TYPES OF STAR A  B  C  D  E  F  G  H  I  J  K  L  M  N  O  P  Q  R  S  T  U  V  W  X  Y  Z           HOME ABOUT CATEGORIES SITE MAP COPYRIGHT ADVERTISE CONTACT entire Web this site O star Alnitak and the Flame Nebula A massive, luminous, blue star of spectral type O. O stars have a surface temperatures of 30,000°–50,000°C and masses of 20–100 solar masses. Their spectra are dominated by emission lines of singly ionized helium (these lines forming the so-called Pickering series). Most other lines are from at least doubly ionized elements, though H and He I lines are also present. O stars are powerful ionizers of the surrounding interstellar medium, giving rise to large, bright emission nebulae and virtually defining the spiral arms of the Milky Way. Most O stars are very fast rotators. They have lifetimes of only 3 to 6 million years. Well-known examples include Alnitak (Zeta Orionis) and Naos (Zeta Puppis). Oe stars have prominent hydrogen emission lines. Oef stars are early-type O stars that show double emission lines in He II at 4686 Å. Of stars are peculiar O stars in which emission features at 4634 to 4641 Å from N III and 4686 Å from He II are present. They have a well-developed absorption spectrum, which implies that the excitation mechanism of the emission lines is selective, unlike that of Wolf-Rayet stars. The spectra of Of stars are usually variable, and the intensities of their emission lines vary in an irregular way. Of stars belong to extreme Population I. All O stars earlier than O5 are Of. Related category    • TYPES OF STARS Also on this site: Encyclopedia of Alternative Energy & Sustainable Living Encyclopedia of History Transport Concepts & Designs (partner site) BACK TO TOP