What is a significant characteristic of T stars?

T stars are characterized by their relatively low temperatures, which distinguishes them from other types of stars in the classification scheme. These stars represent a category of brown dwarfs, which are objects with masses below the threshold needed for hydrogen fusion. The temperatures of T stars typically range from about 700 to 1,500 Kelvins, making them cooler in temperature compared to other stellar types, such as L stars and those higher in the sequence like M stars.

The molecular absorption bands of H2O are indeed strong in T stars, but this is a result of their cooler temperatures. The presence of H2O bands doesn't define the T stars, but rather it is a characteristic consequence of their coolness as these molecules are more prevalent and active at lower temperatures. While T stars also exhibit some ionization of metals, it is not a defining characteristic, as this occurs primarily in hotter types of stars.

Therefore, the significant characteristic of T stars is their status as the coolest stars in the classification, distinguishing them as objects of interest in the study of stellar evolution and the transition between planets and stars.