The black holes are region in space-time with such a high gravitational potential that
even the light cannot escape. The gravitational field of a black hole is so strong that the escape velocity needed is greater than the speed of light. As even light cannot escape, the term ‘black’ hole was coined by physicist John Wheeler in 1967 – previously they had been known as ‘frozen stars’ – a term which failed to grip the imagination! Black holes are surrounded by the event horizon in which the escape velocity is higher than the speed of light. Black holes can characterized only by three parameters: the mass, the spin and the charge.

Black holes are divided, based on their mass, into supermassive black holes (SMBHs) and stellar black holes. Stellar black holes have masses up to 20 times more than the mass of the sun. There may be many, many stellar mass black holes in Earth’s galaxy. Earth’s galaxy is called the Milky Way. Stellar black holes are made when the center of a very big star falls in upon itself, or collapses. When this happens, it causes a supernova. A supernova is an exploding star that blasts part of the star into space.

The SMBHs have masses more than 1 million Suns together. Scientists have found proof that every large galaxy contains a supermassive black hole at its center. The supermassive black hole at the center of the Milky Way galaxy is called Sagittarius A. It has a mass equal to about 4 million suns and would fit inside a very large ball that could hold a few million Earths. The origin of SMBHs remains uncertain. One possible scenario that has been proposed is that supermassive black holes are formed from smaller intermediate mass black holes that merge together. However, they are hard to detect and only a few candidates have been found so far.

Intermediate-mass black holes is a class of black holes with mass 100 up to 100 thousands times the mass of the Sun. These medium-size objects are a long-sought missing link in black hole evolution. They are probably formed when large stars or stellar-mass black holes merge in the centers of globular clusters, so they tend to be obscured within a dense cluster of bright stars. Intermediate black holes usually aren’t active, so we also can’t identify them by their jets or intense x-ray emission. But they should be fairly common – it’s estimated that about 45,000 intermediate-mass black holes could be in the vicinity of our galaxy.

Black holes were predicted by Einstein’s theory of general relativity. A scientist called Karl Schwarzschild provided one of the earliest solutions to Einstein’s equations.

According to Schwarzschild’s solution, a gravitating object will collapse into a black hole if its radius is smaller than a particular distance, known as the ‘Schwarzschild radius’. This radius is calculated by finding the mass of the star before collapse and the radius it would collapse to.

For an object with the mass of our Sun, the Schwarzschild radius is approximately three kilometres, much smaller than the Sun’s current radius of about 700 000 kilometres. It is also significantly smaller than the radius to which the Sun will ultimately shrink after exhausting its nuclear fuel, which is several thousand kilometres. Only more-massive stars can collapse into black holes at the end of their lifetimes.

In 2019 the Event Horizon Telescope (EHT) collaboration released the first image ever recorded of a black hole (image below). Scientists observed the centre of Messier 87, a massive galaxy in the Virgo galaxy cluster. Messier 87 hosts a supermassive black hole with mass 6.5 billion times that of the Sun and is located about 55 million light-years from Earth.  Their analysis revealed a ring-like structure with a dark central region, the black hole’s shadow (see embedded image). Then, they compared their observations with predictions from computer simulation to conclude that the observed image matched extremely well the theoretical predictions!!

Observing a black hole was something thought to be impossible a generation ago. However, thanks to technological discoveries, ingenious algorithms and the collaboration of a large number of scientists, we now have a real, non-simulated image of a supermassive black hole!!