What's on your mind when someone mentions a black hole? A deep and dark hole? Or is it a giant hole that can devour anything nearby?

Most people will immediately assume a black hole as a hungry whirl and able to devour whatever is around. Unfortunately, that assumption is not entirely true.

What is a Black Hole?

A black hole is an object whose gravity is so strong that its velocity is much greater than the speed of light. Oops, what does that mean?

What keeps us grounded on Earth? It is nothing but the force of gravity. But it turns out this gravitational force can be mastered to not hold a person stay on Earth. How to?

Try throwing rocks into the air. It will fly shortly before Earth's gravity slows its motion and pulls it back to the ground. Throw more powerful then it will move faster and higher before then fall back. But the stone could have escaped if it was thrown so strong that it could have enough speed to lose and Earth's gravity would not slow down and stop the stone's motion. In this condition, the stone thrown earlier will have enough speed to escape from Earth. How big is the speed?

On Earth, to be able to escape from the influence of gravity required a freelancing rate of 11 km/sec. Imagine how fast an object must move to get out of Earth's gravitational influence. Elsewhere the required speed will vary depending on the gravity at that location. The greater the force of gravity of an object, the greater the speed of its loose.

For example in the Sun. The Sun's mass is 330000 times larger than Earth and if we could walk on the surface of the Sun, its gravity would attract us 28 times stronger than Earth's gravity. Thus the Sun has a much larger off-Earth speed of 600 km/sec or 3000 times faster than the speed of jet aircraft.

Now, grab an object and squeeze it into small size or grab an object and add mass to it, then its gravity, as well as its loose velocity, will be even greater. If it continues to do, at one time you will have an object whose gravity is so large that its velocity is much greater than the speed of light.

The speed of light itself is the fastest limitation of the speed that exists in the universe that refers to the ability of light to move. If to loose required a greater freelance speed than the speed of light it can be said whatever is in the object will be stuck there forever. Even light will not escape. Like going into an endless tunnel: black hole!

How are Black Holes formed?

The most common way of forming a black hole in the universe is in a supernova explosion. This means that the black hole is the end of the life of the star. When a star with a mass of more than 25 solar masses ends its life, it will explode in a tremendous nuclear explosion. The outer portion ejected at high speed while the core of the star will collapse into a very compressed object. This compressed nucleus can then form a star rich in neutrons and is called a neutron star whose mass of 1 teaspoon of the material alone is 1 billion tonnes.

But if the mass of the falling star core is more than 3 solar masses, then the gravity of the collapsed star core will continue to put pressure on it until the object becomes more compressed and the gravitational field on its surface becomes stronger and eventually becomes a black hole. If this process is complete, the black hole that is formed will have mass several times the mass of the Sun. And the object is formed that is often found in the universe and is known as a star mass black hole.

But there is also a gigantic black hole like a monster, supermassive black hole. This one black hole usually lurks from the center of the galaxy and its size is gigantic. The mass is no longer several times the mass of the Sun but millions to billions of times greater. This black hole is thought to form together with the formation of the galaxy where it lives. But how is the process? This is still a mystery and a question mark. But it is estimated that this supermassive black hole is formed from the explosion of a very large star which then creates a black hole from its crumbling core. Furthermore, this black hole prey on more material including other black holes so that he can have a large mass. Remember, the gravity of a very powerful black hole attracts objects around it. Allegedly, a supermassive black hole is at the center of every galaxy including the Milky Way.

Another way for the formation of black holes is of two neutron stars orbiting each other and resulting in a detectable gamma-ray explosion from the entire observed universe. This gamma-ray burst is what casts the birth of a black hole.

Looking for Black Holes

Black hole, how does it look, does it a dark hole? Unfortunately not. Black holes are not a hole in space like the holes we know on Earth. It is an object that has a very strong gravity and even light cannot be separated from it. That means the light is trapped in it. That is again, these objects that do not emit light. Well, the next question how black holes can be found?

Although not visible, the black hole is not an unidentifiable "magic" thing. One thing is certain, astronomers never detect Black Holes. To be able to find the existence of a black hole, what is detected is a phenomenon that can only be explained by the existence of a close object that corresponds to the image of a black hole. This is the fingerprint of a black hole.

The very strong gravitational force of the black hole affects the motion of the object around it. If astronomers see a star that is circling something but can not see the object then it can be assumed that the invisible object is a black hole. In addition, when the material will be swallowed by a black hole, there is a very strong radiation explosion that can be seen by observers on Earth.

Astronomers can also know the mass of a black hole by measuring the mass of the surrounding star and its star velocity. The same calculations can also be applied to supermassive black holes peeping from the center of the galaxy.

At the center of the Milky Way, there is also a supermassive black hole. This fact is revealed by the behavior of stars and gases in the galactic center. The stars and also the gas around the center of the galaxy move very quickly, this behavior can only be explained by the existence of an object with a mass of several million times the mass of the Sun at the center of our galaxy. The mass must be centered within a 10-daylight radius or about 40 times the Sun-Pluto distance. And thus it can be sure it is a supermassive black hole object.

Black hole fingerprints can also be recognized from paired stars. When one star in the couple becomes a black hole, the two will still continue to orbit each other. By observing the system, astronomers will be able to know the orbits of normal stars and determine the mass of the black hole. But there are only a few systems like this. One other interesting thing is that astronomers have managed to find a sign that could bring astronomers toward a black hole. Is that?

X-ray.

When the black hole consumes the material that falls into it from the star pair, the material gets very hot and emits X-rays and gamma rays. This can then be a sign to indicate a source as a black hole. In fact, the area around the black hole is very good in terms of emitting high energy radiation like this. That's why astronomers then build spies in space that can see the radiation like the Chandra X-ray Observatory and NASA's Fermi Gamma-ray Space Telescope or XMM-Newton X-ray Space Observatory owned by ESA.

Reference :

1. https://www.essaytopics.com/v/237526/what-is-a-black-hole-essay/
2. http://www.astro.umd.edu/~miller/teaching/questions/blackholes.html
3. http://www.sciencemag.org/news/2017/09/top-stories-how-universe-s-biggest-black-holes-formed-and-why-bed-bugs-love-your-dirty
4. http://www.news.com.au/technology/science/space/hubble-spots-tragic-aftermath-of-titanic-black-hole-clash-in-galaxy-3c-186/news-story/86995f1129cc360a53ba868a5f525da1

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