Life Expectancy along the Main Sequence

Along the main sequence of a Hertzsprung-Russel Diagram, stars will spend their lives undergoing hydrogen nuclear reactions in their core, until they run out of hydrogen to fuse and they evolve off of the main sequence. You might think that a larger star will live longer; it has a bigger core, which means that it has more fuel to burn, right? In fact, the opposite is true. You see, a larger star, in order to counteract the effect of gravitational contraction, must create a larger outward pushing thermal pressure. A larger thermal pressure is created by a star undergoing more fusion reactions, which then leads to a higher core temperature, i.e. more thermal pressure. Since we know the mass and lifetime of our own Sun, 10 billion years, we can compare the lifetime of other stars.

Source: https://en.wikipedia.org/wiki/Main_sequence#/media/File:HRDiagram.png

Our Sun lies right about in the middle of the H-R diagram shown above. As mentioned before, the Sun has a life expectancy of 10 billion years, which is the time it takes for the Sun to use up all its hydrogen in its core (about 10% of the total mass of the core). If you took, for example, a sun that had a mass of 10M_sun, it has 10 times as much fuel as the Sun, but according to the H-R diagram uses its fuel 10,000 times faster (it is 10,000 times more luminous). Since luminosity if the energy generated per second, the total energy available divided by the luminosity is equivalent to the lifetime of the star. Therefore, it will live only a thousandth as long as our Sun will, 10 million years. The opposite is true for small suns. For a sun which is a tenth as massive as our sun, it will have a tenth as much fuel, but is a hundredth as luminous. This means it will live 10 times as long as our sun, 100 billion years.

Lifetime also has interesting implications for our continuing search for extraterrestrial life in our universe. The first life on Earth appeared about 1.2 billion years after Earth was formed, and if we use that as a guideline, there are a lot of stars in our universe that do not even live long enough to have a planetary system which can support life. Because of this, our search is narrowed down from the beginning; we can only look at longer living stars such as G, K, and M types even before we begin looking for planets in the habitable zone of the star.

As you can see, life expectancy has the opposite effect that you would respect. Large stars evolve off of the main sequence much faster than smaller stars do. In fact, we still have not seen some of the lower mass classes of stars experience star death, because their life expectancy far exceeds the 14 billion years the universe has been in existence.