Stars are probably born from enormous clouds of gas in space. The particles' own gravitational forces cause most of them to collect together into a sphere. At one point, the core of this mass is receiving so much pressure that it begins a nuclear reaction.
This computer-generated animation shows a crude model of
gravity collecting particles into a sphere-like mass.
Note the few particles which remain orbiting, uncollected.
The equilibrium point (when the material stops contracting) is the point where the pressure below a "shell" of the sphere at a particular radius is equal to the gravity above that shell. (In other words, that particular shell of the star is not falling or rising.) This description is animated below:
When a normal star stops generating thermonuclear energy (it "burns out"), it collapses. The gravitational force at the center of the star is much greater than the amount of pressure (which is now none) being produced from within the star.
The General Theory of Relativity shows that stars which are controlled by quantum effects rather than thermal pressure ("stable degenerate stars") can only exist up to a certain mass, called the "Chandrasekhar mass." This mass is thought to be between 1.5 to 3.0 times the mass of our sun. Stars of lower masses will turn into "white dwarfs" (small cool stars). Stars of slightly higher masses will collapse in a giant explosion called a "super nova." These are called "neutron stars." Both of these types of stars live out their lives at new equilibrium states and slowly cool off.
What happens if a star of a mass much greater than the Chandrasekhar mass collapses? It is thought that the star will collapse into a "singularity," which is surrounded by a "black hole."
Here, small stars die a simple, cooling death. Larger stars explode
into neutron stars. Very large stars collapse into black holes.
("M" is the mass of the star shown, "MC" is the Chandrasekhar mass and "M0" is a solar mass.)
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