M-Class stars, often called red dwarfs, are the most common type of star in the universe, representing about 75% of the stellar population in the Milky Way Galaxy. They are intrinsically faint and emit most of their energy outside the visible light spectrum, making them challenging to observe. The closest star to our own Sun, Proxima Centauri, is an M-Class star, yet it is far too dim to be seen without a telescope.
The Characteristic Color and Temperature Range
M-Class stars appear deep red or orange-red to a viewer. This characteristic hue is a direct consequence of their low surface temperature, which typically ranges from about 2,400 to 3,700 Kelvin. For comparison, our Sun, a G-Class star, has a surface temperature of nearly 5,800 Kelvin.
The color of a star is fundamentally determined by its temperature, following the principles of black-body radiation. Cooler objects, like M-Class stars, emit light with a peak intensity shifted toward the longer, red end of the visible spectrum. Consequently, a significant portion of the energy from an M-Class star is emitted in the infrared, which is not visible to the human eye.
The relatively low temperature of these stars allows for the formation of simple molecules in their atmospheres, unlike hotter stars where molecules are broken apart. This molecular presence further influences the light spectrum, making the stars appear even redder.
Spectral Classification: Placing M-Class Stars
The classification of stars is based on the Morgan-Keenan (MK) system, which categorizes them using the letters O, B, A, F, G, K, and M, running from hottest to coolest. M-Class stars fall at the coolest end of this sequence, defining the boundary for hydrogen-fusing stars. This classification is primarily a measure of surface temperature, which dictates the resulting pattern of absorption lines in a star’s light spectrum.
M-Class stars are specifically identified by the presence of strong molecular bands in their spectra, particularly those belonging to titanium oxide (TiO). These bands are prominent because the star’s cool atmosphere allows these molecules to remain intact. The intensity of these titanium oxide bands is used by astronomers to further subdivide the M class into types, from M0 to M9.
Size, Mass, and Extreme Stellar Longevity
Most M-Class stars are categorized as “Red Dwarfs,” the smallest and least massive true stars capable of sustained hydrogen fusion. Their masses generally fall between 0.08 and 0.6 times the mass of the Sun. This low mass results in a low luminosity, with many M-Class stars emitting less than 1% of the Sun’s light output.
The immense lifespan of M-Class stars stems from a process called full convection, which occurs in stars with less than about 0.35 solar masses. In these stars, the interior plasma is constantly churning, effectively mixing the entire volume of the star. This continuous mixing ensures that all the star’s hydrogen fuel is eventually brought to the core for fusion, unlike stars like the Sun where only the core material burns.
Because they burn their fuel so slowly and efficiently, the smallest M-Class stars have projected main-sequence lifetimes of up to several trillion years. This longevity is far longer than the current age of the universe, meaning that every red dwarf that has ever formed is still an active star.