The Ultimate guide to understanding black holes

Updated: Dec 19, 2020

Black holes, they are one of the fascinating discoveries of modern physics. It is hard to come across something mind-bending, stunning and terrifying at the same time! They have triumphantly entered the mythos of science fiction; writers and creators cannot get enough of them!

The internet has a ton of questions about black holes; 'Is it safe to jump into a black hole?' is one of the most searched. I have tried to answer a few questions myself.

Here is what we would cover,

  • History of black holes

  • What are black holes?

  • How are black holes formed?

  • Anatomy of a black hole

  • Types of black holes

  • What black holes eat?

  • How black holes die?

Excited?! Let's jump right in.

A brief history of black holes:

We took a picture of a black hole pretty recently; that is in 2019, but the discussion about these celestial monsters started way before that. It was back in 1783 John Michell, English natural philosopher and priest, speculated the existence of black holes; he called them frozen-stars. He thought if we have a large enough star, it should have enough gravity to trap light itself from escaping; which is astonishing for a person only used to Newtonian gravity!

In 1915, after ten years of hard labour, Einstein perfected the most elegant, complex and beautiful theory in the history of physics, general relativity. Einstein's equations are till date quite hard to solve, yet the equations were begging for someone to take on the cumbersome task.

In 1916, the astrophysicist Karl Schwarzschild found himself mesmerized by Einstein's equations and solved them. The results were stunning to Schwarzschild and Einstein himself. The solution directly pointed to objects what we call today as 'black holes'.

American Physicist John Wheeler coined the term 'black holes'; the name stuck.

What are black holes?

We keep on hearing the term 'black holes' discussed in NEWS at least once a month recently; what are they?

A black hole is a region of space where an enormous amount of matter crams into a tiny area resulting in extreme gravity; so strong that nothing, not even light—can escape from it. It means mass with enough density can distort space to form a black hole.

If you take earth and crush it down to the size of a soccer ball, you would end up with a black hole.

16x9 GIF with 360-degree rotation and a pause when the view is almost edge on. Credit: NASA’s Goddard Space Flight Center/Jeremy Schnittman
16x9 GIF with 360-degree rotation and a pause when the view is almost edge on. Credit: NASA’s Goddard Space Flight Center/Jeremy Schnittman

How are black holes formed?

The most commonly occurring black holes result from dying stars. As we learnt in the life cycle of stars, stars eventually burn out their fuel and stop shining. Usually, stellar deaths are spectacular events resulting in a planetary nebula or supernova explosions. But when stars with a final mass in the range 2 to 3 solar masses ultimately collapse to form a black hole. We know gravity can do pretty weird things, black holes result from extreme gravity!

Anatomy of a black hole:

Any black hole has three essential components: a central singularity, an event horizon and an accretion disk.

The singularity is the central eye of the black hole. It holds a gravitational singularity in which contains an enormous mass concentrated in an infinitely small space.

The central singularity of a black hole is exotic, it is nothing like we have ever seen, experienced or we know of; all the laws of physics we love and cherish breaks down at the singularity of a black hole.

A black hole is an astronomical object with a gravitational pull so strong that nothing, not even light, can escape it
Anatomy of a black hole

The event horizon is the disk that marks the radius of a black hole a. k. a Schwarzschild radius. It forms when light captured by the gravity of the black hole cannot get away and stays hovering at the edge. Beyond the event horizon lies the point of no return; once you cross the event horizon of a black hole, your fall into the singularity becomes inevitable.

Stuff, such as gas, dust and other stellar wrecks that have come very close to a black hole but haven't slipped into it, creates a pancake-looking disk of matter whirling around the event horizon; we call this the accretion disk.

“The ultimate extreme level of a moment in time is the singularity of a black hole.”
Khalid Masood

Types of black holes:

There are four types of black holes classified based on their mass: stellar, intermediate, supermassive, and miniature.

  • Stellar black holes are the most commonly occurring black holes; they form because of stellar death. These are usually three or four times the mass of our sun.

  • Intermediate black holes contain 100 to 1000 times the mass of our sun. A single star could never form such an enormous black hole; scientists speculate they while two black holes collide and merge. We haven't figured out their formation yet.

  • We find supermassive black holes at the heart of galaxies. They usually contain mass a million to billion times the sun. The black hole at the centre of the Milky way weighs 4 million solar masses.

  • Miniature black holes are hypothetical and said to have formed during the big bang and later evaporated. We do not have any proof of their existence, yet.

What black holes eat?

Black holes are not essentially these cosmic vacuum machines, but they swallow stuff that wanders too close. But when they eat they get fat; the diameter of a black hole is proportional to its mass. If a black hole swallows three times its mass, its diameter grows three times.

In Binary star systems, if one star is enormous enough to form a black hole; the other eventually gets eaten. The star that collapses into a black hole doesn't immediately swallow it up, but it waits like a predator that waits for its prey. When the other star burns up its fuel and expands into a red giant; it gets peeled and eaten layer by layer. It doesn't get spookier than this!

How black holes die?

Black holes don't just suck in everything and live happily ever after; they eventually lose their mass as radiation known as Hawking's radiation.

To understand how this works, we have to understand space first. Space is not a vast emptiness as we think it is; it is teeming with particles popping in and out of existence. These particles aren't real; they are virtual particles. Particles form out of thin air in space all the time, but they annihilate soon after. Cool, right?!

When this happens at the edge of a black hole, one particle gets drawn into the black hole, and the other will escape. Except, the one that departs becomes a real particle, the one that falls in doesn't. So the black hole is losing energy in the form of radiation. This black hole evaporation happens incredibly slowly at first and gets faster as the black hole becomes smaller.

When it has the mass of a mountain, it radiates with about the heat of our sun.

In the last moment of its life, the black hole radiates away with the energy of billions of nuclear bombs diffused at once in a tremendous explosion.

But this process is exceedingly slow; the black holes we know might take up a googol year (a googol year is equivalent to ten raised to the power of a hundred years) to evaporate.

It takes so long that when the last black hole radiates away, nobody will be around to witness it.

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