For the first time, humanity is staring into the dark heart of inexplicable chaos at the center of the Milky Way and focusing on its shadowy form. The object looking back at us, Sagittarius A *, is a monstrous black hole that connects our home galaxy.
On Thursday, researchers at the Event Horizon Telescope (EHT) Collaboration unveiled the first direct visual evidence of Sagittarius A *, or Sgr A *, in coordinated global press conferences. Comprised of more than 300 researchers, the collaboration sparked three years ago to uncover the first image of each black hole and has been trying to depict Sgr A * since 2009.
Today the world bears witness to the fruits of their labor. And this is as revolutionary as expected.
This blinding light, swirling in orange around a shadowy circle, took more than 26,000 years to reach us. It is of luminescence, born on the edge of Sgr A *, when the northern ice sheets of the Earth reached as far as Manhattan, cave bears still roam Europe, and the settlements of Homo sapiens are built of mammoth bones.
“I wish I could tell you that the second time is as good as the first time you paint black holes. But that will not be true. It’s actually better, “said Ferial Ozel, an astrophysicist at the University of Arizona and part of the EHT Collaboration.
Jozel’s feeling comes from the fact that SgrA *’s EHT image is not just a spectacular sight. This is concrete proof that humanity has actually managed to take pictures of the elusive engines that drive our universe. SgrA * has a donut-like structure, similar to the previous picture of the team’s black hole, therefore confirming that these luminous rings are not the product of coincidence or environmental noise.
They are black holes.
Sagittarius Saga A *
It was 1974 when astronomers first discovered evidence of Sgr A *, thanks to a very bright radio signal emitted from the heart of the Milky Way. But then it was unclear whether the replica came from a black hole. He just suspected himself.
Over the next four decades, however, further observations revealed stars orbiting the radio source in extreme orbits and at extreme speeds – both expected to appear around black holes. And until 2018, there was even more comprehensive confirmation that Sgr A * is an absolutely supermassive black hole and one with a mass of over 4 million suns. Two of the scientists who studied Sgr A * were awarded the Nobel Prize in Physics in 2020.
But we still couldn’t see the black hole. So far so good.
An image of the heart of the Milky Way taken by NASA’s Hubble Space Telescope in 2016.
NASA, ESA and Hubble Heritage Team (STScI / AURA, Acknowledgments: T. Do, A. Ghez (UCLA), V. Bajaj (STScI)
The incredible image of EHT is the long-sought visual confirmation of the true nature of Sgr A *, which allows us to finally see the engine behind the Milky Way vortices and refine our ability to study the colossal abysses of the universe and their exotic physics. “This is a big – no, this is a huge – moment for everyone in the collaboration of the Event Horizon Telescope,” said J. Anton Zensus, director of the Max Planck Institute for Radio Astronomy in Germany.
A detailed description of the findings was published Thursday in a series of articles published in The Astrophysical Journal Letters.
An image of the invisible
The gravitational effects of a black hole are so powerful that the gap actually drills a hole in space-time. But black holes are not exactly “black holes”. They are more like invisible cracks in space.
Basically, when a star big enough dies, it collapses to a point with a huge gravitational pull called a singularity. This pull is so unimaginably strong that when gas, dust or light gets inside, the particles can never escape. Nothing can escape, which makes black holes virtually invisible.
In fact, since black holes were first theorized by Einstein in the early 20th century, astronomers were convinced that these gaps existed only because of pure mathematics. But there is one caveat. Although we cannot accurately “see” a black hole, we can visualize the surrounding region, where these forever doomed particles are about to descend to its center.
In other words, just outside the darkness of the mighty void, gas and dust overheat to trillions of degrees Celsius and release light into the electromagnetic spectrum. For us, this light appears as X-rays and radio waves. Both signals can be detected from Earth and so we can see the invisible.
However, to capture these priceless fingerprints from a black hole, you need a telescope the size of our entire planet.
But since this is clearly not feasible, EHT has found a fascinating way to circumvent the premise. It practically connects 11 terrestrial radio telescopes together, all located around the Earth. Over time, these devices searched for the super-hot signatures of the black hole extracted from particles, or rather, the boundary between our universe and the unknown, “invisible” interiors of the black hole.
