What is a Black Hole? An introduction to one of the most mysterious object in the universe

Space is fascinating and among some of the strangest and most fascinating objects in the space, are black holes. Black holes are not holes in space but they are extremely dense objects, formed when a large amount of matter is compressed into a very small area. The gravity of these holes is so strong that not even light can escape once it crosses the boundary known as the event horizon. "It’s a boundary, not a surface," NASA explains, "and it marks the point of no return."

If we talk about the discovery, the concept of black holes was first predicted by Albert Einstein through his general theory of relativity in 1916. However, the exact solution describing an object like such was found a year earlier by Karl Schwarzschild in 1915. The term 'black hole' was later coined in 1967 by American physicist named John Wheeler.

The first black hole ever found was Cygnus X-1 which was discovered in 1964 when a sounding rocket detected X-rays from a celestial source. Another distinct fact about these holes is that they cannot be seen directly, but their existence is inferred from their effects on surrounding matter and light.

According to various studies, there are two known formation pathways for black holes, stellar-mass black holes form when massive stars, those at least 8 to 10 times the mass of the Sun, exhaust their fuel and collapse after a supernova. The other type is called a Direct collapse black hole. These form from collapsing gas clouds, producing much more massive black holes in the early universe.

At the centre of our galaxy lies Sagittarius A, which is a supermassive black hole estimated to be approximately 4 million times the mass of the Sun. It is located 26,000 light-years from Earth. The existence of Sagittarius A was confirmed by tracking the orbits of nearby stars, a discovery that also earned the 2020 Nobel Prize in Physics.

Though black holes do not emit light, they are detected by observing these three things: Accretion disks which are hot rings of gas and dust emitting X-rays, Gravitational lensing which means the bending of light from distant stars and Gravitational waves that ripples in spacetime caused by merging black holes. In 2019, the Event Horizon Telescope captured the first image of a black hole at the centre of galaxy M87, 55 million light-years away.

Stephen Hawking theorised that black holes could emit radiation. This finding is now known as Hawking radiation, slowly losing mass over time. This evaporation process is expected to take longer than the current age of the universe.

Closest: Gaia BH1, just 1,500 light-years away | Farthest: QSO J0313–1806, 13 billion light-years from Earth | Largest: TON 618, estimated at 66 billion solar masses | Smallest: A black hole just 3.8 times the Sun’s mass | Fastest spinner: GRS 1915+105 spins over 1,000 times per second | ‘Spaghettification’: It is a term describing how objects stretch and compress near a black hole’s event horizon.

Despite the science fiction portrayals, Black holes differ from wormholes, which are portals to other dimensions. They do not suck everything in like vacuum cleaners. From a safe distance, their own gravity acts like that of any other object of similar mass in the space.