Astronomers classify stars into groups according to their color and the presence of elements in the stars’ spectral signatures. This star classification system goes like this: O, B, A, F, G, K, M (here’s a way to remember them: “Oh be a fine girl, kiss me”.) M stars are coolest and most common stars in the Universe.
M stars range in temperature from 2,500 Kelvin and go all the way up to 3,500 Kelvin. They look red to our eyes. M stars account for 75% of the stars in our stellar neighborhood, so they’re the most common by far! Most M stars are tiny red dwarfs, with less than 50% of the mass of the Sun, but some are actually giants and supergiants, like the red giant Betelgeuse.
Some familiar M stars include Betalgeuse (red giant), and the red dwarfs Proxima Centauri, Barnard’s star, and Gliese 581
We have written many articles about stars here on Universe Today. Here’s an article about how red dwarf stars have small habitable zones.