This region is actually the namesake of EHT: the event horizon.
This image shows the location of some of the telescopes that make up the EHT, as well as a representation of the long baselines between the telescopes.
ESO / L. Calcade
The Event Horizon telescope sees the horizon of events by synchronizing observations from many of their radio telescopes scattered around the world. It collects light from the area just beyond the horizon using a technique known as “very long basic interferometry” or VLBI.
In short, VLBI requires two separate telescopes to focus on the same place in space at the same time. For example, a telescope in Chile and a telescope at the South Pole can look at the horizon of events. Then, since telescopes are subject to extremely accurate timekeeping, the results of each telescope can be combined into a final composite. Somehow this creates a virtual telescope as large as the distance between the two places. And larger telescopes usually mean higher resolutions.
This view shows several of the ALMA antennas and the central areas of the Milky Way from above.
ESO / B. Tafreshi
Radio astronomers have used this method for decades, but they have expanded the concept to 11 telescopes around the world, and you have a telescope the size of our planet. Ideal for depicting a black hole.
The many EHT telescopes came together at once and observed the black hole for several hours. According to Katie Booman, a computational imaging researcher and member of the EHT, “our radio telescope is shaking hands.” Then these results were combined, all data were passed through an algorithm and – thunder! – We have our picture of a black hole.
“Taking a picture with EHT is a bit like listening to a song played on a piano that has a lot of missing keys,” Booman said. “Because we don’t know when the missing keys need to be pressed, there are an infinite number of possible melodies that could be played. However, with enough keys, our brains can often fill in the gaps to recognize the song correctly.”
In 2019, scientists created the first picture of a black hole in the world. But the new topic of EHT’s black hole poses several additional obstacles.
The first image of a black hole, made in 2019 by Event Horizon Telescope.
National Science Foundation
M87 * vs. SgrA *
The muse of EHT’s first image – a blurry, orange and yellow ring of light stamped on a colorless cosmic void – is M87 *, a supermassive black hole located in the heart of the Messier 87 galaxy with about 55 million light. years from Earth. It has a mass 6.5 billion times greater than that of our sun.
But EHT has always hoped to see Sgr A *, especially because the black hole in our home galaxy is what scientists think most black holes in the universe would look like.
“While M87 * was one of the largest black holes in the universe and launched a jet that pierced its entire galaxy, SgrA * gives us a glimpse into the much more standard state of black holes – quiet and still,” said Michael Johnson, an astrophysicist at Harvard Smithsonian Center for Astrophysics.
However, the SgrA * was much harder to map than the M87, simply because we didn’t have a large angle, and the EHT telescopes had to see through annoying gas and dust, which further obscured the gap from view. When we studied M87 *, these problems were not really present.
Think of it this way. In space cinema, we sat in an empty theater with reclining seats and watched the black hole of Messier 87 on the entire screen of our planet. For Sgr A * we were surrounded by other patrons who constantly got up to pee and interrupt the show.
The other problem was the movie we were trying to watch. The region around the black hole is quite dynamic or in flow due to extreme gravitational mechanics. Because Sgr A * is much closer to Earth and has a smaller event horizon than M87 *, the light it emits to our telescopes changes much faster. It’s more variable. And this variability is a problem for EHT, because the Earth-sized telescope has to observe the black hole for several hours. Sgr A * changes in a few minutes.
“It’s a bit like changing the key to the song as we play it on our broken piano,” Booman said.
It’s “like trying to take a picture of a waterfall with a long shutter speed; the object changes too fast to get a sharp image, “said James Miller-Jones, an astronomer at Curtin University in Western Australia. To see Sgr A *, much more work is needed than the algorithm that collects the final image.
But, alas, they did.
The collaborators collected tens of thousands of different images using a variety of methods – including some fake hard hole simulations based on hard data – to get as much information as possible about SgrA *. They then grouped these photos into four categories and finally averaged them all.
“Through literally …
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