Maarg Manthan · Topic 9.4

Black Holes

Science & Technology › Physics – Modern Physics and Astrophysics · Topic 9.4

A black hole is a region of spacetime where gravity is so strong that nothing, not even light, can escape from it. The boundary beyond which escape is impossible is the event horizon, and at the centre general relativity predicts a singularity, a point of infinite density. Black holes were predicted from Einstein's general theory of relativity of 1915, and Karl Schwarzschild found the first solution in 1916. The name black hole was popularised by John Archibald Wheeler in the 1960s. Black holes are grouped by mass as stellar-mass, from a few to tens of times the mass of the Sun, intermediate-mass, and supermassive, with millions to billions of solar masses at the centres of galaxies. The first image of a black hole, in the galaxy M87, was released in 2019.

5 min read · UPSC Prelims · Science & Technology · Physics – Modern Physics and Astrophysics

At a Glance

Boundary Event horizon
Centre Singularity
First image M87, 2019
Our galaxy Sagittarius A*, imaged 2022
Hawking radiation 1974
Exam link UPSC Prelims, Science & Technology: Physics – Modern Physics and Astrophysics

Where Does It Fit?

This is the fourth topic of the macro on modern physics and astrophysics. It applies general relativity to the end stage of massive stars and leads to gravitational waves and neutron stars. Previous topic: Dark Matter and Dark Energy. Next topic: India-Based Neutrino Observatory.

How Do Black Holes Form and What Are Their Types?

Type	Details
Stellar-mass	Form when a very massive star runs out of fuel, explodes as a supernova, and its core collapses. Cygnus X-1 is an early example
Supermassive	Millions to billions of times the mass of the Sun, in the centres of most large galaxies. Sagittarius A* in the Milky Way has about 4 million solar masses
Intermediate-mass	Hundreds to hundreds of thousands of solar masses, harder to find. Some candidates have been identified in star clusters
Primordial	Hypothetical small black holes formed in the early universe
Formation of the supermassive kind	Still debated. Mergers of smaller black holes and rapid growth by gas are proposed
Mass limit for stars	A white dwarf above about 1.4 solar masses, the Chandrasekhar limit, cannot stay stable, as in Theories and Laws of Physics, Including the Theory of Relativity

What Are the Key Features and How Do We See Them?

Event horizon: the boundary of no return. Its radius for a non-rotating hole is the Schwarzschild radius, about 3 kilometres for each solar mass.
Singularity: the point at which matter is compressed to infinite density in classical theory, which signals the need for a theory of quantum gravity.
Accretion disc: gas spiralling in heats up and glows brightly in X-rays, which is how many black holes are found. Jets of particles can shoot out along the rotation axis.
Gravitational waves: merging black holes ripple spacetime, as in Gravitational Waves.
Event Horizon Telescope: a network of radio telescopes around the world that made the first image of a black hole in M87 in 2019 and of Sagittarius A* in 2022.
Hawking radiation: Stephen Hawking predicted in 1974 that black holes emit faint radiation and slowly lose mass, though it has not been observed.

What Else Should You Remember?

Escape velocity: the speed needed to escape the gravity of an object. At the event horizon it equals the speed of light.
Time near a black hole: time passes more slowly near strong gravity, as general relativity predicts.
Quasars: extremely bright centres of distant galaxies powered by supermassive black holes that are feeding.
White holes and wormholes: hypothetical objects allowed by some solutions of the equations, with no observational evidence.
Indian contributions: Subrahmanyan Chandrasekhar showed the mass limit for white dwarfs, and India’s AstroSat satellite, launched in 2015, has studied X-rays from black holes in binary systems.
Not cosmic vacuum cleaners: a black hole of the Sun’s mass would affect an orbiting planet like any object of equal mass, and it does not suck in everything.
Nobel Prizes: Roger Penrose, Reinhard Genzel and Andrea Ghez received the Nobel Prize in 2020 for work on black hole formation and the object at the centre of the Milky Way.
Related objects: neutron stars and magnetars are the other ends of massive star evolution, as in Sunspots, Magnetars and Neutron Stars.
Exam pattern: definition, types, features and the first image.
Spaghettification: an object falling in is stretched by tidal forces that grow rapidly near the hole, and the effect is gentler for a supermassive black hole than for a stellar one.
Photon sphere: light can orbit a black hole at a certain distance, and this ring of light shapes the bright edge seen in the Event Horizon Telescope images.

