole is defined by its event horizon. This is a distance from the center of the black hole that nothing can escape. Researchers had previously calculated that Sagittarius A* was 16 million miles (26 million kilometers) in diameter. The Milky Way's black hole is huge compared to the black holes left behind when massive stars die.
But astronomers think that almost all galaxies have supermassive black holes at their centers. Compared to most of these, Sagittarius A* is massively underpowered and unremarkable.
2. What does the new image of Sagittarius A* tell the Astronomy and Astrophysics community?
Black holes themselves are completely dark because nothing, not even light, can escape gravity. But black holes are surrounded by clouds of gas, and astronomers can measure this gas to make images of the black holes inside them.
The central dark region in the image is a shadow cast by the black hole onto the gas. The bright ring is the glow of the gas itself. Bright spots in the ring indicate areas of hotter gas that could one day fall into the black hole.
Some of the gas visible in the image is actually behind Sagittarius A*. The light from this gas indicates that the black hole is bent by strong gravity toward Earth. This effect, called gravitational lensing, is a fundamental prediction of general relativity.
3. How did this image of the Black Hole come about?
Supermassive black holes are extremely difficult to measure. They are far away and surrounded by the gas and dust that occludes the center of the galaxies. Also, they are relatively small compared to the expanse of space.
Sagittarius A* is 26.000 light-years away at the center of the Milky Way. Only 10 in 1 billion photons of visible light can reach Earth, and most is absorbed by gas in the path.
Radio waves pass through gas much more easily than visible light, so astronomers have measured radio emissions from the gas surrounding the black hole.
The orange colors in the image are representations of these radio waves. The team used eight radio telescopes spread across the globe to collect data about the black hole over five nights in 2017. It produced so much data every night that the team couldn't share it with each other over the internet. They had to send physical hard drives to where they process data.
Because black holes are so difficult to see, there is a lot of uncertainty in the data collected by telescopes. These must be disposed of.
To turn everything into an accurate image, the team used supercomputers to generate millions of different images.
He built a mathematically viable version of the black hole based on every data collected and the laws of physics. All these images were then put together to produce the final, beautiful and accurate image. The duration of the actions taken was equivalent to running 2.000 laptops at full speed for a year.
4. Why Is The New Black Hole Photo So Important?
In 2019, the Event Horizon Telescope team released the first image of a black hole. This image was located at the center of the galaxy M87. The black hole at the center of this galaxy, named M87, is a giant 2.000 times larger than Sagittarius A and 7 billion times the mass of the Sun.
But because Sagittarius A* is 87 times closer to Earth than M2.000*, the Event Horizon Telescope was able to observe both black holes at a similar resolution – giving astronomers a chance to learn about the universe by comparing the two.
The similarity of the two images is striking because small stars and small galaxies look and behave very differently from large stars or galaxies.
Black holes are single objects that only respond to a law of nature. And they don't care about gravitational scale. For the past few decades, astronomers have thought that almost every galaxy has massive black holes at its center.
While M87* is an unusually large black hole, Sagittarius A* is probably pretty similar to most of the hundreds of billions of black holes at the center of other galaxies in the universe.
5. What Scientific Questions Can the New Black Hole Photo Answer?
It is thought that many scientific publications will emerge from the data collected by the team. According to a remarkable study, the fact that the gas around Sagittarius A* is moving at a speed close to the speed of light may be the focus of scientific studies.
Sagittarius A* is relatively small and flows very slowly towards the center of matter surrounding it. To explain this with an example, we can give the example of Sagittarius A* “if it was the size of a human, it would consume the mass of a single grain of rice every million years”. But by taking many images, it will be possible to monitor the flow of matter around and inside the black hole in real time. These studies will allow astrophysicists to study how black holes consume matter and grow.