Each nebula continues to contract or collapse due to gravity until the atoms are close enough to begin to collide and interfere with each other. This causes them to speed up, which creates a renewed source of heat. When the temperature is sufficient, visible light is radiated from the mass and a star is born. Click on the picture to view a full-size image. After the star is born, it goes through a series of energy and size fluctuations for a period of time.
Gravitational collapse continues to produce energy release which, in turn, causes expansion.
The fusion process releases a huge amount of energy, which stops gravitational collapse and stabilizes the star. Click on the picture to view a full-size image. Stars are of various sizes depending on the amount of matter that was in the original lump of plasma that ultimately became the star. Click on the picture to view a full-size image. The closest star to us is a yellow star called the Sun .