“The black holes of nature are the most perfect macroscopic objects there are in the universe: the only elements in their construction are our concepts of space and time.”– Subrahmanyan Chandrasekhar, Indian-American astrophysicist
A black hole is probably the first thing that we think of when talking about the wildest celestial objects in the universe.
Having their theories first proposed centuries ago, black holes and their understanding have undergone (r)evolutionary changes ever since.
Black holes are easily one of the most (if not the most) intriguing bodies in our universe.
The physics of these monsters is what draws the line between them and the rest of the cosmic crowd.
On 10th May 2022, scientists at the Event Horizon Telescope (EHT) project announced that they had some interesting information to unveil about the supermassive black hole, at the heart of our Milky Way galaxy, called the Sagittarius A* (pronounced as “Sagittarius A-Star”).
It was similar to the announcement made 3 years ago i.e. on 10th April 2019 about the release of the first-ever direct image of a black hole (if you remember).
To our utter delight, it is (again) indeed about the images of a black hole as it turns out that the EHT team was successfully able to produce the first direct image of the supermassive black hole at the center of our galaxy – Sagittarius A*.
History & Background of Black Holes
A black hole is an infinitely dense region of space-time possessing extreme gravity, enough to not let light escape its hold.
The history of black holes dates back to 1784 when physicist John Michell published a paper describing a star so massive (about 500 times bigger than the sun) that its gravity doesn’t even allow light (once in its region) to escape.
Physicists, earlier, didn’t believe that black holes could even exist, speculating that due to the conservation of momentum, the centrifugal force, at some point in time, would be strong enough to counter the effect of gravity and prevent the collapsing of the star at some radius.
Albert Einstein himself didn’t believe that black holes could exist, even though his theory of General Relativity did predict the existence of black holes.
Later, theories were proven through experiments as our understanding of black holes evolved, but we still couldn’t observe these black holes directly.
In 2009, scientists collaborated and launched the Event Horizon Telescope project, an international collaboration that aimed to observe & capture (images of) two supermassive black holes – Messier 87* (at the center of the Messier 87 galaxy) and Sagittarius A* (at the center of our Milky Way galaxy).
The EHT project did successfully capture images of the Messier 87*, which were made public on 10th April 2019, but couldn’t do the same with Sagittarius A* (then) due to several factors.
Event Horizon Telescope Project
Event Horizon Telescope (EHT), launched in 2009, is a global network of radio telescopes that involves linking radio dishes across the globe to create a super-telescope of the size of the Earth.
The ETH project is an international collaboration, dedicated to studying and observing black holes, that combines and correlates data from several Very Long Baseline Interferometry (VLBI) stations from around the world.
The various telescopes involved in the EHT project work collectively to gather extremely large amounts of data.
The data is then correlated, studied, and analyzed on large computers (running at a speed of about 40 Gbit/s) at places like MIT Haystack Observatory and Max Planck Institute for Radio Astronomy.
The data that is collected on hard drives is so big that it cannot be shared online, and so it is transported by commercial freight airplanes to the observatories.
The hindrances caused by transportation were a major cause for the failure of imaging Sagittarius A* by the data collected in 2017.
The EHT project has expanded its arsenal of both telescopes and observatories in the past decade.
Targets of Event Horizon Telescope
The primary observational targets of the EHT project include observing and studying two supermassive black holes –Sagittarius A* (or Sgr A*) and Messier 87* (or M87*).
Sagittarius A* is the closest supermassive black hole to the Earth.
It interested the team because it resides in the center of our galaxy, approximately 26,000 light-years (156 quadrillion miles) away from us.
Although it’s not the only black hole in our galaxy, it is the one that appears the largest from Earth.
Sagittarius A*, despite being comparatively closer to us, couldn’t be imaged properly due to several limiting factors involved (refer to Challenges Involved in Capturing Sagittarius A*).
Capturing Messier 87* was a relatively easy target (as it is quite different from Sagittarius A*), and its image was released back in 2019 (as we know it).
It is with great delight and pride that both of the targets of the EHT project have now been successfully met.
This large array of telescopes has been responsible for two of the most groundbreaking discoveries in the past 3 years, one of which came only recently; but let us start with the first one.
Capturing Messier 87*
The Event Horizon Telescope had been extensively studying and collecting data about the supermassive black hole residing at the center of the Messier 87 galaxy.
It announced its results in six simultaneous press conferences worldwide on 10th April 2019.
It featured the first-ever direct image of a black hole, unveiling the Messier 87* (pronounced as “Messier 87-Star”).
This image provided a test for the theory of general relativity under extreme conditions.
Einstein had predicted a dark shadow-like region caused by gravitational bending and capture of light, which matched the observed image.
