Normal stars

An O-type spectrum is characterized by the simultaneous presence of lines of neutral and ionized helium. If ionized helium is absent, the spectrum corresponds to a B-type. The lines of ionized helium strengthen toward earlier stars, whereas neutral helium lines and hydrogen lines decline toward earlier types. Table 8.1, taken from Conti (1973), illustrates this by way of the equivalent widths of some of the stronger lines.

Because of this antagonistic behavior of He ii and He i, we can use them for classification. The MK system uses the ratio He ii λ4541/He i λ4471 as the criterion for spectral type for the interval 03–09 (for an illustration, see figure 8.1). The He ii lines are grouped in the so-called Pickering series (analogous to the Bracket series of hydrogen) with lines at λλ5414,4850,4542,4339,4200,4100, 4025 and 3968,…, and the Fowler series (analogous to the Paschen series of hydrogen) with lines at λλ4656,3204,…

Besides helium, some other elements are also present in O-type spectra. Among these we have Si iv(λ4089) and C iii(λλ4068,4647 and 4651) in late O-type stars. N iii(λλ4634,4640) if present is only seen in emission.

In these stars emission lines are very often present and can appear with varying degrees of intensity, from barely perceptible to rather strong, although they are never as broad and intense as in Wolf–Rayet stars. They can appear in different objects as a weak central emission on a wider absorption, or as a complete fill-in, or as a moderately strong emission.