Here's why:

* Low-mass stars are much smaller and cooler than the Sun, with masses less than half that of our star. They burn hydrogen slowly and have lifespans of billions of years.

* High-mass stars are much larger and hotter than the Sun, with masses several times greater. They burn hydrogen quickly and have lifespans of only a few million years.

The Sun's mass is about 1.989 × 10^30 kg, making it fall comfortably within the range of medium-mass stars. It has a lifespan of about 10 billion years.