Explain why cooler stars are red and hotter stars are blue

More cool stars produce much of their light in the red part of the spectrum, so you see them, and bam, the color red. More hot stars, however, produce much more of their light in the green and or yellow spectrums, with much more tinier amounts of red / blue. This balance of the colors, your eye, sees simply as white. The more hot something is, the greater frequency of radiation it produces! Blue light has a higher frequency than red light, so the stars that glow red are cooler, than the stars that glow blue. :)

Hope this helped!

The masses of the objects and their densities

A) acceleration. this seems to be the one that makes sense.