A black hole devours anything that enters its event horizon

Discover the amazing secrets of a black hole

Black holes are one of the many mysterious objects in the universe. There are millions, if not billions, of black holes scattered all over the universe. Many of us have heard about these terrifying and equally baffling objects. The matter is so densely packed inside a black hole that gravity becomes so immense that even the light can not escape it. Even with all the technological advancements, we still have many answered questions about the black holes, such as what happens to a body inside a black hole or do the black holes act as portals to another universe and so on. This article sheds some light on key concepts and theories of the black holes.

At the centre of every galaxy, there resides a black hole
CENTRE OF OUR MILKY WAY HAS A SUPERMASSIVE BLACK HOLE

What is a black hole??

Many of us assume that gravity is a force that attracts two bodies. For example, a stone falls back to the ground when thrown into the sky. Contrary to our notion, gravity is not a force. It is a curvature in spacetime—massive objects such as the earth curve space to look like it attracts things toward themselves. When an object’s mass goes up, the curvature in spacetime it creates also increases.
If you start piling up the matter in a small region, after a certain point, it becomes so massive that nothing, even the light, can escape it, which is called a black hole.
So a black hole can be defined as a region in spacetime with an escape velocity greater than the speed of light. Technically, no object can travel more than the speed of light; therefore, nothing can escape a black hole.

How to detect a black hole??

Well, scientists detect their presence indirectly. When the matter is sucked into a black hole, it emits x-rays, gamma rays, etc. This heating up of matter is due to the tremendous friction created while the object falls into a black hole. As a result, a hot, spiralling disc is created around the black hole called an accretion disc.
Also, when two black holes merge, they create cosmic gravitational waves and are observed by large observatories such as the LIGO. In
2015, LIGO detected the gravitational waves emanating from the merger of two stellar black holes, and the waveform of the signals matched with ones created numerically using relativity. In 2017, Rainer Weiss, Barry Barish and Kip Thorne won the Nobel prize in Physics for their valuable contribution to the LIGO detector and observation of gravitational waves.

How are black holes formed??

A black hole is formed when a star, several times more massive than our sun, runs out of fuel and collapses under its gravity. This phenomenon when gravity overcomes the electron degeneracy pressure, the outer layers get ejected, leaving the dense core behind. It is called a white dwarf star. The outer layers are ejected in a supernova explosion for more massive stars.

Check out my post on life cycle of a star: http://walktoinfinity.com/life-cycle-of-a-star/
If the star’s core is more than three solar masses, then the immense gravity will overcome the neutron degeneracy pressure, and it collapses further to form a stellar black hole.
Many accept the gravitational collapse of giant star results in forming a black hole. Also, some even predict that in the early stages of the universe formation, when there was enough heat and pressure, the gigantic dust clouds collapsed to form black holes. Such hypothetical black holes are called primordial black holes.

A black hole is formed at the final stage of the life cycle of a massive star

A BRIEF HISTORY OF BLACK HOLES

Albert Einstein’s general theory of relativity postulated that gravity bends spacetime. A German physicist named Karl Schwarzschild proposed solutions to Einstein’s field equations. Later, Johannes Droste, a student of Hendrik Lorentz, presented the same but revealed that at Schwarzschild’s radius, some values of Einstein’s equations became infinite. It was thought that a star with Schwarzschild’s radius might have contributed to this anomaly.
A few years later, Indian born scientist S. Chandrasekhar discovered that matter with a certain mass would overcome electron degeneracy pressure and showed no stable solutions to Einstein’s equations. In 1939, Robert Oppenheimer and Hartland Snyder proposed that a massive star might collapse under its gravity to form what we now refer to as black holes.
In the first half of the 20th century, many scientists were opposed to the idea of a black hole’s formation but were genuinely curious. Science journalist Ann Ewing first coined the term black hole in 1964.
In 1967, Jocelyn Bell Burnell discovered the first pulsar(A rapidly spinning neutron star with an enormously powerful magnetic field). A few years later, Stephen Hawking predicted that black holes emit radiation like a black body at a temperature proportional to the black hole’s specific gravity. This effect was later known as Hawking radiation. Black holes dissipate slowly by emitting this form of radiation. In 2019, scientists are able to capture first image of supermassive black hole in the centre of Messier 87 galaxy.

First image of a black hole taken by telescopes placed all around the globe
FIRST IMAGE OF A BLACK HOLE

What are the different types of black holes??

Black holes vary in size. They are few times to several million times massive than our sun. Schwarzschild defined a black hole as a body with a radius smaller than its Schwarzschild radius. Based on the mass of a black hole, they can be classified into three types.

