M-type stars, also known as red dwarfs, are the coolest and smallest type of main-sequence stars in the Hertzsprung-Russell diagram. They have surface temperatures ranging from about 2,400 to 3,700 Kelvin and are characterized by their reddish color. M-type stars make up about 70-80% of the stars in the Milky Way galaxy. These stars have a long lifespan, often lasting tens of billions of years, which is much longer than larger stars like O-type or B-type stars. Due to their low luminosity, M-type stars are often difficult to observe from Earth, but they are important in the search for exoplanets and potential extraterrestrial life.