Say hello to Sagittarius A*, the black hole at the center of the Milky Way galaxy

On May 12, 2022, astronomers from the Event Horizon Telescope team published an image of a black hole called Sagittarius A* which lies at the center of the Milky Way galaxy. Chris Impey, an astronomer at the University of Arizona, explains how the team got this image and why it’s so important.

1. What is Sagittarius A*?

Sagittarius A* sits at the center of our galaxy, the Milky Way, towards the constellation Sagittarius. For decades, astronomers have been measure radio wave bursts from an extremely compact source there.

In the 1980s, two teams of astronomers began tracking the movements of stars near this mysterious source of radio waves. They saw stars whirling around a dark object at speeds up to a third of the speed of light. Their movements suggested that at the center of the Milky Way was a black hole 4 million times the mass of the Sun. Reinhard Genzel and Andrea Ghez then shared the Nobel Prize in Physics for this discovery.

The size of a black hole is defined by its event horizon – a distance from the center of the black hole inside which nothing can escape. Scientists had previously been able to calculate that Sagittarius A* is 16 million miles (26 million kilometers) in diameter.

The Milky Way’s black hole is huge compared to the black holes left by the death of massive stars. But astronomers believe there are supermassive black holes at the center of almost every galaxy. Compared to most of them, Sagittarius A* is skinny and unremarkable.

2. What does the new image show?

It is impossible to take a direct image of a black hole because no light can escape its gravity. But it is possible to measure the radio waves emitted by the gas that surrounds a black hole.
EHT cooperation, CC BY-SA

Black holes themselves are completely dark, since nothing, not even light, can escape their gravity. But black holes are surrounded by clouds of gas, and astronomers can measure this gas to infer images of the black holes inside. The central dark region of the image is a shadow cast by the black hole onto the gas. The glowing ring is the glowing gas itself. The bright spots in the ring show 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 is bent by the powerful gravity of the black hole towards Earth. This effect, called gravitational lensis a basic prediction of general relativity.

A red mass of gas and stars at the center of the Milky Way.

Galactic nuclei, like the center of the Milky Way seen in this photo, are filled with gas and debris, making it very difficult to get direct images of stars or black holes.
NASA/JPL-Caltech, CC BY-NC

3. What happened to produce this image?

Supermassive black holes are extremely difficult to measure. They are far away and shrouded in the gas and dust that clogs the centers of galaxies. They are also relatively small compared to the vastness of space. From where Sagittarius A* sits, 26,000 light-years away from the center of the Milky Way, only 1 in 10 billion photons of visible light can reach Earth – most of it is absorbed by gas along the way. Radio waves pass through gas much more easily than visible light. Astronomers therefore measured radio emissions from the gas surrounding the black hole. The orange colors in the image are representations of these radio waves.

Lines connecting point on the globe connecting eight different areas across the Earth.

The researchers used eight telescopes around the world – located at the points where the white lines intersect – to act as one massive telescope.
ESO/L. Calcada, CC BY-ND

The team used eight radio telescopes distributed around the world to collect data on the black hole over five nights in 2017. Each night generated so much data that the team couldn’t send it over the internet – they had to ship physical hard drives to where they processed the data.

Because black holes are so hard to see, there’s a lot of uncertainty in the data that telescopes collect. To turn all of this into an accurate image, the team used supercomputers to produce millions of different images, each a mathematically viable version of the black hole based on the collected data and the laws of physics. They then blended all of these images together to produce the final, beautiful and accurate image. The processing time was equivalent to running 2,000 laptops at full speed for a year.

4. Why is the new image so important?

In 2019, the Event Horizon Telescope team released the first image of a black hole – this one at the center of the galaxy M87. The black hole at the center of this galaxy, named M87*, is a behemoth 2,000 times larger than Sagittarius A* and 7 billion times the mass of the Sun. But because Sagittarius A* is 2,000 times closer to Earth than M87*, the Event Horizon Telescope was able to observe both black holes at similar resolution, giving astronomers a chance to learn more about the universe by comparing the two.

Two side-by-side images of donut-shaped red gas clouds surrounding black holes.

M87*, left, is 2,000 times larger than Sagittarius A*, right. The thin white circles indicate the sizes of the orbits of the planets in the solar system.
EHT collaboration (thanks: Lia Medeiros, xkcd), CC BY-ND

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 the only objects in existence that obey only one law of nature: gravity. And gravity doesn’t care about scale.

For the past few decades, astronomers have thought there were massive black holes at the center of almost all galaxies. While M87* is an unusually huge black hole, Sagittarius A* is probably quite similar to many of the hundreds of billions of black holes at the center of other galaxies in the universe.

5. What scientific questions can this answer?

Much remains to be done from the data collected by the team.

An interesting avenue of research stems from the fact that the gas surrounding Sagittarius A* moves at a speed close to the speed of light. Sagittarius A* is relatively small and counts sinks in very slowly – if it were 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 would be possible to observe the flow of matter around and in the black hole in real time. This would allow astrophysicists to study how black holes consume matter and grow.

A picture is worth a thousand words, and this new picture has already generated 10 scientific papers. I expect there will be many more to come.

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