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

This article was originally published in The Conversation. The publication published an article in Expert Voices: Op-Ed & Insights on

Chris Impey, Distinguished Professor of Astronomy at the University of Arizona

On May 12, 2022, astronomers from the Event Horizon Telescope team released an image of a black hole called Sagittarius A* 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* is at the center of our Milky Way galaxy, towards the constellation Sagittarius. For decades, astronomers have been measuring radio wave emissions from an extremely compact source.

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

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

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

2. What does the new image show?

It is impossible to get a direct image of a black hole because no light can escape from its gravity. But it is possible to measure the radio waves emitted by the gas surrounding the black hole. (Image courtesy of EHT collaboration, CC BY-SA)

Black holes themselves are completely dark, as nothing, not even light, can escape their gravity. But black holes are surrounded by clouds of gas, and astronomers can measure that gas to bring up images of the black holes inside. The central dark area in the image is the shadow cast by the black hole on the gas. The bright ring is the luminous gas itself. The bright spots on the ring show regions of hotter gas that could one day fall into the black hole.

Some of the gas seen in the image is actually behind Sagittarius A*. The light from this gas is deflected by the black hole’s powerful gravity toward Earth. This effect, called gravitational lensing, is the main prediction of general relativity.

Galactic nuclei, like the center of the Milky Way seen in this photo, are full of gas and debris, making it very difficult to get any direct images of stars or black holes out there. (Image credit: NASA/JPL-Caltech, CC BY-NC)

3. What went into creating this image?

Supermassive black holes are extremely difficult to measure. They are far away and shrouded in gas and dust clogging the center of galaxies. They are also relatively small compared to the vastness of space. From where Sagittarius A* is, at a distance of 26,000 light-years 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 travel through gas much more easily than visible light, so astronomers measured the radio emission from the gas surrounding the black hole. The orange colors in the image represent these radio waves.

The researchers used eight telescopes from around the world located at the intersections of the white lines to act as a single massive telescope. (Image credit: ESO/L. Calçada, CC BY-ND)

The team used eight radio telescopes scattered around the globe to collect black hole data over five nights in 2017. So much data was generated every night that the team couldn’t send it over the Internet—they had to send a physical hard drive. drives to where they processed the data.

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

4. Why is a new image so important?

In 2019, the Event Horizon Telescope team released the first image of a black hole, this time at the center of the M87 galaxy. The black hole at the center of this galaxy, named M87*, is a monster 2000 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 the same resolution, giving astronomers a way to learn about the universe by comparing them.

M87* on the left is 2000 times larger than Sagittarius A* on the right. Thin white circles indicate the size of the orbits of the planets in the solar system. (Image credit: EHT collaboration (credit: Leah Medeiros, xkcd), CC BY-ND)

The similarity between 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 several decades, astronomers have believed that there are massive black holes at the center of almost every galaxy. While M87* is an unusually huge black hole, Sagittarius A* is likely very similar to many of the hundreds of billions of black holes at the centers of other galaxies in the universe.

5. What scientific questions can it answer?

Based on the data collected by the team, much more scientific research remains to be done.

One interesting avenue of investigation has to do with the fact that the gas surrounding Sagittarius A* is moving at close to the speed of light. Sagittarius A* is relatively small, and matter seeps into it very slowly—if it were the size of a human, it would absorb the mass of one grain of rice every million years. But by taking a lot of pictures, it would be possible to observe the flow of matter around the black hole and into it 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 image has already spawned 10 scientific papers. I expect there will be many more.

This article is republished from The Conversation under a Creative Commons license. Read the original article.

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