O starsStars of spectral type O that are very hot massive blue stars, emitting copious quantities of ultraviolet radiation, and with surface temperatures (T eff) from about 28 000 to over 40 000 kelvin. Strong lines of singly ionized helium (He II) dominate the spectra. Lines of doubly and triply ionized elements are also present with lines of neutral helium (He I) and hydrogen strengthening in later subdivisions. No O0, O1, or O2 stars have been discovered and only a few O3 and O4 stars are known. Most O stars are found in the spiral arms of galaxies, often in OB associations, and are very fast rotators. They have short lifetimes of about three to six million years. See also Of stars; ultraviolet stars.
Collins Dictionary of Astronomy © Market House Books Ltd, 2006