Stellar Nebulae

• Stars begin in stellar nebulae--giant pockets of gas and dust in a galaxy. The gases clump together naturally by gravity. As the gases condense, they form a ball, growing larger and beginning to spin.
  
  

Protostar

• The gas ball attracts more matter into itself, developing an increasingly strong gravity. The gravity becomes so powerful that hydrogen atoms fuse together in the center, producing light and heat.
  
  

Main Sequence

• During the star's main sequence, it shines brightly as it continues to fuse hydrogen into helium. For a midsized star, this period lasts about 10 billion years.
  
  

Red Giant

• When a star has used up all its hydrogen, it begins to fuse helium into carbon and oxygen. The star swells up to 100 times its earlier size and cools, its light changing from yellow-white to red. At this stage, it is called a red giant.
  
  

White Dwarf

• After a red giant uses up its store of helium, it shrinks to about 1 percent of its original size, cooling and dimming over a few billion years.