Stars Do Not Shine Forever. Simple reasoning explains why. Stars are of finite size. For instance, the Sun's size is 100 times that of the Earth. This is big, but it's still finite. Finite size means that stars have only finite amounts of nuclear fuel. Eventually they will exhaust their fuel and fade.

You can compare this to driving your car. One tank of fuel lets you drive only a certain number of miles. When the fuel is exhausted, the car will stop.

How Long Do Stars Shine? That depends on how much mass they have.

Mass determines the amount of nuclear fuel stars have at birth. Mass also determines how bright stars are, or how rapidly they consume fuel.

The greater a star's mass, the greater is the amount of its nuclear fuel. However, the more massive stars are fuel guzzlers. They shine much brighter than the less massive stars and use up their fuel very fast. Hence, the more massive stars have shorter lives.

The most massive stars -- those with masses 25 to 50 times that of the Sun -- race through their lives in just a few million years. The Sun will last for about 10 billion years. Stars less massive than the Sun will last even longer.