A supergiant of spectral type O in which the absorption lines of carbon are unusually strong relative to the nitrogen lines, hence the suffix ‘C’. They are probably stars in which nitrogen produced by the carbon–nitrogen cycle of nuclear burning in the star's core has not reached the surface in any great quantities, as is the case with most type O supergiants. Two factors, age and speed of rotation, are thought to be behind the difference in composition between OC and O stars. The OC stars are younger than normal O stars, so there has been less time for nitrogen to reach the surface; and they are slower rotators, so that material from their interiors is not brought to the surface as quickly as in faster rotators.
Subjects: Astronomy and Astrophysics.