Introduction to Black Holes

What is a Black Hole ?

Briefly, a Black Hole is nothing but an empty space that is a great amount of matter packed into a very small area where the gravitational field of a black hole is so strong that nothing – even light – can escape.

Discovery of Black Holes

An English amateur scientist and cleric named John Michell showed that Newton’s law of gravity suggested such objects could exist, in 1783. Yet, Michell suggested that despite being invisible, such objects might reveal themselves if they have a star in orbit about them.

During the 1930s, theorists using Einstein’s more sophisticated theory of gravity (a.k.a General Relativity) showed that sufficiently massive stars could collapse under their own gravity at the end of their life, and turn into black holes (actually Einstein himself never accepted that such strange objects could really exist).

In the early 1970s, British astronomers (L. Webster & P. Murdin) announced the discovery of a massive but invisible object in orbit around a blue star over 6,000 light-years away. The object, an intense X-ray source codenamed Cygnus X-1, is now regarded as the first black hole to be identified.

Today, scientists can’t directly observe black holes with telescopes that detect x-rays, light, or other forms of electromagnetic radiation. However, they can infer the presence of black holes and study them by detecting their effect on other matter(s) nearby. For instance, if a black hole passes through a cloud of interstellar matter, it will draw matter inward in a process known as accretion or similar process can occur if a normal star passes close to a black hole when the black hole can tear the star apart as it pulls it toward itself. As the attracted matter accelerates and heats up, it emits X-rays that radiate into space. 

Recent researches offer that the effect on neighborhoods around a black hole devour nearby stars by emitting powerful γ- ray bursts and spurring the growth of new stars in some areas while stalling it in others.

How A Star Actually Die ?

The formation of a star may be written as an another article but basically, stars are formed in clouds of gas and dust named as nebulae. Nuclear reactions at the core of stars provides enough energy to make them shine brightly for many years. The exact lifetime of a star depends on its size. Massive (large) stars burn their fuel much faster than smaller stars and may only exist a few hundred thousand years. Smaller stars (e.g. Sun), however, will last for several billion years, because they burn their fuel much more slowly.

The fate of a star depends on its mass but simply they die because they drain their nuclear fuel. Red Giants, from average stars become planetary nebula and at the end they become white dwarfs. Red Supergiants, from massive stars- becomes supernovas and due to their mass they may become neutron stars or black holes i.e when a massive star dies, it leaves behind a small, dense remnant core. If the core’s mass is more than about three times the mass of the Sun, the force of gravity overwhelms all other forces and produces a black hole.

Life After Death

Theoretically, when the total mass of the star is large enough (about three times the mass of the Sun), no force can keep the star from collapsing under the influence of gravity. Yet, there is fascinating process that happens when a star becomes a black hole. As the surface of the star nears an imaginary surface (event horizon) time on the star slows relative to the time kept by observers far away. When the surface reaches the event horizon, time stands still and the star can collapse no more i.e it becomes a frozen collapsing object.

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Another long-standing mystery about black holes, they appear to exist on two radically different scale. There are countless black holes that are the remnants of massive stars. Throughout the Universe, these “stellar mass” black holes are generally 10 to 24 times as massive as the Sun. Most of these stellar black holes lead isolated lives and are impossible to detect. Judging from the number of stars large enough to produce such black holes, scientists estimate that there are as many as ten million to a billion such black holes in the Milky Way alone.

The other size spectrum are the giants known as “supermassive” black holes, which are millions, if not billions 🙂 , of times as massive as the Sun. Astronomers believe that supermassive black holes lie at the center of virtually all large galaxies, even our own Milky Way – can detect them by watching for their effects on nearby stars and gas -.

Astronomers, have long believed that no mid-sized black holes exist.  However, recent evidence from Chandra, XMM-Newton and Hubble strengthens the case that mid-size black holes do exist. Another possible mechanism for the formation of supermassive black holes involves a chain reaction of collisions of stars in compact star clusters that results in the build-up of extremely massive stars. The star clusters then sink to the center of the galaxy, where the intermediate-mass black holes merge to form a supermassive black hole.

TBC, O.S. Tapsin

Resources: https://science.nasa.gov/astrophysics/focus-areas/black-holes — https://www.sciencefocus.com/space/who-really-discovered-black-holes/ — https://www.sciencefocus.com/space/how-do-stars-die/ — https://www.schoolsobservatory.org/learn/astro/stars/cycle —