why do I see in the sky stars with different colors?what does it means the different color of the stars?
The stars have different colors, from red (for example Antares or Aldebaran) to blue (like Betelgeuse or Sirius). That difference in colors is due to a “superficial” temperature of the stars. We can consider with good approximation the stars as a black body. A black body is an object that absorbs every incident radiation (and then it is black) and re-emit the radiation absorbed as a funcion of its temperature. It is very difficult to have a perfect black body, but in the other hand we can consider most of the objects like black bodies with a good approximation. This means that every body above the 0K emits radiations as function of its temperature. The wavelenght of maximum emission is given by a simple formula: λ=0.29/T (cm) where T is the temperature of the body (K). Using this formula we can extimate the emission of a human body (310K); it is easy to show that the emission is in the IR light, and indeed the thermal visor of army are able to see persons in the night using their black
