**The first-ever image of a Black Hole was taken by the Event Horizon Telescope (EHT) in 2017.**
Credits – European Southern Observatory (ESO)/Source /CC BY 4.0

Table of Contents

Introduction

Many curious people are fascinated by strange astronomical objects in space called Black Holes. And they have a lot of reasons to be interested since a Black hole is still a mysterious object even for the most modern science and technological tools.

Initially a vague theoretical construct, modern science has allowed us to take a photo of it. Some of the greatest minds in Physics have worked on Black Holes. Let us look into some of the fascinating details about Black Holes.

What is the history behind Black Holes?

Pre-General theory of Relativity

Astronomical pioneer John Michell briefly proposed the idea of a massive body that even light could not escape in a letter published in November 1784.

Same density as the Sun.
Star’s diameter exceeds the Sun’s by a factor of 500.
the surface escape velocity exceeds the usual speed of light.
Non radiating bodies.
They are detectable through their gravitational effect on nearby visible bodies.

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Post-General theory of Relativity (1915) history of black holes Timeline

German physicist Karl Schwarzschild, Physicist who first found discovered the black hole theoretically. — **German physicist Karl Schwarzschild.**

Schwarzschild, Johannes Droste, a student of Hendrik Lorentz, independently gave the same solution for the point mass in Einstein’s field equation.

This solution indicates Schwarzschild radius, where it became singular, means some of the terms in the Einstein equations became infinite.

Subrahmanyan Chandrasekhar (1931) calculated, using Special Relativity a non-rotating body of electron-degenerate matter above a certain limiting mass has no stable solution, which is presently called Chandrasekhar limit at 1.4 M_☉.

In 1939, Oppenheimer and his co-authors interpreted the singularity at the boundary of the Schwarzschild radius, the boundary in which time stopped.

In 1958, David Finkelstein identified the Schwarzschild surface as an event horizon.

In 1963, Roy Kerr found the exact solution for a rotating black hole.

Ezra Newman found the asymmetric solution for a black hole that is both rotating and electrically charged.

Stephan Hawking, in 1974, showed that quantum field theory implies that black holes should radiate like a black body now; it is called Hawking radiation.

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What is the definition of a Black Hole?

A black hole is a region of spacetime exhibiting gravitational acceleration so strong that nothing, no particles, or even electromagnetic radiation such as light can escape from it.

How do Black Holes form and evolve?

Gravitational collapse

Gravitational collapse occurs when the star’s internal pressure is insufficient to resist its gravity.

Reasons

Star has little fuel left to maintain its temperature through stellar nuclear synthesis, or a star would receive extra matter in a way that does not raise its core temperature.
The condensation of matter into an exotic dense state.
The gravitational collapse of heavy stars is assumed to be responsible for stellar-mass black hole formation.
A star with a mass greater than 20 times the mass of our Sun may produce a black hole at the end of its life.

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What is a Primordial Black Hole?

Stephen Hawking. Genius physicist. — **British physicist Stephen Hawking.**
Credits – elhombredenegro/CC BY 2.0/Source

Primordial black holes are a hypothetical type of Black holes that formed soon after the Big bang.

In the early universe, shortly after the Big bang, densities are much higher, creating a situation for the possible formation of a Black hole.

Their masses can be far below stellar mass. Hawking calculated that primordial black holes could weigh as little as 10⁻⁸ kg.

Can a high-energy collision produce a Black Hole?

The Large Hadron Collider at CERN where high energy collisions take place, where the micro black hole could potentially form and evaporate. A paper published in 2010 by Choptuik and Pretorius in Physical Review Letters, presented a computer-generated proof that micro black holes must form from two colliding particles in the energy range of LHC operation if additional dimensions are present other than the customary four. — The Large Hadron Collider at CERN where high energy collisions take place, where the micro black hole could potentially form and evaporate. A paper published in 2010 by Choptuik and Pretorius in Physical Review Letters, presented computer-generated proof that micro black holes must form from two colliding particles in the energy range of LHC operation if additional dimensions are present other than the customary four. Credits – Chris Mitchell/CC BY-SA 4.0/Source

By high energy collision, it that should achieve sufficient density.

Einstein’s theory of general relativity predicts that a black hole can be made by particle collision.

By computer simulation, If the two particles collide with a total energy of about one-third of the Planck energy*

No such events are observed directly or indirectly

* Choptuik and Pretorius Physical Review Letters.

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How are Black Holes originated?

After the formation of the Blackhole, it will continue to grow by absorbing additional matter like gas and interstellar dust from its surrounding; this is the probable reason for the formation of Supermassive Black holes and Intermediate Black holes.

Black holes can also merge with other stars or even other Black holes.