Why Does It Matter for the Exam?

Feature questions: event horizon, singularity and accretion disc.
Current affairs link: new images, mergers detected by LIGO and Nobel Prizes.
Concept questions: how stars die and what decides a black hole.
Indian link: Chandrasekhar’s work and AstroSat observations.
Common errors: believing that black holes suck in all matter or that light inside them can escape.

As an analytical point, black holes test the limits of physics, where general relativity and quantum mechanics must both apply. Observations now make them objects of direct study.

Why Is This Topic Important?

Facts Types and images asked
Concept Horizon and singularity
Link Gravitational waves

What Came Next?

The next topic covers the India-Based Neutrino Observatory. Next topic: India-Based Neutrino Observatory.

Key People and Terms

Event horizon Boundary of no escape
Singularity Point of infinite density
Accretion disc Gas swirling into a black hole
Schwarzschild radius Radius of the event horizon
Quasar Bright galactic nucleus
Hawking radiation Predicted faint emission

Exam Corner

Points to Remember

First image: M87, 2019.
Sagittarius A*: about 4 million Suns.
Hawking radiation: 1974.
Term popularised by Wheeler.
Chandrasekhar limit: 1.4 solar masses.

Do Not Confuse With

Event horizon and singularity Boundary versus centre
Stellar and supermassive Star collapse versus galactic centre
Black hole and neutron star No surface versus dense star with surface

Memory Hook

Horizon the border, singularity the core; stellar from supernova, supermassive in the galaxy’s heart; M87 imaged in nineteen.

Mains Angle

Questions ask about the formation and detection of black holes. These points add depth.

Formation Stellar death
Detection X-rays, waves, image
Theory Relativity and quantum gravity

A Question You May Face

An original practice question, not a past paper question.

“Explain how black holes form, how they are classified, and how astronomers detect and image them.”

How to Answer

Formation Collapse.
Types Three.
Detection X-ray, waves.
Image EHT.

GS Relevance

Prelims: features, types, formation, images, Hawking radiation, Indian links. GS Paper 3: science and technology.

Frequently Asked Questions

What is a black hole?

A black hole is a region of spacetime where gravity is so strong that nothing, not even light, can escape from it. It forms when a massive star collapses at the end of its life, has an event horizon as its boundary and, in classical theory, a singularity at the centre.

What is the event horizon?

The event horizon is the boundary around a black hole beyond which nothing can escape because the escape velocity there equals the speed of light. For a non-rotating black hole its radius, called the Schwarzschild radius, is about 3 kilometres for each solar mass.

How are black holes detected?

Black holes are detected indirectly by the X-rays from hot gas in their accretion discs, by the motion of stars orbiting them, and by gravitational waves from merging black holes, as detected by LIGO. In 2019 and 2022 the Event Horizon Telescope made images of the shadow of the black holes in M87 and the Milky Way.

What are the types of black holes?

Stellar-mass black holes of a few to tens of solar masses form from collapsed massive stars. Supermassive black holes of millions to billions of solar masses sit at the centres of galaxies, and intermediate-mass black holes lie between them and are harder to find.

What is Hawking radiation?

Hawking radiation is a faint thermal radiation that Stephen Hawking predicted in 1974 should be emitted by black holes because of quantum effects near the event horizon. It would cause them to lose mass slowly over a very long time, but it is far too faint to have been observed so far.

PYQ Practice — Statement Analysis

1 Nothing, not even light, can escape from within the event horizon.

True

It is the point of no return.

2 The first black hole image was of Sagittarius A*.

False

It was of M87 in 2019.

3 Supermassive black holes lie at the centres of galaxies.