ultracool
Ultracool dwarfs are a class of very cool, low-mass objects whose spectra lie at the cool end of the stellar sequence. The term covers the spectral classes L, T, and Y. They have effective temperatures much lower than the Sun’s and, for the most part, do not sustain hydrogen fusion in their cores. As a result, many ultracool dwarfs are brown dwarfs, though some occupy the very low-mass end of the stellar regime.
Temperatures for ultracool dwarfs span from roughly 1,500–2,400 K for early L types down to well below
Ultracool dwarfs are found throughout the Milky Way, and a substantial fraction lie within the solar neighborhood.
Study of ultracool dwarfs informs models of cool, high-pressure atmospheres and condensate clouds, contributes to understanding