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Spectroscopic Atlas for Amateur Astronomers 33

9 Wolf Rayet Stars

9.1 Overview

The French astronomers Charles Wolf and Georges Rayet discovered in 1867 very rare
stars, whose spectra are standing out by massively broadened, intense helium emission
lines and the almost complete absence of hydrogen. These objects are grouped in an extra
class, marking the final stage of massive O-type stars approx. ≥25 M . Below this mass
limit of <25 M , the SN explosion takes place directly at the end of the Red Giant stage,
without to pass a subsequent WR phase.

On the northern hemisphere mainly in the constellation Cygnus, a concentration of such
extreme stars can be found, which are members of the Cygnus OB associations. Here some
23 WR stars are located – 14 are classified as WN-type, 8 as WC-, and just one as WO-
type (WR 142). The two brightest ones reach an apparent magnitude in the range of 6–7m
and are therefore well accessible with a slit spectrograph, even for moderately sized ama-
teur telescopes. By chance they just represent the two main types WN and WC.

At the begin of the WR phase, the star blasts away its entire outer hydrogen shell by a huge
stellar wind with velocities of up to 2000 km/s (see Tables 87, 88). On the further path to
the SN of the category 1b or 1c (sect. 25), similar to the peeling of an onion, also the layers
of the stellar core are removed from the top down. This causes an annual mass loss rate of
about 10−5 to 10−4 M [236]. On the surface of the star the extremely hot, former nuclear
H- and He fusion zones become exposed where, in addition to helium, the metals C, N, and
O have been formed. The total duration of the entire WR stage is estimated to be about
200,000 years [238].

9.2 Spectral Characteristics and Classification

Therefore, the spectrum differs now completely from the previous O-class stage. The broad
lines of ionised helium He ll appear now together with the emissions of highly ionised C, N,

or O – depending on the “currently” exposed surface layer of the former H and He fusion
layers [236]. These metals determine the classification of the WR stars and their high stage
of ionisation is an indicator of the involved, extremely high temperatures and the corre-
sponding excitation energies (see table sect. 34). Similar to the other spectral types also
the WR stars are subdivided into decimal subclasses.

WN: WR stars with nitrogen emissions (Late subtypes WNL with H-lines; Early subtypes
WNE without H-lines [236])
WC: WR with carbon emissions (by partial helium fusion [236])
WO: WR with oxygen emissions (by complete helium fusion, very rare[236]).

Limited to the very early stages of the WR evolution, the H-Balmer series of the not yet
completely repelled hydrogen shell, can still be detected exclusively within blends at the
very late WN (WNL) types [2] [236].

In the later stages of WR-development, the "early" WC subclasses show, though still
weakly, the highly ionised oxygen O VI doublet at λλ 3811/3834.

In the final WO phase, this O VI doublet appears strongly developed, together with other,
highly ionised oxygen emissions (Table 6). Pure WO stars are extremely rare and the corre-
sponding final phase probably very short – in addition associated with a significant X-ray
radiation.