What is a Black Hole? and History of Balck Hole

Black Hole

What is a black hole? The name might fool you into thinking’ black hole’ as an empty space, but it’s not. Rather, it has a lot of matter packed into a small space. Imagine a star ten times bigger than the Sun shoved into a sphere with a diameter approximately equal to that of New York City. The gravitational field in such a confined space is so intense that nothing can escape it. 

The notion of a massive and dense object in space and light cannot escape it has existed for centuries. The black holes were most famously talked about by Einstein’s general relativity theory. It showed that when a big star disintegrates, it leaves behind a dense and small remnant core. If the mass of this core is more than three times the mass of the Sun, the gravitational force predominates all other forces and creates a black hole Sun. 

The black holes can not be directly observed by scientists using telescopes that detect light, X-rays, or other forms of electromagnetic radiation. However, the presence of black holes can be inferred and studied by detecting their effect on the nearby matter. For example, if a black hole crosses a cloud of interstellar matter, it will attract matter inward by a process called accretion. The same happens if a normal star passes close to a black hole Sun. Here, the star will be torn apart by the black hole as it tries to pull it toward itself. As the matter attracted towards the black hole accelerates and its temperature increases, it radiates X-rays into space. 

Recent discoveries have shown the black hole Sun has a dramatic influence on its surrounding neighborhoods. They emit powerful bursts of gamma rays devouring the nearby stars and stimulating new stars to grow in some areas while stalling the growth of stars in other areas.

One Star’s End is a Black Hole’s Beginning

The majority of black holes are created from the remnants of the death of a large star in a supernova explosion. If the total mass of the star is about three times the total mass of the Sun, nothing can prevent it from collapsing under the influence of the gravitational force. A strange thing occurs as the star collapses. As the star’s surface comes closer to the event horizon, an imaginary surface, the time on the star slows down compared to the time kept by observers far away. As the surface approaches the event horizon, time pauses, and the stars can no longer collapse. It becomes a frozen collapsing body.  

Stellar collisions can give rise to even bigger black holes. Soon after NASA’s Swift telescope was launched in December 2004, it observed the powerful light flashes called gamma-ray bursts. NASA’s Hubble Space and Chandra’s telescopes later gathered information from the event’s “afterglow,” and the data led astronomers to conclude that the collision of a black hole and a neutron can result in powerful explosions, giving rise to another black hole. 

Some black holes have been found to have non-stellar origins. Astronomers believe that huge volumes of interstellar gas accumulate and collapse to form supermassive black holes. These black holes are located at the centers of galaxies and quasars. A gas mass rapidly falling into a black hole is speculated to give off about a hundred times more energy than an equal amount of mass via nuclear fusion. One such non-stellar black hole is the Sagittarius A, which lies at the Milky Way Galaxy’s center. This black hole is super massive, with a mass equivalent to more than 4,000,000 Suns. 

The English astrophysicist Stephen Hawking proposed that another kind of non-stellar black hole exists. According to his theory, several small primordial black holes with masses equal to or less than that of asteroids might have been formed during the Big Bang. The Big Bang was a state in which the universe was formed 13.8 billion years ago. The temperatures and density of the universe were extremely high in this state. These small black holes are believed to lose mass over time by a mechanism called Hawking radiation and ultimately disappear. 

Babies and Giants

Although the fundamental process of the formation of black holes is fairly understood, an eternal mystery in the science of black holes is that their sizes differ radically. There are countless black holes that are roughly 10 to 24 times more massive than the Sun. These are the remnants of massive stars and are spotted by astronomers when another star gets close enough for some matter around it to be pulled by the gravitational force of the black hole. However, most black holes of this type are difficult to detect. As the number of stars massive enough to produce such black holes is high, it is estimated that the Milky Way alone has l around ten million to a billion such black holes. 

On the other hand, there are these giants on the other end of the size spectrum. They are called “supermassive” black holes, as they are millions of times more massive than the Sun. It is believed that such types of black holes lie at the center of all large galaxies, including the Milky Way. These black holes can be detected by astronomers by observing their effects on the surrounding gas and stars. 

Astronomers have historically been believing that there are no existing mid-sized black holes. Recent evidence from XMM-Newton, Chandra, and Hubble, however, shows that mid-sized black holes exist. When supermassive black holes are formed, a chain reaction of star collisions in compact clusters of stars occurs. This results in the accumulation of very massive stars that later collapse, forming mid-massed black holes. The clusters of stars then drop to the galaxy’s center, where the mid-massed black holes combine to form a supermassive black hole sun.

History of Black Holes

Black Holes were discovered well before the 20th century. In 1784, hundreds of years after Newton gave the theory of gravitation, a British astronomer called John Michell put forward the possibility that the gravitational force of a massive star can be so intense that nothing could escape it, not even light. 

The correct explanation of a black hole sun can be given by the general theory of relativity by Einstein. Based on this theory, in 1916, Karl Schwarzschild found a solution for equations formulated by Einstein regarding black holes. Some years later, a few other physicists, including Robert Oppenheimer, gave more detailed calculations. The name “black hole” was coined by John Wheeler. 

Seeing Black Holes

Black holes are very compact and do not emit visible light. So, how can we observe what a black hole is? Well, there are some tricks. Stellar black holes form part of a binary system where two stars revolve around each other. From Earth, we can see a visible star revolving around nothing (it appears to be nothing). In actuality, the star is revolving around a black hole. The mass of the black hole can be inferred by observing how the visible star revolves around it. The gravitational force is more intense for massive black holes, which puts a greater effect on the visible star. 

Another way of seeing what a black hole is: by assessing how x-rays are formed in its surroundings. As black holes have very intense gravitational forces, a black hole in a binary system can literally tear its companion star apart. The gas from the torn-apart star swirls into the black hole and can be compared to how water goes down a drain. This swirling gas is known as an accretion disk. This gas heats up due to the friction of the fast-moving gas molecules as it gets closer to the black hole sun. The temperature of the gas outside the event horizon of the black hole can go upto millions of degrees and release x-rays. 

Conclusion

Many questions regarding black holes, like how the material directly goes into the black hole, why only some black holes have jets, and what are the exact masses and spins of black holes are still unanswered. Hopefully, the future will bring answers to such questions and more to help us understand our universe better. 

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