Why Do Stars Decrease in Size and Mass Moving Toward Red on the Main Sequence?
Look at the two stars on the left. More massive stars are hotter inside because of the weight of the outer layers pressing down on the center.
If the center is hotter, nuclear reactions converting mass to energy occur more frequently and more efficiently, which means the rate that the star produces energy is faster. This makes the star brighter.
So if a star is heavy, it will automatically be brighter and hotter (further toward blue) on the main sequence.
Mass and brightness are closely related.
In fact, the mass of a star determines its color, temperature, lifetime and end. This is the singlemost important thing to know about a star.