Stellar Evolution

The balloon has features in common with our sun. The sun is not the solid object it appears. Abetter model is that the sun is a bag of gas, somewhat like a balloon, but with the constraining force of the rubber replaced by gravity. The sun looks like it has a distinct surface, but that is an illusion created by the fact that the gas molecules in the sun, which are mostly hydrogen, become ionized and opaque at temperatures in excess of 3000 Kelvins. As you go down into the sun, the temperature of the gas increases. The distance at which it reaches 3000 Kelvins, the gas changes from transparent to opaque and that is what we see as the surface of the sun.
For the past five billion years, the sun has gotten energy from the conversion of hydrogen nuclei to helium nuclei. And there is another five billion years worth of hydrogen left before the sun runs out of fuel. At that point the sun will do something spectacular. It will expand so that the earth will be orbiting near the sun’s surface. But eventually the sun will settle down and begin to cool off.
As the sun cools, it will contract very much like the balloon in our demonstration. And like the balloon, the collapse will be halted when the atoms are so close together that the repulsive core of the molecular forces prevents further contraction. At that point the sun will have become what is called a white dwarf, an object about the size of the earth slowly cooling until it becomes a dark ember.
If the sun were just a bit bigger, about 1.4 times its current mass, the gravitational collapse would not be halted by the molecular forces between atoms. That is the mass at which gravity is strong enough to crush the atoms together, to overcome the atomic repulsive cores, with the result that you end up with a neutron star.