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Apr 23, 2019

Black Hole

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By Allen Martis, USA [ Published Date: April 29, 2005 ]

Imagine throwing a tennis ball into the air. The harder you throw the tennis ball, the faster it travels when it leaves your hand and the higher the ball will go before turning back. If you throw it hard enough it will never return, the gravitational attraction will not be able to pull it back down. The velocity the ball must have to escape is known as the escape velocity and for any object to be thrown and not return back to ground due to the gravitational pull, it would have to leave the earth at about 7 miles per second.

As a body is crushed into a smaller and smaller volume, the gravitational attraction increases, and hence the escape velocity gets bigger. Things have to be thrown harder and harder to escape. Eventually a point is reached when even light, which travels at 186,000 miles per second, is not travelling fast enough to escape. At this point, nothing can get out as nothing can travel faster than light.  This point from which light can no longer escape from the black hole is called an Event Horizon. Nobody knows as to what really happens inside the event horizon. One needs to have a theory on quantum gravity to explain that. Classically, matter collapses to a point, called a singularity (infinite density), but what really happens is not known. The event horizon refers to the location from the black hole where the escape velocity equals the speed of light. In other words, no particle (even light) can escape from within the event horizon.  This is the concept behind a black hole.  A Black Hole is a region of space-time from which nothing can escape, even light. 

Black holes used to be called as "dark stars" or sometimes "frozen stars" before a physicist John Wheeler introduced us to the term "black hole" in 1967.  How did he come up with this name?  No one knows.  Until recently, black holes used to exist as a concept backed up by strong scientific understanding and complex mathematical formulas.  Mathematically, the size of the black hole is given by GM/c2 where G is the gravitational constant, M is the mass of the black hole and c is the speed of light.

When a large star has burnt all its fuel it explodes into a supernova. The stuff that is left collapses down to an extremely dense object known as a neutron star. We know that these objects exist because several have been found using radio telescopes.  If the neutron star is too large, the gravitational forces overwhelm the pressure gradients and collapse cannot be halted. The neutron star continues to shrink until it finally becomes a black hole. A supernova occurs in our galaxy once every 300 years, and in neighboring galaxies about 500 neutron stars have been identified. Therefore we are quite confident that there should also be some black holes.

Now let us try to understand how black holes are discovered in the universe.

1. Any body which is above absolute zero (-273 Celsius) radiates thermal energy, and the peak wavelength of emission depends on the temperature of the object. For example, the sun's surface is about 6000 Kelvin so that its peak emission is in green light. If an object's temperature is about a million degrees, then its peak emission will be in X-rays.

2. Normally stars are prevented from collapsing from gravity due to thermal gas pressure and radiation pressure. However, if the thermal energy source (nuclear fusion reactions) stop, then the star will collapse.  Astrophysicist Chandrasekar proved that there is a maximum mass beyond which nothing can beat gravity. So, if we detect a compact object in space which is more than this critical mass, then we can be confident that it is a black hole.

Consider a binary (binary = two) system of stars where one of the stars is a black hole and the other a normal star. If the normal star's envelope gets close enough to the black hole, then the fierce gravity of the black hole can rip out gas from the normal star which is then swallowed by the black hole.

However, due to the conservation of angular momentum, the gas cannot plunge straight into the black hole, but must orbit it for some time before it gets sucked. Thus, a disc like structure is formed around the black hole from which gas is pulled slowly into the black hole. When the gas orbits the black hole in the disc, its temperature is raised to several millions of degrees which emits radiation in the X-ray part of the spectrum (by the first note that I explained above). Thus, when we detect X-ray sources in the sky, then we know that there is gas which has been heated to several million degrees, and one of the mechanisms to achieve that is the accretion disc around the black hole.

If the system giving out X-rays turns out to be a binary star, then a case can be made that one of the stars is a compact object (a neutron star or a black hole). Binary stars are very useful to astronomers because it allows us to measure the mass of the stars in the system. If the mass of the compact object turns out to be more than the critical mass mentioned above, then one can be sure that it is a black hole. So that is how black holes are discovered.

This artist impression from the photos above illustrates the tremendous gravitational pull of a giant black hole on a passing star. The doomed object is first stretched by tidal forces until it is torn apart. Most of the gas making up the star is lost from the system but some of it is trapped by the black hole and forms a disc of gas around it. In the disc, the gas is heated to millions of degrees and emits in the X-rays, before disappearing forever, swallowed by the black hole.  A super-massive black hole has ripped apart a star and consumed a portion of it, according to data from NASA's Chandra X-ray observatories. These results are the best evidence yet that such a phenomenon, long predicted by theory, does actually happen.

While one class of black holes have "small" masses (greater than 5 times the mass of the sun), there are others which have gigantic masses (more than a million times the mass of the sun), called supermassive black holes.  Since our Sun is a star, could it ever turn into a black hole? The answer is No.  The Sun will never burst and will not become a black hole. Stars end their lives in two different ways: those with mass of the Sun will end their lives in a gentle way, becoming a planetary nebula and leaving behind a remnant called "white dwarf". Stars much more massive than the Sun explode as a supernova leaving behind either a "neutron star" or a "black hole".

However, in another 4.5 billion years (which is a long time…so enjoy your life in the meantime), the Sun will expand into a "Red Giant" star and will come very close to the Earth's orbit. At that point, Earth will become so hot that there will no longer be any oceans and will probably be the end of life as we know it. However, that is way into the future and right now, there is more danger that we will cause our own destruction (by wars and pollution) than anything happening due to the Sun.

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