| Term: | Of stars |
| Definition: |
An O star whose spectrum displays strong N III 4634-4640-4642 emission and strong He II 4686 emission. The N III lines are always much stronger than C III 4647-4650-4651 when the latter are present. Historically, Of stars were considered to belong to the peculiar category, hence the f notation (see below). In his thesis work, Walborn removed them from that category and established them as the normal O-type supergiants. He also used the notation ((f)), (f), and f to describe the progression from strong He II 4686 absorption, through weakened/absent, to emission, respectively, correlated with increasing N III emission strength, subsequently showing that it is a luminosity sequence -- the first such for stars earlier than O9. |