M stars are characterized by which type of absorption bands?

M stars, also known as red dwarfs, are the coolest and least massive stars on the main sequence, classified by their relatively low surface temperatures. These stars typically have temperatures ranging from about 2,400 to 3,700 Kelvin. The low temperatures play a significant role in determining the characteristics of their absorption spectra.

In M stars, molecular absorption bands are prevalent, specifically from molecules such as titanium oxide (TiO) and vanadium oxide (VO). These molecules form under the cooler conditions found in M stars, leading to strong absorption features in the optical and near-infrared regions of the spectrum. The prominence of these molecular bands is a key indicator of the spectral type of these stars, helping astronomers classify them accurately.

This characterization stands in contrast to the absorption features seen in other types of stars. For instance, more massive stars with higher temperatures will show different absorption lines, such as ionized metals or hydrogen lines, rather than the molecular features prominent in M stars. Thus, the presence of TiO and VO bands is definitive for M star classification.