**This artist’s impression shows a possible seed for the formation of a supermassive black hole.**
Credits – NASA/CXC/M. Weiss/CC BY 4.0/Source

Do Black Holes evaporate?

In 1947, Hawking predicted that a Black hole emits a small amount of thermal radiation by applying quantum field theory at a temperature ℏc³/(8 π G M k_B); this effect became Hawking radiation.

A supermassive black hole in the constellation Centaurus emitting jets and radiation. Credits – ESO/WFI — **A supermassive black hole in the constellation Centaurus emitting jets and radiation.** Credits – ESO/WFI. Source. CC BY 4.0.

What is the structure of a Black Hole?

All the regions inside a black hole. Starting from the singularity. — **Structure of the Black Hole.**
Credit: Ask The Van / UIUC Physics Department

1. Quiet Region: negligible gravitational Influence:

2. Ergo sphere:

3. Event horizon:

4. Gravitation space-time distortion:

5. Singularity:

6. Photon sphere:

7. Innermost stable circular orbit (ISCO)

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What are the types of Black Holes?

Black holes are commonly classified according to their mass, and it does not depend on angular momentum, whereas Schwartzchild radius depends on mass.

CLASS	APPRX MASS	APPRX RADIUS
*Super massive black hole* *(SMBH)*	10⁵–10¹⁰ M_Sun	0.001–400 AU
*Intermediate-mass black hole (IMBH)*	10³ M_Sun	10³ km ≈ R_Earth
*Stellar black hole*	10 M_Sun	30 km
*Micro black hole*	up to M_Moon	up to 0.1 mm

When was the first Black Hole observed?

The first direct visual image of a black hole in Messier 87, a supergiant elliptical galaxy in the constellation Virgo taken by the Event Horizon Telescope (EHT). — **The first direct visual image of a black hole in Messier 87, a supergiant elliptical galaxy in the constellation Virgo.** Credits – Event Horizon Telescope. CC BY 4.0, Source

The first image of the Black Hole was taken by astronomers using indirect observations using the Event Horizon Telescope (EHT). In April 2017, data was collected, and after two years of processing, in April 2019, the first image of the Supermassive Black hole was released. The supermassive black hole (SMBH) at the core of supergiant elliptical galaxy Messier 87, with a mass ~7 billion times the Sun’s.

Can the gravitational waves from the merging Black hole be detected?

On 14 September 2015, the LIGO gravitational observatory made the first-ever successful direct observation of gravitational waves which are produced by the merger of two Black holes.

Black holes colliding, gravitational wave generation. — **Collision and merger of two black holes, resulting in the first detection of gravitational waves, GW150914, by LIGO.** Credits – Simulating eXtreme Spacetimes Lensing (SXS). CC BY 4.0. Source.

Is there a Black Hole in the center of our galaxy?

Sagittarius A* is a bright and compact astronomical radio source at the center of the Milky Way. Since 1995, astronomers have tracked the motions of 90 stars orbiting an invisible object coincident with the radio source Sagittarius A*. It is around 4.3 million M_☉.

Supermassive black hole in the center of our galaxy. — **A simulation of the central massive Sagittarius A* black hole by the European Space Observatory.** Credits – Event Horizon Telescope project • CC BY-SA 4.0 • Source

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What are X-ray binaries?

X-ray binaries are a class of binary stars that are luminous in X-rays. The X-rays are produced by matter falling from one component, called the donor, to the other component, called the accretor, which is very compact: a neutron star or black hole.

X-ray binary stars. X-ray binaries are a class of binary stars that are luminous in X-rays. The X-rays are produced by matter falling from one component, called the donor, to the other component, called the accretor, which is very compact: a neutron star or black hole. — **Artist’s impression of an X-ray Binary.**
Credits – ESA, NASA, and Felix Mirabel • Public Domain • Source

References

Basu, B., Chattopadhyay, T., & Biswas, S. N. (2011). An Introduction to Astrophysics (2^nd ed.). PHI Learning.
NASA Citizen Scientists. (n.d.). Black holes. Nasa.Gov. Retrieved June 12, 2022, from https://science.nasa.gov/astrophysics/focus-areas/black-holes
What is LIGO? (n.d.). LIGO Lab | Caltech. Retrieved June 12, 2022, from https://www.ligo.caltech.edu/page/what-is-ligo
Canizares, C., Schulz, N. S., Marshall, H. L., Houck, J., Dewey, D., & The MIT/Chandra Science Binary Team. (n.d.). Norberts’s X-ray Binaries Page. MIT Kavli Institute for Astrophysics and Space Research. Retrieved June 12, 2022, from https://space.mit.edu/~nss/binaries.html

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