STELLAR BLACK HOLES

Stellar black holes are formed after a supernova explosion, i.e., when a massive star collapses. When a core weighs more than three solar masses, it will overcome neutron degeneracy pressure to form a black hole.
However, according to Einstein’s theory of general relativity, black holes can be found of varying masses. Stellar black holes have higher average densities than Supermassive black holes.
Schwarzschild radius is defined by the equation

Rₛ = 2GM/c²

Schwarzschild radius is proportional to its mass, and therefore, we can conclude that the lesser the mass of a black hole, the greater the average density. So stellar black holes have more average density than supermassive black holes. Scientists have discovered the presence of numerous stellar black holes in the universe.

INTERMEDIATE BLACK HOLES

These types of black holes are a hundred to a few thousand times more massive than our sun. In 2015, two black holes merged to form a black hole that was almost 140 times more massive than our sun. Usually, intermediate black holes are not created by a star’s collapse but by merging two stellar black holes.

SUPERMASSIVE BLACK HOLES

When a stellar black hole engulfs massive objects such as nearby stars, planets, other matter, it grows in size and mass. These black holes are many millions to billions of times more massive than our sun. For example, the centre of our milky way galaxy has a supermassive black hole, Sagittarius A, 4.1 million times our solar masses.
Supermassive black holes have a lesser average density than stellar black holes.

What is the structure of a black hole??

Black holes often vary in size and mass. The enormous mass of the black hole is concentrated in a small region in space. Most black holes have a specific structure, from ISCO(Innermost Stable Circular Orbit) to singularity. They are among the most violent entities in our universe,

ISCO

According to general relativity, the Innermost Stable Circular Orbit is the smallest possible radius. A particle can revolve around a black hole without being sucked into the black hole.
Usually, for a non-spinning black hole, The ISCO radius is equivalent to three times the Schwarzschild radius. At the same time, the ISCO radius is approximately 4.5 times its Schwarzschild radius for an uncharged and rotating black hole.

ERGOSPHERE

The ergosphere is the region around a black hole where a particle experiences the inward dragging force of the black hole and gradually starts heading towards it. If a black hole is non-rotating, the particle will fall directly into it. For a spinning black hole, the angular momentum of the rotation causes a frame-dragging effect, and the particle will rotate in the same direction in which the black hole spins for a faraway observer. The particles can still escape the pull of the black hole as they are outside of the event horizon.
As the angular momentum is highest at the equator of a spinning body and gradually decreases when approaching its poles, the ergosphere assumes an oblate shape.

PHOTON SPHERE

Due to the immense gravitational pull of the black holes, the light is forced to rotate in orbit around the black hole. As this path is unstable, a photon is faced with two choices: to fall into or escape from the black hole. When a particle travels along the photon sphere, the centrifugal force it experiences is zero. For a non-spinning black hole(Schwarzschild black hole), the photon sphere radius is 1.5 times its Schwarzschild radius.

EVENT HORIZON

When a body approaches the event horizon, it will reach a point of no return. It is a boundary beyond which nothing, not even light, can escape. Technically, an object has to travel more than the speed of light to escape, which is not possible. Physics says any object can be turned into a black hole if its entire mass is compressed within a radius of the event horizon.

SINGULARITY

A singularity is a region in spacetime where the curvature becomes infinite. It is a point of no return. Gravitational pull is so extreme that nothing can escape it. The entire mass of a black hole is compressed to a single point until it has infinite density.
After crossing the event horizon, particles end up falling into a singularity. Imagine a man falls into a stellar black hole with the head facing the black hole, his head experiences higher gravitational pull than his legs until he gets stretched like a spaghetti. This phenomenon is called spaghettification.

The centre of a black hole is called a singularity
SINGULARITY IS A POINT OF NO RETURN

Do black holes die??

Nothing in this vast universe lasts forever, not even black holes. According to Hawking, black holes are not black bodies, so even they emit radiation. This radiation is called Hawking radiation. A black hole releases a tiny amount of radiation and slowly dissipates into nothingness. Well, for this to happen, it may take billions of billions of years. Because our universe is still 13.7 billion years old, we haven’t noticed the death of a black hole.
There’s a lot to know about black holes. With the advancements in technology, we will be able to unravel many mysteries of black holes in the near future.

How a black hole is formed?

A black hole devours anything that enters its event horizon

A black hole is formed when a star, several times more massive than our sun, runs out of fuel and collapses under its gravity. This phenomenon when gravity overcomes the electron degeneracy pressure, the outer layers get ejected, leaving the dense core behind. It is called a white dwarf star. The outer layers are ejected in a supernova explosion for more massive stars.

What is inside a black hole?

A black hole

A black hole doesn’t mean it is black. When enough matter is compressed into a point, overcoming degeneracy pressure, such that the density becomes infinite, a black hole is formed. A black hole devours everything in its path, starting from gas and dust to massive stars. No one knows for sure what’s inside a black hole.

Can a person survive in a black hole?

Singularity of massive black hole

Due to a black hole’s extremely high gravitational pull, nothing, not even light, can escape. The gravity is very high that even massive stars get sucked in. It is next to impossible for a person to survive the tidal forces of the event horizon.

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