The mystery at the center of the Milky Way has finally been solved. This morning, at simultaneous press conferences around the world, astronomers from the Event Horizon Telescope (EHT) discovered the first image of Sagittarius A *, a supermassive black hole in the center of the Milky Way. This is not the first picture of a black hole that this collaboration has given us iconic image M87 *which they are revealed April 10, 2019. But it is the one they wanted most. Sagittarius A * is our private supermassive black hole, the fixed point around which our galaxy revolves.
Scientists have long thought that a supermassive black hole hidden deep in the chaotic central region of our galaxy was the only possible explanation for this. amazing things happening there – for example, giant stars slingshots around the invisible something in space with a noticeable fraction of the speed of light. However, they did not dare to say it directly. For example, when astronomers Reinhard Hansel and Andrea Gez shared a piece 2020 Nobel Prize in Physics for work on Sagittarius A *, them the quote is indicated that they were rewarded for “discovering a supermassive compact object at the center of our galaxy” rather than for discovering a “black hole”. The time for such a precaution is over.
At the National Press Club in Washington, DC, this morning Professor of Astronomy and Physics at the University of Arizona and member of the EHT Scientific Council Feryal Özel presented a drawing, a dark ring framed by three shiny knots of trillions. -degree gas. “I met [Sagittarius A*] 20 years ago and since then I have loved it and tried to understand, ”Ezel said. “But so far we haven’t had a direct picture.”
Black holes trap everything that falls into the trap, including light, so they are, in a very real sense, invisible. But they distort space-time around them so much that when illuminated by brilliant streams of matter crushed in their gravitational grip, they cast a “shadow”. A shadow is about two and a half times larger than the event horizon of a black hole: its boundary and defining line, a line in space-time through which nothing that passes can ever return.
EHT captures images of this shadow using a technique called very long basic interferometry (VLBI), which brings together radio observatories on several continents to form a virtual telescope the size of Earth, the highest resolution instrument in all of astronomy. In April 2017, EHT staff spent several nights showing off this virtual tool on Sagittarius A * and other supermassive black holes. We have already seen the first finished product from this effort: M87 *. The team also received raw data for the image of Sagittarius A * in the same company, but the conversion of these observations into a real picture took much longer.
This is because Sagittarius A * is constantly changing. M87 *, the black hole in the center of the galaxy Messier 87, or M87, is so huge that the matter orbiting it takes many hours to complete full orbit. Practically speaking, it means that you can look at it for a long time, and it will hardly change. Sagittarius A * is more than 1,000 times less massive, so it changes about 1,000 times faster as matter moves in denser and faster orbits around the black hole. Katie Booman, a computer scientist and astronomer at the California Institute of Technology who heads the EHT imaging task force, said matter revolves around Sagittarius A * so fast that it changes “minute by minute.” Imagine that you are taking a picture of a time-lapse bullet rushing – it is not easy to do. That’s why extracting a clear image of Sagittarius A * from data collected during observations in 2017 has been a work of several years.
If the mercury nature of Sagittarius A * made it difficult for him to imagine, it also makes it an exciting laboratory for future research on black holes and Einstein’s general theory of relativity, his consecrated theory of gravitation. After decades of studying with various telescopes, astronomers already knew the basic measurements of Sagittarius A * (its mass, diameter and distance from Earth) with great accuracy. Now, finally, they have the opportunity to observe its development – to observe how it feeds on the erupting, flickering streams of matter – in real time.
Raising the multilayer veil
Scientists began to suspect that a black hole was hiding in the heart of the Milky Way in the early 1960s, shortly after the discovery of active galaxy nuclei – extremely bright areas in the nuclei of some galaxies illuminated by voracious supermassive black holes. From our point of view here on Earth, active galactic nuclei are a thing of the past – we only see them in the distant universe. Where did they all go? In 1969, English astrophysicist Donald Linden-Bell argued that they were nowhere to be found. Instead, he said, they just went to bed after a heavy meal – dormant supermassive black holes, as he predicted, dozing all around us in the hearts of spiral galaxies, including our own.
