| O star |
| A star of spectral type O. O stars have surface temperatures in the range 28,000-50,000 K and are bluish white in colour. Their spectra are characterized by lines of both neutral and ionized helium; emission lines are also commonly present. The four brightest O stars in the sky are Delta (d) and Zeta (z) Orionis, the easternmost stars in Orion's belt, and the southern stars Zeta (z) Puppis and Gamma2 (&gamma2) Velorum. |