Y-type star

A Y-type star is a star within the Y-class, a spectral class indicating a star with a noticeable absorption line at wavelength 1.55 μm and a surface temperature under 700 K, which encompasses the coolest brown dwarfs (Y dwarfs). The spectral energy distribution (SED) peaks in the mid-infrared range. There are multiple possible molecules that might be contributing to such a 155 μm spectral feature and confirming which are actually contributing is an area of research. Other than that feature, their spectra is not well-characterized. As of 2013, seventeen bodies were known that fit the criteria. Such cool brown dwarfs are not that different from T-dwarfs or hot planets and discussion remains regarding whether this classification is useful: it is presumably for objects classified as brown dwarfs based upon their mass that fall within this temperature range. These cool classes are also often cited for objects that are considered to be planets: the classes basically indicate the objects' surface-temperature regimes evident from the classes' defining spectral features.