The article was originally published at The Conversation.
Chris Impey is a professor at the University of Arizona.
On May 12, 2022, the event horizon telescope team released an image of a black hole at the center of the Milky Way. Chris Impey is an astronomer at the University of Arizona.
The center of the Milky Way is located in the direction of the Sagittarius constellation. Astronomers have been measuring radio waves from an extremely compact source there for decades.
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The motion of stars near this mysterious source of radio waves was tracked by two teams of astronomer in the 1980s. The stars were moving at up to a third of the speed of light. The motions suggested that the center of the Milky Way was a black hole. The discovery was shared by Genzel and Ghez.
The event horizon is the distance from the center of the black hole to which nothing can escape. Sagittarius A* is 26 million kilometers in diameter.
Black holes left behind when stars die are larger than the black hole in the Milky Way. Astronomers think there are black holes at the center of the universe. Sagittarius A* is mediocre compared to most of these.
Nothing, not even light, can escape the gravity of a black hole. Black holes are surrounded by clouds of gas, which can be measured to see the black holes. There is a shadow cast by the black hole on the gas. The gas is glowing. There are bright spots in the ring that show hotter gas that could fall into the black hole.
The gas is behind Sagittarius A*. Light from that gas is being reflected by the black hole. This effect is a core prediction of general relativity.
Black holes are hard to measure. They are far away from the center of the universe. They are small compared to space. Only a small amount of visible light can reach Earth from where Sagittarius A* sits, at the center of the Milky Way. Astronomers measured the radio emissions from the gas surrounding the black hole, because radio waves pass through gas much more easily than visible light. The orange colors are representations of radio waves.
The team used eight radio telescopes spread across the globe to collect data on the black hole over the course of five nights. The team had to ship physical hard drives to where they processed the data because they couldn't send it through the internet.
There is a lot of uncertainty in the data the telescopes collect because black holes are so hard to see. To turn it all into an accurate image, team used supercomputers to produce millions of different images, each one a viable version of the black hole based off the data collected and the laws of physics. The final, beautiful, accurate image was produced after they blended all of the images together. For a year, the processing time was equivalent to running 2,000 laptops at full speed.
The first image of a black hole was released in 2019. The black hole at the center of this galaxy is larger than the sun and 2,000 times larger than Sagittarius A*. The event horizon telescope was able to observe both black holes at the same resolution because Sagittarius A* is 2,000 times closer to Earth.
Small stars and small galaxies look and behave differently than large stars or galaxies, so the similarity of the two images is striking. Black holes are the only objects that can answer to gravity. gravity doesn't care about scale.
Astronomers have believed for the last few decades that there are massive black holes at the center of every galaxy. Sagittarius A* is likely similar to many of the hundreds of billions of black holes at the center of other galaxies in the universe.
The team collected a lot of data.
The gas surrounding Sagittarius A* is moving close to the speed of light, making it an interesting avenue of inquiry. If it were the size of a human, Sagittarius A* would consume the mass of a single grain of rice every million years. It would be possible to see the flow of matter around and into the black hole in real time by taking many images. astrophysicists would be able to study how black holes consume matter.
A picture is worth a thousand words, and this new image has already generated 10 scientific papers. I think there will be many more to come.
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