There are strange things happening in the center of the universe. There is a place where stars shoot around at a fraction of the speed of light. Until this year, scientists hadn't said that a black hole could explain the stars' movements. The discovery of a supermassive compact object at the centre of our galaxy, not the revelation of a black hole, was the subject of the 2020 Nobel Prize in physics. The object is a star.
The first image of a black hole at the center of the Milky Way was unveiled by the event horizon telescope. The first picture of a black hole captured by this collaboration was the famous image of M87*. They wanted it the most. Our private black hole is Sagittarius A*, which is the point where our universe revolves.
Everything that falls into a black hole is unseeable. When they are illuminated by glowing streams of infalling matter shredded in their gravity, they cast a shadow. The shadow is two and a half times larger than a black hole's horizon, which is the boundary in spacetime through which nothing can ever return.
A virtual Earth-size telescope is an instrument with the highest resolution in all of astronomy, thanks to a technique called very long baseline interferometry. The virtual instrument was pointed at at Sagittarius A* and other black holes. Scientists spent years analyzing the data and converting it into an image.
It took so long because of the global destruction caused by the COVID Pandemic. The biggest challenge was that Sagittarius A* is always changing. The observatory's previous target, the black hole at the center of the galaxy M 87, is so large that it takes many hours to complete a full circle. It will barely change because you can stare at it for a long time. The appearance of Sagittarius A* changes about 1,000 times faster because it is less massive. The matter in Sagittarius A* is so fast that it can change minute to minute. It is difficult to take a time-lapse photograph of a speeding bullet.
Sagittarius A* is an ideal laboratory for understanding black holes and Einstein's theory of gravity because of its unpredictable nature. Astronomers have been studying Sagittarius A*'s mass, diameter and distance from Earth for decades. They now have the ability to watch it evolve in real time.
In the early 1960s, scientists began to suspect that a black hole was present in the center of the Milky Way. In the distant, ancient universe, active galactic nuclei are a thing of the past. What location did they all go to? DonaldLynden-Bell argued that they did not go anywhere. They just went to sleep. He said that black holes are sleeping all around us in the hearts of spiral galaxies.
In 1974 American astronomer Bruce Balick and Robert Brown pointed radio telescopes in Green Bank, W.Va., at the center of the Milky Way and discovered a small black hole. They found it in a part of the sky called Sagittarius A. Clouds of hydrogen were surrounded by Radiation from the New Source. Brown borrowed the name Sagittarius A* from atomic physics, in which excited atoms are marked with an ant.
For the next two decades radio astronomy improved their view of Sagittarius A*, but they were limited by a lack of suitable telescopes, relatively clunky technology, and the inherent difficulty of looking into the crowded space center.
The Sagittarius A* is hidden. There are 27,000 stars, gas and dust in the first layer of the galactic plane. The scattering screen, a turbulent patch of space where density variations in the interstellar medium can affect radio waves, is the second layer of the veil. Infalling matter surrounds the black hole itself. It's similar to peeling off an onion's skin. The same wavelength light is emitted by the outer layers ofmatter. It was a major technological challenge to make VLBI work with shorter wavelength light.
Astronomers were able to gather indirect evidence that Sagittarius A*'s "speck" was a seething black hole using other techniques. In the 1980's physicist Charles Townes and his colleagues showed that gas clouds in the center of the universe were moving in ways that made sense only if there was a mass behind them. In the 1990s, Genzel and Ghez independently began to track the motion of giant blue stars in the center of the universe.
There was an improvement in the situation for radio astronomy. A new generation of short-wavelength radio telescopes started to come online in the late 1990s and early 2000s, allowing them to observe the microwave frequencies thought to shine from Sagittarius A*'s shadow. The computing revolution that led to solid-state hard drives and smartphones in every pocket vastly increased the amount of data that each observatory could record and process.
A group of telescopes in Hawaii, California and Arizona were used to pierce the veil surrounding Sagittarius A*. The project saw something light from the long-sought shadow.
In 1973, physicist James Bardeen predicted that a black hole in front of a bright background would create a silhouette. There seems to be no hope of seeing this effect. In 2000, Heino Falcke, Fulvio Melia and Eric Agol demonstrated that a radio telescope could see the shadow of Sagittarius A*.
A few dozen of the astronomy and astrophysicists laboring in this obscure corner of astronomy agreed to build a virtual planet-scale radio telescope to observe that shadow. The event horizon telescope was born after the first official meeting in January of 2012
After growing into a collaboration of more than 200 scientists with eight participating observatories across the globe, the EHT took its first shot at seeing the shadow of Sagittarius A*. Over the course of 10 days in April of last year, telescopes in North America, South America, Hawaii, Europe and Antarctica collectively flew in on the black holes, gathering 65 hours of data on 1,024 eight-terabyte hard drives. The world saw that the experiment worked five years later.
The Astrophysical Journal Letters published a special issue on the Sagittarius A* image. The scientists presented a multidimensional portrait of our black hole.
The basics were confirmed. Sagittarius A* is almost 30,000 light-years away. The black hole's mass is the equivalent of four million suns, thanks to years of tracking the stars around Sagittarius A*. The expected size of the black hole's shadow can be calculated using distance and mass as inputs. The image is in line with the prediction. The shadow is the size of a doughnuts on the moon and it has a diameter of 52 microarc seconds. The shadows of Sagittarius A* and M87*, black holes that differ in mass by three orders of magnitude, were found to be universal features of black holes.
The Chandra and Nu-STAR x-ray telescopes and other instruments are helping to answer questions about Sagittarius A*'s environment. Astronomy has determined that the matter around the black hole is a diffuse gas of electrons and protons. We now have a better idea of where it came from. The Chandra x-ray telescope shows that the black hole pulls matter from the atmosphere of stars. It doesn't pull very much. Less than 1% of the stuff captured by the black hole's gravity makes it to the horizon. The black hole is dim due to that. Sagittarius A* is only 100 times brighter than our sun.
It was not always so weak. As recently as 60 or 70 years ago, Sagittarius A* appears to have gone on a feeding binge, with light bouncing off nearby clouds of dust and gas. It has its active moments. There was a bright x-ray flare from Sagittarius A*. The magnetic fields around the black hole are similar to the ones that cause solar flares in our own star. Future campaigns would like to directly observe these flares.
Sagittarius A* has a lot more to offer. The black hole is spinning but we don't know how fast. Future observations aim to show how the black hole eats infalling matter and how movies of the black hole evolve over time.
The first EHT picture of Sagittarius A* shows what the object is not. Sagittarius A* has an event horizon because of the shadow. We have known for a long time that it's not a dense star, a wormhole, or a point of infinite density unconcealed by an event horizon. It's nothing new to see a black hole, and now it's closer to revealing its secrets.
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