A star can shine for only as long as it has nuclear fuel to process in its core. The intense gravitational pressure generated by the weight of the star pressing inward on itself can only be held off by burning materials in the core, thus generating an outward force strong enough to support the structure of the star against collapse.

A star's lifetime is thus determined by the amount of fuel, found primarily in the form of hydrogen, that is present.

One might think that the more fuel a star has, the longer it will live (much like a car runs longer on a full tank of gas than on a quarter of a tank). While a car burns gasoline at a constant rate, all star do not burn hydrogen at a single rate. The more massive a star is (the more hydrogen it contains), the greater the core pressure, and the more efficiently it converts hydrogen into helium.

The most massive stars thus have the shortest lifetimes, lasting only a few million years. They burn their fuel at the fastest rate, outshining less massive stars like our Sun by factors of a million in brightness. But though they are brighter candles, the Sun will outlive them by more than nine billion years.

Remembering the fable of the tortoise and the hare, we note that the race does not always go to the brightest competitor.