Most stars fit a predictable pattern. These stars have predictable lifetimes and are consistent in their energy generation. As you can see, there is a sort of diagonal line of stars going from right to left. These are main sequence stars. About nine out of every ten stars are on the main sequence.
Most stars have less mass than our sun.
Finally, the star burns up most of the hydrogen in its core and begins to die. The sun will lose perhaps half its mass, and grow smaller and fainter as it cools. Life forms adapt to their environments, and so must have a dependable environment to adapt to. Main sequence stars are stable as they live out the mature period of their lives. Stars are grouped by temperatures.