Introduction
Before the light was seen as both waves and particles it was impossible to prove that it was subject to gravity. But if light can be seen as particles, then it is obviously subject to gravity like all objects and matter. When light was thought to be travelling at an infinite speed, it was also difficult to imagine that gravity could slow it down, but now we know that light is travelling fast, but still at a finite velocity. In these conditions gravity can affect light.
Collapsing stars
In 1783, the British scientist John Mitchell followed by the French Marquis de Laplace, suggested that a massive and compact star would have such a strong gravitational field that light could not escape it. The light emitted by the star surface would be attracted back by gravity. There are many such stars but, unfortunately, we cannot see them, as they do not radiate light to us. However, we can observe and measure their gravitational attraction. These stars are now called black holes.
A star is formed when a large amount of gas -mainly hydrogen- collapse on itself under the influence of its gravity. As the gas contracts the atoms collide with each other, and the gas heats up. When the gas reaches a certain temperature, the hydrogen atoms coalesce to form helium. This is similar to the nuclear reaction that occurs in a hydrogen bomb; and we know that nuclear reactions release a large amount of energy. This release of energy makes the star shines, it also increases the gas pressure and, at some point, this effect balances the gravitational attraction. Once they have reached this condition, the stars remain stable for a long time. Much later on the star will run out of hydrogen and starts to cool down and contract.
Strange as it seems at first the heavier the star is at first -or the more fuel it has- the sooner the nuclear reactions will stop. In effect, the heavier the star is, the hotter the gas must be heated to balance gravity and, as a result, the faster it uses its fuel. Scientists believe that the Sun has enough fuel to last another 5*10E+9 years, but more heavy stars can burn out in 1*10E+8, and this is less that the age of the universe.
Subrahmanyan Chandrasekhar theory
While travelling from his country, India, to Britain, Chandrasekhar calculated how big a star could be and still not collapse under its own gravity after all its fuel is used. Chandrasekhar's idea was that when the star collapses on itself, the particles forming its matter come very close to each other. According to Pauli's Exclusion Principle, they must have different velocities and, as a result, they move apart and tend to make the star expand. According to this theory the star can remain stable -keep the same diametre- due to the balance between the attraction of gravity and the repulsion due to the Exclusion Principle (before the balance was between the effects of gravity and heat).
The Exclusion Principle's effect is limited as the theory of relativity limits the maximum difference in the velocities of the particles in relation to the speed of light. When the star has collapsed enough and has become very dense, the repulsion due to the exclusion principle becomes smaller that the attraction of gravity, and the collapse is total.
Chandrasekhar calculated that a cold star of more than 150% of the mass of the sun would not be able to avoid total collapse under its own gravity. This is known as the Chandrasekhar limit. The Russian scientist Landau arrived to the same conclusion at about the same time.
"White Dwarf" stars
If a star's mass is less that the Chandrasekhar Limit, it can stop contracting and stabilize as a "White dwarf". White dwarf are characterized by a low luminosity, a mass on the order of that of the Sun (1.99 10E+33 grams), and a radius comparable to that of the Earth (about 10,000km). Because of their large mass and small dimensions, such stars are dense and compact objects with average densities approaching 1,000,000 times that of water. White dwarfs have exhausted all their nuclear fuel and have no residual nuclear energy sources. Their compact structure prevents further gravitational contraction. In a White Dwarf the force of gravity is balanced by the force resulting from the exclusion principle repulsion between its electrons.
There are many White Dwarfs.
Neutron Stars
Landau found that there was another possible family of stars, with a limiting mass of one to two times the mass of the sun, but smaller that the white dwarfs. In this case the force of gravity is balanced by the force resulting from the exclusion principle repulsion between its neutrons and protons. They have a radius of about 15 to 20 kilometres, a mass roughly the same as the Sun's, and their mean densities are extremely high (about 10E+14 times that of water).
Pulsars
In 1967 Jocelyn Bell, a student at Cambridge University, found objects in the sky emitting pulses of radio waves. After discarding her first thought that she had made contact with aliens on another planet, scientists agreed that the signals came from rotating neutron stars known now as pulsars. They are emitting pulses of radio waves due to the interactions between their magnetic field and the matter around them.
If a neutron star can collapse to such a small size -a few times the critical radius at which a star becomes a black hole- it is believable that other stars could collapse to a smaller radius and become black holes.
Heavy stars
Stars with a mass above the Chandrasekhar limit may explode, or manage to
expel enough matter to reduce their mass below the above limit after they
have exhausted their fuel, and avoid gravitational collapse. However, why
would a star looses mass and, moreover, enough of it to reduce its mass
below the critical limit. Robert Oppenheimer was the first scientist to
suggest what happens to a collapsing heavy star. This problem was forgotten
until 1960 when scientists became again interested in it.