The mystery at the center of the Milky Way has been solved. The first image of Sagittarius A*, the black hole at the center of the Milky Way, was revealed this morning at simultaneous press conferences around the world. The image of M87 was the first picture of a black hole that this collaboration has given us. It is the one they wanted the most. Our private black hole is Sagittarius A*, which is the point around which our universe revolves.
Scientists have long believed that a black hole hidden deep in the chaotic central region of our galaxy was the only explanation for the strange things that happen there. They have been hesitant to say that. The discovery of a supermassive compact object at the centre of our galaxy was the subject of the work done by Reinhard Genzel and Andrea Ghez.
Everything that falls in, including light, is trapped by black holes. The shadow is about two and a half times larger than a black hole, because they warp spacetime around them so severely.
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, which combines radio observatories on multiple continents to form a virtual Earth-size telescope. In April of last year, the EHT collaborated to point out the virtual instrument at Sagittarius A* and other black holes. The first finished product from that effort was M87*. The team captured the raw data for the Sagittarius A* image in the same campaign, but it took a long time to convert those observations into an actual picture.
Sagittarius A* is constantly changing. The black hole at the center of the galaxy M87 is so large that it takes many hours to complete a full circle. It will not change because you can stare at it for a long time. Sagittarius A* is 1,000 times less massive, so it changes 1,000 times faster as matter moves around the black hole. It isn't easy to take a time-lapse photograph of a speeding bullet. It has taken several years to get a clear image of Sagittarius A* from the data collected in the observing run.
It is an exciting laboratory for future studies of black holes and Einstein's theory of gravity because of Sagittarius A*'s unpredictable nature. Astronomers have been studying Sagittarius for decades and already knew its basic measurement of mass, diameter and distance from Earth to great accuracy. They have gained the ability to watch it evolve, to watch as it feeds on flaring, flashing streams of matter in real time.
After the discovery of active galactic nuclei, scientists suspected that a black hole was present in the heart of the Milky Way. We only see active galactic nuclei in the distant universe from our perspective. Where did they all go? Donald Lynden-Bell argued in 1969 that they didn't go anywhere. He said that they just went to sleep after their heavy meals, and that they were sleeping all around us.
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 dim speck they suspected was the central black hole. There was a speck in the sky known as Sagittarius A. Brown borrowed from atomic physics, in which excited atoms are marked with an asterisk, and named the new speck Sagittarius A*.
For the next two decades, radio astronomy improved their view of Sagittarius A*, but they were limited by a lack of suitable telescopes, relatively primitive technology and the inherent difficulty of looking into the galactic center.
A veil hides Sagittarius A*. The first layer is made of gas and dust that blocks light. The scattering screen is a turbulent patch of space where density variations in the interstellar medium knock radio waves off course. The black hole's obliterated matter is the final layer concealing Sagittarius A*. Peering through that barrier is like peeling an onion. The black hole's event horizon can be seen with a closer-in view because the outer layers emit longer-wavelength light. That was a major technological challenge.
Astronomers using other techniques had more success than they did with VLBI. Charles Townes and his colleagues showed in the 1980s that gas clouds in the center of the universe were moving in ways that only made sense if there was a mass behind them. In the 1990s, Genzel and Ghez independently began tracking the motion of giant blue stars in the galactic center, mapping their motion around a heavy but hidden pivot point.
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The situation for radio astronomy improved. A new generation of high-frequency radio telescopes started to come online in the late 1990s and early 2000s, and could operate at the microwave frequencies thought to shine from the edge of Sagittarius A*. The computing revolution that led to solid-state hard drives and smartphones in every pocket vastly increased the amount of data that each observatory in a network of radio telescopes could record and process.
In 2007, a small precursor for the EHT used a trio of telescopes in Hawaii, California and New Mexico to pierce the veil surrounding Sagittarius A*. They saw something, even though they were far from making an image.
Scientists have known for a while that a black hole should cast shadows. In 1973, physicist James Bardeen predicted that a black hole in front of a bright background would show its silhouette, but he decided that there was no hope of observing it.
Half a decade later, a few dozen of the astronomy and astrophysicists laboring in this obscure corner of astronomy agreed on the formal goal of building a virtual planet-scale radio telescope to observe that shadow. The first official meeting for the project took place in January of 2012
Five years later, after growing into a collaboration of more than 200 scientists with eight participating observatories across the globe, the team took its first realistic 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 andAntarctica all gathered 65 hours of data on 1,024 eight-terabyte hard drives, which were shipped to supercomputer banks in Massachusetts and Germany. The world has seen that the experiment worked after five years.
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