In 1974, American astronomers Bruce Balick and Robert Brown sent radio telescopes to Green Bank, Virginia, to the center of the Milky Way and discovered a dim spot they suspected was the central black hole of our galaxy. They found a spot in a piece of sky known as Sagittarius A. Radiation from a new source illuminated – or “captured” – around a cloud of hydrogen. Brown borrowed from the nomenclature of atomic physics, in which excited atoms are marked with an asterisk, and named the newly discovered spot Sagittarius A *.
Over the next two decades, radio astronomers gradually improved their understanding of Sagittarius A *, but they were limited by the lack of suitable telescopes, relatively primitive technology (mention drum-to-drum magnetic tape), and inherent difficulty in peering into the center of the galaxy.
Sagittarius A * is hidden by a multi-layered veil. The first layer is the galactic plane – gas and dust for 26,000 light-years that block visible light. Radio waves pass unimpeded through the galactic plane, but they are covered by the second layer of the curtain – the scattering screen, a turbulent part of space where changes in density in the interstellar medium slightly distract radio waves. The last layer, which hides Sagittarius A *, is the erased matter that surrounds the black hole. Looking through this barrier is like peeling a bulb. The outer layers emit light with a longer wavelength, so the RSDB works with a shorter wavelength of light to get closer views of the black hole event horizon. However, this was a serious technological problem.
Astronomers who used methods other than VLBI were initially more successful, constantly gathering circumstantial evidence that Sagittarius A * “dot” was in fact a raging supermassive black hole. In the 1980s, physicist Charles Townes and colleagues showed that gas clouds at the center of the galaxy moved in a way that only made sense if they were under the influence of a large, invisible gravitational mass. And in the 1990s, Gez and Hansel independently began tracking the orbits of giant blue stars at the center of the galaxy, displaying their movement around a difficult but hidden turning point.
Meanwhile, the situation for radio astronomers has improved. In the late 1990s and early 2000s, a new generation of high-frequency radio telescopes began to appear on the Internet – telescopes that, if supplemented by a large number of special equipment, could participate in RSDL observations at the frequencies of the microwave, which are believed to shine from the edge of Sagittarius’ shadow A *. At the same time, the computing revolution that has led to solid state drives and smartphones in every pocket has greatly increased the amount of data that every observatory in the radio telescope network can record and process.
In 2007, the small predecessor EHT took advantage of these trends and used a trio of telescopes in Hawaii, California and New Mexico to break the veil around Sagittarius A *. They were far from making an image, but they saw it something.
Scientists have long known that a black hole under certain circumstances should cast visible shadows. In 1973, physicist James Bardin predicted that a black hole on a bright background would show her silhouette, although he decided that “there seems to be no hope of observing this effect». And in 2000, astrophysicists Heino Falke, Fulvio Melia and Eric Agol showed that an Earth-sized radio telescope that collects microwave ovens must be able to see the shadow of Sagittarius A * against the glow of the surrounding ring of destroyed matter.
Fifty years later, several dozen astronomers and astrophysicists working in this obscure corner of astronomy agreed to formally build a planetary-scale virtual radio telescope to observe this shadow. The first official kick-off meeting for the project took place in January 2012, and EHT was born.
Five years later, in collaboration with more than 200 scientists from eight observatories around the world, the team took its first realistic image, seeing the shadow of Sagittarius A *. During the 10 days of April 2017 Telescopes in North America, South America, Hawaii, Europe and Antarctica together approached the center of the galaxy and other black holes, collecting 65 hours of data on 1,024 eight-byte hard drives that were sent to supercomputer banks in Massachusetts and Germany for Korea. . Five years later what, joyful EHT researchers showed the world that their experiment worked. “We’ve been working on this for so long that we have to pinch ourselves from time to time,” Booman said this morning. “It’s a black hole in the center of our galaxy!”