A black hole is essentially invisible because its huge gravity always traps any light falling within its horizon. But just outside the black hole's no return path, light still exists, and its pattern is like a photo negative, which can reveal the latent existence of the black hole Now, an international team of astronomers has captured the light around the supermassive black hole, revealing for the first time the image of the black hole sagitarrius a (SGR a , pronounced 'sage ay star') in the center of the Milky Way galaxy
The image was created by the event horizon telescope (EHT), a global network of radio telescopes whose movements are arranged into a virtual, planet sized telescope. The researchers focused the EHT array on the center of the Milky Way Galaxy 27000 light-years away from the earth, passing through our planet's atmosphere and turbulent plasma outside the solar system.
The resulting image reveals SGRA *, in the form of a luminous, doughnut shaped halo, for the first time. This ring structure is just outside the event horizon, or the point where light cannot escape, which is the result of light being bent by the huge gravity of the black hole. The halo surrounds a dark center and is described as the "shadow" of a black hole.
The ring's white hot plasma temperature is estimated to be 10 billion Kelvin, or 18 billion degrees Fahrenheit. Judging from the size of the ring, the mass of SGRA * is about 4 million times that of the sun, and it is very compact. Its size can be put into the orbit of Venus.
This image is the first visual confirmation of the existence of a black hole in the center of our Milky Way galaxy. Astronomers have previously observed stars spinning around an invisible, huge, extremely dense object - all signs point to a supermassive black hole. The images disclosed today provide the first visual evidence that the object is a black hole, and its size is consistent with the prediction based on Einstein's general theory of relativity. The result is a milestone in our understanding of black holes in general, and black holes in the center of our Milky Way Galaxy in particular.
Today, some papers in the special issue of astrophysics introduce the image and the corresponding analysis results. These findings are the work of more than 300 researchers from 80 institutions around the world, who jointly formed the event horizon telescope cooperation organization.
Chasing the tail of a black hole
Before obtaining the new image of SGRA , eht obtained the first ever image of a black hole in 2019. The original image is M87 , a supermassive black hole at the center of Messier 87, a galaxy 53 million light-years away from earth.
Compared with SGRA , M87 is a giant with a mass of 6.5 billion suns (more than 1000 times heavier than the black hole in the center of the Milky way), which can easily swallow the entire solar system. The image of M87 shows a bright ring structure, much like SGRA . The similarity between the two images confirms another prediction of general relativity: all black holes have the same shape, regardless of their size.
The images of the two black holes are based on data from respective sources taken by eht in 2017. However, because SGRA * is small and located in our Milky Way galaxy, it takes more time and effort to make it the focus.
Astronomers suspect that hot gas bypasses the two black holes at the same speed, close to the speed of light. Since SGRA is 1500 times smaller than M87 , its speed of light is more difficult to distinguish. It's harder to shoot a dog running around the park at the same speed.
M87 is located in a galaxy that deviates from our galaxy, which makes it easier to see. SGRA is located in our own galaxy, which is also a challenge in imaging. SGRA * is located at the center of the plane of the Milky Way galaxy, where there are some heated gas or turbulent plasma, which can distort any radiation from the black hole that reaches the earth. It's like trying to see through the warm air from a jet engine. It's very complex, which is why it takes longer to generate this image.
Jump data
In order to capture a clear image of SGRA *, astronomers coordinated eight radio stations around the world as a virtual telescope, which they pointed to the center of the Milky Way Galaxy in a few days in April 2017. Each Observatory uses a high-speed recorder developed by haystack observatory to record incident light data. These recorders are designed to process large amounts of data at a rate of 4 gigabytes per second.
After collecting a total of 5pb of data, including observations of SGRA and M87 , the hard disk filled with recorded data was transported away, half to MIT haystack Observatory and the other half to Max Planck Institute of radio astronomy in Germany. Both places have heavy equipment: giant supercomputers that "correlate" data, compare data flows between different stations, and convert the data into signals that can be seen by planet sized telescopes.
Then they calibrate the data - a detailed process that eliminates noise from sources such as instrument effects and the earth's own atmosphere, so as to effectively align the "mirror" of the virtual telescope with the specific signal of SGRA *.
Then, the imaging team undertook the task of transforming these signals into representative images of black holes - a more difficult challenge than M87 imaging. M87 is a larger and more stable source with little change in a few days, while "SGRA * will change in a few minutes and the data will jump everywhere," said Vincent fish, a research scientist at haystack and a cooperative member of EHT. "This is the basic challenge of black hole imaging."
Akiyama, who leads the EHT calibration and imaging team, has developed a new algorithm to pair with those used to image M87 . The researchers fed data into each algorithm to generate thousands of black hole images. They averaged these images and generated a main image showing that SGRA is a luminous ring structure.
In the next few years, scientists expect that with the expansion of EHT, more telescopes will be added to its virtual array to collect more data on SGRA and other black holes. With the expansion and improvement of the telescope array, the technology developed for Sgr A paves the way for spectacular eht images and Science in the future.
"The next step is, can we get a clearer image of this ring?" Akiyama said. "Now we can only see the brightest features. We hope to capture weaker substructures, too. Then we expect to see more details, which are significantly different from the first doughnut."