If you see a red nebula surrounding a blue star, is it an emission or a reflection nubula? Can you explain?

Answer 1

Red, specifically 656.2 nm, is characteristic of hydrogen emission. Blue stars (type O or B) emit enough ultraviolet energy to stimulate the surrounding gas to emit.

However, you are unlikely to "see" the red color. Human vision is relatively insensitive to light that far into the red, so our visual perception of the nebula is dominated by the blue-green emission at 486.1 nm. In any case, emission nebulae are generally too faint for the eye to detect ANY color, but the red color of hydrogen emission nebulae shows up in photographs.