Furthermore, the image provided new and improved measurements for the mass and diameter of M87*.
Its mass was measured to be 6.5 ± 0.7 billion solar masses and the diameter of its event horizon was measured to be approximately 40 billion kilometers.
From the enhanced brightness of the southern part of the ring due to the relativistic beaming of approaching wall jet emission, EHT concluded that the black hole, which anchors the jet, spins clockwise as seen from the Earth.
On 24th March 2021, the EHT collaboration also unveiled the polarised view of M87*, which, although requiring loads of effort, was immensely satisfying.
The above image shows how the light is polarized due to the magnetic effect around the black hole.
This polarization helps the researchers to map the magnetic field lines at the edge of the black hole and also understand how the black hole “eats” matter and launches jets; thus, giving deeper insights into the physics of M87*.
Just 3 years after the picturing of M87*, EHT successfully captured the Sagittarius A* whose picture was revealed to us on 12th May 2022.
Capturing Sagittarius A*
Successfully photographing the monstrous supermassive black hole sitting at the center of our galaxy is the most recent achievement for not just the Event Horizon Telescope but also in all of astronomy.
It is the first direct evidence of a black hole at the center of our galaxy, which was once mere speculation on paper.
Discoveries like these make us realize that we have come a long way indeed, but before we move on any further, let’s have a look at the Sagittarius A* –
Similar to the picture of M87*, the Sagittarius A*, herein, can be seen as a central blob of darkness surrounded by rings of glowing gas and light that is orbiting the black hole.
Although the two images might look similar, the two black holes are very different from each other, and so was the process of capturing them.
What Does the Image of Sagittarius A* Tell Us?
Considering the color scheme, the image of Sagittarius A* is mostly black and orange.
The black blob in the center, interestingly, is the shadow of the black hole itself.
It is evidence of the fact that the center of our galaxy does hold a black hole, which is self-explanatory as the light that’s going in the center does not return (thus the black blob).
Now, this is surrounded by a lot of yellow-orange(ish) stuff, which is the accretion disk of the black hole.
This mostly contains the glowing gasses and electromagnetic radiation that are rapidly orbiting the black hole.
Overall, the mass of Sagittarius A* is found to be 4 million solar masses.
The new measurements obtained in the image of Sagittarius A* indicate that the infrared size of the black hole shadow (black central blob) is consistent (within 10%) with the predictions from the theory of general relativity given by Albert Einstein.
Based on the observations, the simulation library of the team of astronomers also implies that Sagittarius A* is a spinning black hole; but that is not conclusive and remains a subject of intense study.
How Is Sagittarius A* Different From M87*?
The differences in the process of capturing both black holes owe to the differences in their physics.
The consistencies observed in the images show that Sagittarius A* and M87* are accreting matter at hugely different rates.
It has been observed that Sagittarius A* is accreting a million times lesser matter than the M87*.
Although both black holes fall in the category of supermassive black holes, M87* (6.5 billion solar masses) is quite a lot bigger than Sagittarius A* (4 million solar masses).
Analogically, if Sagittarius A* (including its accretion disk) would compare to the size of the orbit of Mercury, then M87* (just the event horizon; not the accretion disk) would compare to the size of the entire solar system.
M87* accumulates matter at a much faster rate than Sagittarius A*, whereas Sagittarius A* is a very slow eater.
Due to its big size, the gas around M87* takes many days to completely orbit the black hole, making it a steadier, calmer, and observation-friendly black hole (the bigger the black hole, the calmer it is).
On the other hand, Sagittarius A* is just the opposite, where the gas around the black hole takes just minutes to completely orbit it.
You might wonder if the two black holes are so distinguishable in size, why do their images look alike i.e. of the same size?
Well, behind it stands the same reason as the sun and the moon appearing equally big to our naked eyes.
Although M87* is 1500 times more massive than the Sagittarius A*, it is also 2000 times farther from us than Sagittarius A* is. Due to this, the images captured appear to be of the same size.
What Went Behind Capturing Sagittarius A*?
The pictorial evidence of Sagittarius A* was made possible by the array of radio telescopes linked across the globe, under the EHT project.
Each of the telescopes observes the object for several hours over a very long period, and the data, synchronized with an atomic clock, is combined using supercomputers called correlators.
The telescopes work in collaboration with each other; the more the telescopes, and the farther apart they are, the clearer shall be the image obtained.
After gathering and combining the data from different telescopes and following a series of complex analyses, the team prepared thousands of images of the Sagittarius A*, which were then classified into four types, depending upon the similarities and dissimilarities in the images.
From here, the team began eliminating the images that did not fit the data (or even theories) of the Sagittarius A*.
