What causes the different colors of stars?

The different colours of stars are caused by their varying temperatures and the principle of black body radiation.

Stars, like our Sun, are gigantic balls of gas that emit light and heat due to the nuclear reactions happening in their cores. The colour of a star is primarily determined by its surface temperature, which is a result of these nuclear reactions. This is based on the principle of black body radiation, a fundamental concept in physics.

Black body radiation refers to the spectrum of light that an idealised physical body would emit when it is heated. As the temperature of this body increases, the peak wavelength of the light it emits decreases. In simpler terms, the hotter the object, the bluer the light; the cooler the object, the redder the light. This is known as Wien's Displacement Law.

Stars are considered as approximate black bodies because they absorb all incident radiation, regardless of wavelength or direction. Therefore, they follow the same principle. Cooler stars, with temperatures less than 3,500 degrees Celsius, appear red. Stars with medium temperatures, like our Sun which is about 5,500 degrees Celsius, appear yellow or white. The hottest stars, with temperatures over 6,000 degrees Celsius, appear blue.

However, it's important to note that this is a simplification. The colour of a star can also be influenced by other factors such as its chemical composition and the presence of dust or gas between the star and the observer. These factors can absorb or scatter certain wavelengths of light, altering the star's apparent colour. For example, our Sun appears yellow from Earth, but from space, without the Earth's atmosphere scattering the shorter blue and green wavelengths, it would appear white.