Black holes are regions of space with gravitational fields so intense that nothing, not even light, can escape from them. This extreme gravity occurs because a significant amount of mass is concentrated in a very small area, creating a singularity – a point where density and curvature of spacetime become infinite. They form from stellar collapses or grow over time to become supermassive black holes at the centres of galaxies.

While they cannot be directly observed they can be detected through their interaction with surrounding matter. Their presence is inferred through their gravitational effects, X-ray emissions, and gravitational waves.

Black holes are crucial for understanding fundamental physics, including general relativity and the nature of spacetime.

1. 1. Formation and Types

• • • Stellar-Mass Black Holes: Formed from the remnants of massive stars that have ended their life cycles. When such a star exhausts its nuclear fuel, it can undergo a supernova explosion, and if the core’s mass is sufficient, it collapses into a black hole, typically with masses ranging from about 5 to 20 times that of the Sun.
    • Supermassive Black Holes: Located at the centres of most galaxies, including the Milky Way, with masses ranging from hundreds of thousands to billions of times the mass of the Sun. Their formation is less understood, but they may grow by accumulating mass from surrounding matter and merging with other black holes.
    • Primordial Black Holes: Hypothetical black holes that could have formed immediately after the Big Bang, with a wide range of possible masses.
    • Intermediate-Mass Black Holes: Hypothetical black holes with masses between stellar-mass and supermassive black holes. Their existence is still debated, but they may form through the merging of smaller black holes.
    • Micro Black Holes: Theoretical tiny black holes with masses much less than stellar black holes. They are predicted to evaporate quickly via Hawking radiation.

 

1. 2. Key Properties

• • • Event Horizon: The boundary surrounding a black hole beyond which nothing can escape. The event horizon is not a physical surface but a theoretical boundary where the escape velocity equals the speed of light.
    • Singularity: At the core of a black hole, where matter is thought to be infinitely dense where the curvature of spacetime is extreme and the laws of physics as we currently understand them break down.
    • Accretion Disk: A disk of gas and dust that forms around a black hole as material falls into it. Friction in the accretion disk heats the material, emitting radiation that can be observed.

 

1. 3. Detection and Observation

• • • Gravitational Effects: Black holes can be detected by observing their gravitational influence on nearby objects, such as stars or gas clouds.
    • X-ray Emission: As matter falls into a black hole and heats up, it emits X-rays that can be detected by telescopes.
    • Gravitational Waves: The collision and merger of black holes produce ripples in spacetime known as gravitational waves, which can be detected by observatories like LIGO and Virgo.

 

1. 4. Notable Examples

• • • Sagittarius A: The supermassive black hole at the centre of the Milky Way Galaxy.
    • M87: A supermassive black hole in the galaxy M87, famously imaged by the Event Horizon Telescope in 2019.
    • Cygnus X-1: A well-known stellar-mass black hole in the Cygnus constellation, identified through its X-ray emissions.

 

1. 5. Theoretical Implications

• • • General Relativity: Black holes are a key prediction of Einstein’s theory of general relativity, which describes how gravity affects the fabric of spacetime.
    • Information Paradox: A theoretical dilemma concerning what happens to information when it falls into a black hole, with ongoing debates about whether it is lost or preserved in some form.