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Spectroscopic Atlas for Amateur Astronomers 100
Table 62: A montage of three broadband spectra (200L grating) is presented to demon-
strate the spectral development within the S–Class. To optimise the clarity, only the promi-
nent lines are labelled here.
Omicron1 Orionis (4 Ori) HD 30959, (540 ly).
This star, with an apparent magnitude of about 4.74m, is only slightly variable. With M3.2S
it forms the transition class between the M(e) and S–types. Alternatively, the star is also
classified as S3.5/1–. According to the very low C/O index ZrO is barely visible in this low-
resolution spectrum. TiO bands here still dominate the profile.
Chi Cygni HD 187796, (340 ly).
With a classification of S6/1e (alternatively S7/1.5e), the C/O index is here only slightly
higher compared to Omicron1 Orionis. Thus the zirconium oxide absorptions can overprint
here the still dominant TiO bands only within a few small sections of the profile at λλ
5298/05, 5375/79, 5839/49 and 6475/81/94 (blend with TiO λ 6478). This star shows
with Vvar ≈ 3.3m – 14.3m, the highest brightness variation of all known variables – period
length about 408 days (LPV).
R Cygni HD 185456.
As already shown in Table 61, the ZrO absorptions have fully overprinted here the TiO
bands, due to the high C/O Index.
Observation hint: R Cygni is easy to find. It’s located just nearby of Theta Cygni, HD
185395 (mV ≈ 4.5m).
Table 63: Shows a montage of two broadband spectra (200L grating) to compare an “in-
trinsic” and an “extrinsic” S–class star. Both stars have nearly the same spectral classifica-
tion. Despite of some minor differences the two profiles run almost identically – regardless
of the suspected different origin of the ZrO in the stellar atmospheres.
The intrinsic HR Pegasi, HD 216672, (970 ly), is only slightly variable Vvar ≈ 6.1m – 6.5m
with a period of some 50 days (Type SR, semi-regular). It is classified as S4/1+ with a very
low C/O Index.
The extrinsic BD Camelopardalis, HD 22649, (510 ly), is only slightly variable mV ≈ 5.1m. It
forms a component of a close binary system, is classified as S3.5/2 symbiotic and there-
fore relatively hot.
