By Anushree Kagalkar
On 12th May, 2022, the scientific world made a huge breakthrough by revealing the first ever image of a supermassive black hole: Sagittarius A*. The black hole belongs to our home galaxy, the Milky Way. It was taken by the Event Horizon Telescope collaboration. This is only the second picture of a black hole, ever. The first one was the supermassive black hole at the center of the Messier 87, about 55 million light years away from Earth. Sagittarius A*, on the other hand, is only 27,000 light years away, but since it is 1,000 times smaller than M87*, it only appears slightly larger in the night sky. We managed to capture the image of M87’s black hole before ours due to the dust and gas between us and Sagittarius A*.
Over the past three decades, we have managed to observe the core of our galaxy, and detect the black hole. A video from the European Southern Observatory showed a collection of stars zipping around on different eccentric orbits at very high speeds - one star was going at the speed of 24 million meters per second, which is 8% the speed of light! These stars appeared to be orbiting something massive and compact, which we believe to be a supermassive black hole. Using the motion of the stars around it, we can infer that the black hole is about 4 million times the mass of the sun, but all crammed into one tiny point: the singularity.
Infrared light was used to penetrate the debris between us and the black hole. Radio telescopes were used to observe radio waves with a wavelength of 1.3 millimeters. When a source emits radio waves, they travel radially in all directions, but due to the distance, they are almost flat and parallel by the time they reach the Earth, forming plane waves. When we point a telescope directly at a radio source, the waves travel the same distance at the same time, causing them to be in phase, and they constructively interfere. When we move our telescope away from the source, some waves travel a bit further than others, and they are out of phase, so they destructively interfere, causing the signal to drop to zero.
However, the size of the black hole in the night sky is too tiny (as tiny as looking at a donut on the moon!), so there is no noticeable difference in the angular resolution as we observe the black hole. To create a sharp image, the drop in the signal needs to be as steep as possible, so peak intensity is achieved only when the telescope is aimed directly at the source and drops rapidly if it moves a tiny bit away. To increase the angular resolution, we will need a telescope the size of the Earth! Instead of this, we can use small individual radio telescopes that are separated by distances up to the Earth’s diameter. A network of telescopes across the globe observed Sagittarius A* at the same time, down to a femtosecond. To make an image, we need pairs of telescopes at all different orientations and distances apart, and each pair has to make a different interference pattern. By aligning these patterns, we are able to map the image we see now.
But what exactly are we looking at? If even light fails to escape a black hole, what is the shiny yellow ring in the image? Using the Schwarzschild radius, which is the gravitational radius of an object, we observe the light around the black hole. The matter orbiting the black hole, such as the stars we observed, orbit at or over 3 Schwarzschild radii. If any matter goes inside this orbit, it gets sucked in. Light however, can orbit at 1.6 Schwarzschild radii, since it has no mass. There is a sphere of photons orbiting the black hole (if you were standing in that orbit, you would be able to see the back of your head, as the photons would complete the orbit). If we want to observe the black hole at its closest, we use a light ray at 2.6 Schwarzschild radii. This light ray simply grazes the photon sphere at its closest approach and then goes off to infinity, without falling into orbit. Using various light rays aimed at different angles, all the bent light rays from the back of the black hole reach us, forming the image we see now.
The Event Horizon Telescope collaboration will soon be awarded a Nobel Prize in Physics. Since this is only the second picture of a black hole, the scientific world awaits more images and videos which might show the black hole churning objects up! With the help of the James Webb Space Telescope, we hope to unveil many more mysteries surrounding the beginning of the universe, and maybe, discover extraterrestrial life!
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