metals (Mn I, Fe I, Si I). By late K, molecular bands of titanium oxide become present. Mainstream theories (those rooted in lower harmful radioactivity and star longevity) would thus suggest such stars have the optimal chances of heavily evolved life developing on orbiting planets (if such life is directly analogous to earth's) due to a broad habitable zone yet much lower harmful periods of emission compared to those with the broadest such zones. Spectral standards: K0V – Sigma Draconis K0III – Pollux K0III – Epsilon Cygni K2V – Epsilon Eridani K2III – Kappa Ophiuchi K3III – Rho Boötis K5V – 61 Cygni A K5III – Gamma Draconis Class M Class M stars are by far the most common. About 76% of the main-sequence stars in the solar neighborhood are class M stars. However, class M main-sequence stars (red dwarfs) have such low luminosities that none are bright enough to be seen with the unaided eye, unless under exceptional conditions. The brightest-known M class main-sequence star is Lacaille 8760, class M0V, with magnitude 6.7 (the limiting magnitude for typical naked-eye visibility under good conditions is typically quoted as 6.5), and it is extremely unlikely that any brighter examples will be found. Although most class M stars are red dwarfs, most of the largest-known supergiant stars in the Milky Way are class M stars, such as VV Cephei, Antares, and Betelgeuse. Furthermore, some larger, hotter brown dwarfs are late class M, usually in the range of M6.5 to M9.5. The spectrum of a class M star contains lines from oxide molecules (in the visible spectrum, especially TiO) and all neutral metals, but absorption lines of hydrogen are usually absent. TiO bands can be strong in class M stars, usually dominating their visible spectrum by about M5. Vanadium(II) oxide bands become present by late M. Spectral standards: M0IIIa – Beta Andromedae M2III – Chi Pegasi M1-M2Ia-Iab – Betelgeuse M2Ia – Mu Cephei ("Herschel's garnet") Extended spectral types A number of new spectral types have been taken into use from newly discovered types of stars. Hot blue emission star classes Spectra of some very hot and bluish stars exhibit marked emission lines from carbon or nitrogen, or sometimes oxygen. Class W: Wolf–Rayet Once included as type O stars, the Wolf–Rayet stars of class W or WR are notable for spectra lacking hydrogen lines. Instead their spectra are dominated by broad emission lines of