This is because of gravitational redshift. Now , what is it? Let's take the example of the sun. Sun is an average sized star. Nuclear fusions inside the core causes the electrons of the orbit to move to higher orbits. When the energy has transformed to another energy or virtually finished then they move to the lower / real positions and emit photons (ight).These photons are pushed with great , they have kinetic energy Since the space around the sun is virtually curved due to its gravity , the photons constantly struggle to move away and this energy is utilized. Since energy is directly proportional to frequency and inversely proportional to wavelength , and red has the longest wavelength therefore the sun appears to be red. Wavelength means the length between the two waves . larger the wavelength longer the distance covered.