New Knowledge ｜ Take a photo of the ＂behemoth＂ at the center of the galaxy

This is the first photo of the black hole at the center of the Milky Way released by astronomers on May 12. Xinhua News Agency

At 21:00 on May 12, Beijing time, the Event Horizon Telescope (EHT) cooperation organization held a press conference in seven cities around the world, including Shanghai, and announced the first photo of the supermassive black hole (Sagittarius A *) at the center of the Milky Way. For the first time in human history, the true face of the center of the Milky Way has been revealed.

This black hole photo was co-organized by the Event Horizon Telescope (EHT) and "taken" through a network of radio telescopes distributed around the world. Its release gives evidence that Sagittarius A * is the black hole in the center of the Milky Way, providing valuable clues for understanding this "behemoth" that is generally believed to inhabit the center of most galaxies. Its birth opened a new chapter in human exploration of the black hole in the center of the Milky Way.

Getting to know black holes for the first time

For the mysterious and vast universe, human beings are always full of imagination, and most of them come from the most primitive curiosity of human beings. With the development of science and technology, many imaginings have become scientific predictions, and through the unremitting exploration of generations of scientists, they have further become reality.

Modern astronomy tells us that the beautiful Milky Way we see at night is actually a flat disk-shaped spiral galaxy composed of hundreds of billions of stars. And our solar system, as an ordinary member of it, revolves around the center of the Milky Way 27,000 light-years away from the center of the Milky Way.

The "Galactic Center" was located in the direction of Sagittarius, which was covered with a large amount of gas and dust. It was difficult to observe in the visible light band, and the infrared and radio bands with longer wavelengths were less affected, which became the main way to study the "Galactic Center".

In 1933, when Carl Jansky of Bell Labs published a paper announcing the discovery of radio radiation possibly originating from the center of the Milky Way galaxy, it was the first time that humans had captured radio waves in the depths of space, but scientists at the time knew nothing about the secrets behind this intense radio radiation. Over the next three decades, Jansky’s findings were not only confirmed, but also traces of intense radio radiation were found in other, more distant galaxies.

Similar findings have led theorists to make bold predictions that galaxies harbor supermassive black holes at their centers, which are sources of intense radio radiation.

Black holes are cosmic celestial bodies that make many people curious. But few people really understand this cosmic "monster".

What is a black hole? "In layman’s terms, a black hole is actually the remains of a huge star," said Qin Ruiqiang, president of the Hebei Astronomy Amateur Association.

According to the introduction, the interior of a star is constantly undergoing thermonuclear reactions. When a star ages, it uses up the fuel in the center due to thermonuclear reactions, and there is not much energy produced by the center. At this time, it can no longer bear the huge weight of the shell. Therefore, under the weight of the shell, the core begins to collapse, and matter will inexorably "march" towards the center point. At this time, a supernova explosion will occur for a long time, until finally the star forms a star with a volume close to infinity and an almost infinite density, which is a black hole.

To fly from Earth to space, a rocket needs to reach a speed of 7.9 kilometers per second. As the mass of the Earth continues to increase, its surface attractive force will also increase, and the rocket will escape the Earth at an increasing speed. When this speed reaches 300,000 kilometers per second, nothing can escape the celestial body, including light.

A black hole is this celestial object that even light cannot escape, and this region is known as the black hole’s event horizon.

Inside, no light can escape, and all matter and mass gather at the singularity at the center of the black hole, where the curvature of space-time is infinite and the attractive force is infinite. Once inside the event horizon, all the laws of physics we know about the world break down.

In 1971, astronomers Nader Lindenberg and Martin Rees first predicted that there was a supermassive black hole at the center of the Milky Way. Because of the nature of black holes, it cannot be directly observed. But as the black hole swallows other matter, it forms an accretion disk, in which a large amount of matter rotates, rubs, and warms around the black hole, making the temperature here reach millions of degrees Celsius and appear very bright.

Black holes are one of the rarest cases in the history of science, developed to the point of exhaustion as a mathematical model without any observational evidence to prove their theory correct. In the early 20th century, black holes became widely known by Einstein’s general theory of relativity, but scientists did not discover the first black hole until 1970.

Although scientists have long simulated the appearance of black holes in supercomputers, they have long hoped to take a picture of them.

Photographing a black hole

With the continuous advancement of technology, humans can finally take pictures of black holes. Scientists chose two supermassive black holes as "models".

Released this year is Sagittarius A *, the supermassive black hole at the center of the Milky Way, 27,000 light-years from Earth and more than 400 times the mass of the Sun. The black hole image released in 2019 is from the galaxy M87, which is farther and larger, 55 million light-years away from Earth and 6.50 billion times the mass of the Sun.

"People observe these two black holes from the earth, they are very small, and the apparent diameter of the ring structure is only about 40 to 50 microarc seconds." Qin Ruiqiang introduced that a microarc second is about the size of the period at the end of an article on the earth when viewed from the moon. The two black holes seem to be a "doughnut" on the surface of the moon 380,000 kilometers away from the ground.

