If you are an amateur astronomer you may have observed that stars are not as uniform as they first appear. If you were to observe the stars you would notice that they are different colors. So why are star’s different colors? The difference in colors actually depends on a lot of different factors. The first is the composition of a star. While stars are all basically composed of atoms some stars have other trace elements in them that can alter the wavelengths of light that they emit. The next factor is surface temperature. This is the most significant contributor to a star’s color. The change in temperature changes the wavelength of light a star emits. The last factor is distance in relation to the Doppler Effect.
Composition does play a role in the color of light a star emits. This is because not all stars are formed exactly the same way. The reason is due to being formed in different nebulas. Nebulas in the interstellar medium are largely composed of hydrogen the main fuel for star creation. However they do carry other elements. The mix of these other elements varies from nebula to nebula. The change in color these elements add to stars is not very obvious. However it is important in the field of spectroanalysis. Depending on the kind light an element burns scientist can analyze a star’s light and basically determine its elemental composition.
The most important factor to a star’s color has to be its surface temperature. If you have ever seen an open flame you would understand why. A blue flame is flame burning at very high temperatures. A yellow flame has temperatures that are cooler than blue flames and red flames are the coolest of all. The same thing happens with stars. Very hot stars tend to be blue stars. Stars with an average surface temperature become yellow stars like our sun. The coolest stars are Red Dwarfs.
Last there is Doppler Effect also known as red or blue shift. If you are familiar with the Doppler Effect you known that distances can extend or contract waves. It’s the fire truck analogy where a siren sounds closer as the fire truck approaches and sounds lower as it leaves. With light it becomes bluer if the distance between a star and an observer is shrinking and redder if it is growing. This evidence was used to determine that the universe is expanding as galaxies and stars showed a red shift.
We’ve done many episodes of Astronomy Cast about stars. Listen here, Episode 12: Where Do Baby Stars Come From?