O Stars

O stars are hot, massive stars. O stars are characterized by the presence of hydrogen (H), neutral Helium (HeI), and singly ionized helium (HeII) in the optical spectra. Other spectral features may include SiIV (4089Å) and CIII (4068, 4647, 4651Å). In O stars the HeI and HeII line strengths are a closely correlated with photospheric temperature, with HeII strength increasing with temperature and HeI strength decreasing with temperature. The HeII (4541Å) / HeI (4471Å) ratio define the spectral subclass.

Of are a sub-class of O stars, possible representing the O supergiants. The defining feature of Of stars is the presence of NIII (4630-4634Å) in emission. Of stars also have HeII (4686Å) emission, while O(f) stars have a weak HeII feature, and O((f)) stars have HeII absorption. Other features may include H emission, and CIII (5696 Å) emission.

Oe stars are the hotter extension of the Be stars. They are typically main sequence stars, variable, rapid rotators (broad lines), and show H emission that is often double-peaked. Stellar mass loss has produced a disk and/or shell of material around the star.

Click here for a catalog of selected bright O stars.


O7n O8V



O5.5f O5.5f O5.5f

O7f O8f