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Black Holes

Updated: Oct 8, 2023

Author: Kavya Pullanoor

Many people often mistake black holes as empty space. Remember- black holes are anything but empty space. All of us are familiar with what they basically are. If not, read further to get a brief idea regarding the existence of one of the most fascinating objects in astrophysics.

What are black holes?

Imagine an object ten times the size of the sun squeezed into a sphere approximately the diameter of New York City. Could you imagine the gargantuan amount of energy that lies within this? This is what a black hole is. The gravitational field of a black hole is so strong that not even light can escape it.

Black holes are formed when massive stars die. These stars exhaust their internal thermonuclear fuels in their cores at the end of their lives. Due to this, the core becomes unstable and gravitationally collapses inward upon itself, and the outer layers of the star are blown away. The weight of all the constituent matter falling in from all directions compresses the star to a point of zero volume and infinite density called ‘singularity’.

Black holes can be formed only from the most massive stars, having more than three solar masses. Stars with a smaller amount of mass evolve into less compressed bodies, such as white dwarfs or neutron stars.

Why can’t even light pass through a black hole?

The structure of the black hole was drawn from Albert Einstein’s General Theory of Relativity. The singularity of the black hole constitutes its center which is covered by the ‘surface’ of the black hole, also known as ‘event horizon’. At and beyond the event horizon, the escape velocity exceeds the velocity of light, the reason due to which not even light can pass through a black hole.

Schwarzschild and Michell

In the late 1700s, a not-so-well-known scientist John Michell was the first person to conceive of the possibility of a gravitational mass so large that light could not escape from it, and was then able to come up with an estimate of how large such a body must be. Michell’s calculation did not produce the right answer as he was working with Newton’s laws, not Einstein’s, and the speed of light was not known to high accuracy at the time.

More than a century later, Karl Schwarzschild would be the first to correctly analyze the relation between the size of a black hole and its mass. Inspired by Einstein’s General Theory of Relativity, German physicist Karl Schwarzchild worked on this concept. His work revealed the limit at which gravity exceeds other physical forces to form a black hole. This value is called the ‘Schwarzchild radius’.

First picture of a black hole

Everyone believed black holes weren’t real, until April 2017, when a picture of a supermassive black hole at the center of M87, a galaxy 54 million light-years away, was taken after five days of observation using eight telescopes around the world by a collaboration known as the Event Horizon Telescope.

The picture of a black hole contains black matter surrounded by bright lights. This ‘black matter’ is the event horizon, having a gravitational field so strong that even light cannot pass through it, as mentioned above. Previously, researchers had captured a jet of light emerging from where the M87 black hole was predicted to be—but they couldn’t definitively see the black hole because their instruments were nowhere near as sharp as the Event Horizon Telescope. This black hole is about 6.5 billion times the mass of the sun.

To see the black hole’s boundary between light and dark, the astrophysicists captured radio waves 1.3 millimeters in wavelength, invisible to the human eye. These radio waves are emitted by the gas or the bright light swirling around the black hole. The researchers chose this particular wavelength because it can sail through entire galaxies and even Earth’s own atmosphere without being absorbed. But they still needed good weather at all eight of their telescope sites to see the black hole.

M87's black hole is relatively close to Earth, as the light coming from it was only emitted 54 million years ago. So we’re seeing it at a more mature moment in its existence.

It took two decades of work to capture the image. Part of that effort was designing, building, and hauling the hardware to various telescope sites. Everything they’ve observed so far about M87- its mass and the size of its event horizon, is consistent with Einstein's theory.

This image is just the beginning, says Feryal Özel, a collaborator of Event Horizon Telescope (EHT) that released the first image of a black hole. The team plans to take more, better-quality pictures of this black hole to understand it in more detail. Now that they’ve finally stared into the eyes of the beast, it’s time to watch how it behaves. Cheers to the team!

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