Eventually, we were left with just a handful of images of the black hole, which were then simulated and combined into the final image.
As blurry as it looks, the picture of the Sgr A* is one of the sharpest images that we’ve ever obtained.
A team of 300 researchers from over 80 institutes, 8 telescopes, and about 100 petabytes of data (equivalent to 100 million gigabytes) collected over several years, are the factors that are responsible for this breathtaking look at the heart of our galaxy.
Challenges Involved in Capturing Sagittarius A*
Capturing Sagittarius A* was not a piece of cake for the astronomers, even with state-of-art equipment and manpower.
The team had to overcome many challenges before getting their hands on the final image.
Peeking at the supermassive black hole of our galaxy meant peeking through stellar systems, nebulae, and dense clouds of gasses, all the way towards the very center of our galaxy, which is about a distance of 26,000 light-years from our planet.
Moreover, the dynamic environment of Sagittarius A* made it difficult for astronomers to picture it.
As mentioned above, Sagittarius A* changes rapidly as the glowing gas orbits the black hole in mere minutes.
These quickly changing patterns in the environment of the black hole made it quite challenging for the EHT to observe it, like capturing a moving target.
This was one of the reasons why Sagittarius A* couldn’t get processed into a final image back in 2019 (at the time of capturing M87*), even after the collection of essential & raw data.
Nonetheless, we learned from our failures and adopted a better scientific methodology to better process the data, simulations, and pictures of the Sagittarius A*; and we passed with flying colors.
What’s Next For The Event Horizon Telescope?
Although the Event Horizon Telescope has successfully photographed both Sagittarius A* and M87*, there’s no stopping here.
The team looks forward to simulating movies for both observed black holes to better understand their behavior and also that of their surroundings.
It’s not just about capturing a still black hole, but also about understanding the evolution and movement of the black hole with time.
Although the researchers have collected a lot of data in the past few years, it still isn’t enough to create the movies they want.
So both the collection of data, and refinement of the existing data would be one of the future endeavors for the team.
Moreover, the team also wants to soon capture the Sagittarius A* in polarized light as it did with M87*.
This would offer key insights into understanding the behavior of light and magnetic field lines around the black hole, and of course, the black hole itself.
The researchers are also astonished at how well the data and observations correspond with the existing theories like that of general relativity.
However, the team would be openly involved in the deeper analysis to find any possible cracks in the prevailing theories with the help of observations.
If the theories do happen to break down somewhere, it’ll help us reach a more profound understanding of gravity and the nature of spacetime.
The researchers are also looking forward to adding more telescopes to the EHT collaboration to dig into higher resolutions and sharper features of the observations.
We hadn’t thought that we would be able to live the day that sees the first image of the supermassive black hole of our galaxy, yet here we are.
There is a lot that has been achieved, but more importantly, there is A LOT that’s waiting to be discovered, to be understood, and unveiled.
Ultimately, progress is what science has always been about.
“Astronomy compels the soul to look upward, and leads us from this world to another.”– Plato, a Greek philosopher
Frequently Asked Questions (FAQs)
Which Black Hole’s Images have been recently captured?
The black hole that has recently been captured by the Event Horizon Telescope is Sagittarius A*.
It is the supermassive black hole that is located at the center of the Milky Way galaxy.
It is the second black hole to have been imaged in history; the first one was Messier 87* (captured in April 2019) which is located at the center of the Messier 87 galaxy.
How was the image of the Black Hole taken?
The Sagittarius A* was imaged by the Event Horizon Telescope collaboration.
The Event Horizon Telescope is a group of telescopes consisting of 8 radio telescopes around the globe that work in pairs to view the black hole and form high-resolution pictures of the accretion disk around it.
The data collected by the Event Horizon Telescope is combined, studied, and analyzed to better understand the behavior of black holes.
Why is the Image of the Black Hole Blurry?
The Sagittarius A* is 26,000 light-years away from us, separated by dense clouds of ionized gasses, plasma, and other remnants.
So, the large distance between the earth and the galactic center paired with dense cosmic material amidst us and the black hole is what diminishes the resolution.
Why is the Image of the Black Hole Important?
The image of a black hole might just look like an orange donut but to the eyes of an astronomer, it’s filled with data.
The thousands of images and measurements taken from the black hole and its event horizon will help in improving or even completely changing our understanding of black holes, gravity, and the nature of spacetime in general.
Will the Sagittarius A* Swallow Up our Galaxy (or our Planet Earth)?
It is a common misconception that black holes just suck in everything, they just pull in everything that gets too close to them.
So unless matter gets too close to a black hole, it will keep orbiting it the same way the planets orbit the sun.