Why do some stars end up as black holes?

[Milan Milošević 0]

Why do some stars end up as black holes and other do not?

[Milan Milošević 0]

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The answer involves the gravity and the internal pressure within the star. These two things oppose each other -- the gravitational force of the star acting on a chunk of matter at the star's surface will want to cause that matter to fall inward, but the internal pressure of the star, acting outward at the surface, will want to cause the matter to fly outward. When these two are balanced (i.e., equal in strength) the star will maintain its size: neither collapse not expand. Such is the case for the Sun at the moment, and even, for that matter, for the Earth.

However, when a star runs out of nuclear fuel, and therefore continues to lose energy from the surface (it is emitting light energy), while not replacing the lost energy through nuclear fusion (no more nuclear fuel), gravity will win out over internal pressure and the star will contract slowly or collapse quickly depending upon the details of the internal structure and composition. Gravity wins out over the internal pressure of the star, because that pressure was produced by a normal, hot gas, and that gas is losing energy as the star radiates energy from the surface.

The star may thus end up as a black hole. It just depends upon whether or not the collapse is stopped at some smaller size once another source of pressure (other than what is produced by a normal, hot gas) can become sufficiently strong to balance the inward gravitational force. There are other forms of pressure besides that produced by a hot gas. Pressing your hand upon a desk top will let you experience one of these other pressures --- the desk pushes up against you, indeed it can support your weight (gravitational force)! The pressure that keeps the desk rigid against your weight is caused by forces between the atoms in the desk.

Furthermore, electrons within atoms must avoid each other (for example, they cannot all be in the same atomic "orbit" --- this is called "the exclusion principle"). Therefore, if we had a collection of freely moving electrons they would also avoid each other: the harder you compress the collection (the smaller the volume they are confined in) the more they rebel against the squeeze --- a pressure opposes your confinement of the electrons.

This "electron avoidance" pressure can only become strong enough to oppose the gravitational forces within a star of about the mass of the Sun when the star is compressed by gravity to about the diameter of the Earth. Thus a star as massive as the Sun can be prevented from becoming a black hole when it collapses to the size of the Earth, and the internal "electron avoidance" pressure (called the "degenerate electron pressure") becomes strong enough to hold the star up. This sort of pressure does not depend upon the energy content of the star ---- even if the star continues to lose energy from its surface, the pressure will continue to hold the star up. Our Sun can never become a black hole.

However, if the star is more massive than something like 3 to 5 solar masses, its gravitational forces will be larger, and its internal degenerate electron pressure will never be sufficient to stop its collapse. It turns out that neutrons can also obey the exclusion principle and neutrons will be produced in abundance when a massive star collpses, but even neutron degeneracy cannot stop the collapse of massive stars --- anything over 3 to 5 solar masses cannot be stopped, it will become a black hole according to current thinking.

Source: http://www1.phys.vt.edu/~jhs/faq/blackholes.html#q1

 

[Milan Milošević 0]

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Stars with more than 15 solar masses burst up in the form of hypernova with a gamma ray burst resulting in a stellar remnant a BLACK HOLE. Now as a star grows it core contracts and outer layer expands due to yhe increasing heat. Then as the helium fusses to form higher elements a core of carbon is formed. This iron core brings an end to the fusion and the core is very dense, enough to break the neutron degeneracy pressure( neutrons cannot occupy the same quantum state.) which leads to the gravitational collapse of the core and they blast up to form a black hole.

Time dosen’t really change or stop . As per the theory of relativity light has the same speed everywhere which is 3,00,000 km/s. Now as you accelerate near the speed of light the time decreases. In a black hole the gravity is very high than light cannot escape it which means you would need a higher escape velocity. In this case time has to decrease. So the time inside a black hole becomes very less and you cannot notice it.

Source: https://www.quora.com/Why-do-some-stars-end-up-as-black-holes-How-does-time-change-in-black-holes