Stars contain enormous amounts of mass, and since an object's gravity is determined by its mass, stars have a great amount of gravity. In the interior of stars, all the mass pressing inwards is enormous, so great that smaller atoms in the center of the star fuse together and form different, heavier atoms. This process, termed "fusion," produces a tremendous amount of energy that pushes in an outward direction, balancing the inward force of gravity and preventing the star from collapsing.

Eventually however, the star runs out of lighter atoms and fusion can no longer take place. It is a bit like a balloon: the tension in the balloon walls tries to collapse the balloon, while the air pressure in the balloon presses outwards and prevents the balloon's collapse. Stopping fusion in the star's center is like taking the air out of a balloon: the star collapses inward. But in a process that is poorly understood, the collapse of the star and the great compression of the star's interior somehow unleashes an amount of energy that for a brief moment may be greater than all the energy produced by all the stars in a galaxy. The release of energy causes the star to explode outwards in a phenomenon known as a supernova. The supernova disperses the stellar material around the galaxy. The material from multiple supernovae, floating around the galaxy, will eventually coalesce due to gravity, and when enough material coalesces, the gravity of the new object gradually increases until there is enough pressure in the center of the sun to push atoms together, fusion begins anew, and the object is considered a new star, a Sun. Something like that.