A star of spectral type O, the brightest, hottest, and most massive of all normal, hydrogen-burning stars on the main sequence, appearing blue in colour and emitting most of its energy in the ultraviolet. Their temperatures range from 30 000 K to above 50 000 K, and they have luminosities from 100 000 to more than a million times the Sun's. With masses of 20–100 solar masses or more, O stars burn their nuclear fuel at a prodigious rate and have lifetimes of only 3–6 million years. Because of their high temperatures, hydrogen lines in their spectra are weak, the dominant lines being those of singly ionized helium (the Pickering series). O stars form infrequently and have short lifetimes, so they are very rare; only a handful of O2, O3, and O4 stars are known (no stars are classified earlier than O2). The brightest naked-eye O star is Zeta Orionis, an O9.5 supergiant. O stars are often found with B stars in OB associations. See also Oe Star; Of Star.