Such a small apparent diameter challenges the angular resolution of telescopes.

"Angular resolution is a term that describes the capabilities of a telescope," Mr. Qin said. The smaller the angular resolution, the better the telescope’s ability to distinguish distant celestial bodies, and the clearer the images we can obtain.

The optical telescope is affected by the lens process and the earth’s atmosphere, and its resolution is weak. Even the 39.3-meter European Extremely Large Telescope under construction has a theoretical limit angular resolution of only 1,000 microarcseconds, which is still far from 50 microarcseconds.

Therefore, scientists thought of radio telescopes.

The aperture of a single radio telescope can reach tens of meters or even hundreds of meters. Although the resolution is much lower than that of optical telescopes of the same aperture due to its longer observation wavelength, scientists can use a VLBI (Very Long Baseline Interferometry) technology to form two or more radio telescopes into a larger aperture radio telescope, and improve the resolution by increasing the aperture.

According to reports, the black hole photos released in 2019 were also contributed by the Shanghai Tianma Telescope. It formed the East Asia VLBI Network with several stations in South Korea and Japan to conduct observations. The addition of the Tianma Telescope has improved the quality of East Asia VLBI maps by 53%.

Although Sagittarius A * is closer to Earth, photographing it is much more difficult than M87 *. The achievement was achieved by the whims of more than 300 researchers from 80 research institutions around the world.

The EHT collaboration "combined" eight radio telescopes in six geographic locations around the world to form a so-called "Event Horizon Telescope" with unprecedented sensitivity and resolution.

In 2017, the eight telescopes, which travel north to Spain and south to the South Pole, cast a "big net" at selected targets to "retrieve" massive amounts of data to give us a picture of what a black hole looks like. But the observation window left to scientists is very short, only about 10 days per year.

In addition to the limitation of observation time, photographing black holes is also extremely demanding in terms of weather conditions. "Water in the atmosphere has a significant impact on this observation band, and water affects the intensity of radio waves, which means that precipitation can interfere with observations." As a result, experts say, the eight telescopes are located in areas with high altitudes, very little rainfall and a very high probability of sunny days.

In addition, successful imaging requires all telescopes to be fully synchronized in time. During observations, each radio telescope collects and records radio wave signals from the vicinity of the target black hole, which are integrated to obtain an image of the event horizon. To ensure signal stability, the EHT uses atomic clocks to ensure that the signals it collects and records are synchronized in time.

Chi-kwan Chan, a scientist at the Steward Observatory in the United States, the Department of Astronomy and the Institute of Data Science at the University of Arizona, has described the difficulty of observing Sagittarius A * this way: "The surrounding gas is rotating around Sagittarius A * and M87 * at a high speed at almost the speed of light. It takes a few days to several weeks for the gas to circle M87 *, but for the much smaller Sagittarius A *, the gas can circle once in a few minutes. This means that as the EHT observes Sagittarius A *, the brightness and pattern of the gas surrounding the supermassive black hole are also changing rapidly over time. Taking a picture of Sagittarius A * is a bit like taking a clear picture of a puppy chasing its tail."

In the future, EHT will make a black hole "movie"

Using existing images of two black holes with masses more than 1,500 times different, scientists can further examine the attractive forces in extreme environments.

The photo of Sagittarius A * was made from a combination of many photos extracted from the EHT observation data in 2017. Since then, in 2018, 2020 and 2021, the EHT cooperation organization has not stopped the pace of observation research. Moreover, the number of EHT stations continues to increase. In 2018, three new stations were added, including the Greenland Telescope, and the coverage of the observation array was further expanded.

In future plans, more telescopes will be used to expand the EHT observation array.

In addition to the increase in the number of telescopes, starting in 2023, the EHT will conduct observations in the 0.87mm band, which will improve the resolution by nearly 50% compared to observations in the 1.3mm band under the same circumstances. Sagittarius A * and M87 * remain the most important scientific targets of the EHT collaboration, and astronomers are exploring how they change over time and studying the surrounding magnetic field.

The continued expansion of the EHT population and technological innovation will allow scientists to share more compelling astronomical photos, and even "movies" of black holes.

"Using the next-generation EHT to film such a’movie ‘of the black hole at the center of the Milky Way is the pursuit of astronomers." Some astronomers said so.

Scientists in our country have long been concerned about high-resolution black hole imaging research. In this EHT cooperation, scientists in our country actively participated in the joint promotion of early EHT international cooperation, the joint application of EHT observation time, the observation operation of the Maxwell Telescope in Hawaii, and the post-data processing and analysis.

Scientists will also aim their telescopes at other sources to make more "doughnuts," experts say, a goal that could be achieved by building space telescopes because other "doughnuts" are smaller. (Wang Ludan, Hebei Daily reporter)