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Spectroscopic Atlas for Amateur Astronomers 141

Overall, the cluster contains about 500 stars. According to a study by Abt and Levato [328],
the brightest 50 stars of M45 spread over the various spectral classes as follows:
B: 17, A: 24 and F: 9. The remaining stars are classified as type F or later. Anyway it is still
an enigma, why in this rather small cluster the number of massive stars, classified in the
range of the middle to late B-Class, exceeds by orders of magnitude their expected average
incidence of just some 0.125% (see sect. 6.4)! The famous reflection nebulae are not, as
originally believed, the remaining "birth shells" of the brighter stars but rather a galactic gas
cloud, which is randomly located in the trajectory of M45.

Tables 78A and B:

Both tables impressively show the very similar spectra of the 10 brightest stars within
M45. The middle to late B class can be recognised here mainly due to the relatively intense
Balmer lines in combination with the very weak Ca II absorptions at λ 3933.66 (sect. 10).
Striking features of all profiles are also the small emission humps in the continuum at λλ ~
5840 and ~ 5350. Further visible here is the increase in intensity of the H-Balmer lines be-
tween the classes B6 to B8.

Table 78A shows a montage of those four spectra, which are classified to show emission
lines (index e). Three of them are even classified as Be-stars (CDS). However in March
2014, just two of them showed intensive emission lines and Merope, 23 Tau, a very weak
filling-in in the core of its Hα absorption. As the only one the Be- star Pleione shows in its
profile coarser absorptions between the H-Balmer lines. Recording info: C8/DADOS, Grat-
ing 200L/mm, Atik 314L+, average 1x60sec, 2x2 binning.

27.7 Age estimation of M45

The earliest main sequence (MS) classification is here B8. All earlier B types have already
migrated to the giant branch. According to the table in sect. 27.5 this yields an age of some
more than 100 million years. The accepted value is in the range of 100 – 130 million years.

Cecilia Payne Gaposchkin (see sect. 7) discovered in M45 several White Dwarfs. This
would mean that some stars of the middle to late B-type with < 8M , have already repelled
Planetary nebulae. Under debate is the hypothesis, if in exceptional cases, also early B stars
with > 8M , could end up as White Dwarfs. If such stars would lose sufficient matter, dur-
ing the Giant- or Post AGB Phase, their mass could sink this way below the Supernova limit.

27.8 Globular Clusters – Analysis by Integrated Spectra

Due to their enormous distances of about 25,000 – 40,000 ly the brightest individual stars
of globular clusters reach at most an apparent magnitude of mv ≈11m (eg. at M5 and M13).
Thus for an individual star analysis with an 8-inch telescope only the few brightest speci-
mens could be recorded with a slit spectrograph. Therefore recommendable for amateurs is
the recording of the composite spectrum in the integrated light, as it is applied also for the
galaxies in sect 26. Due to the very old stellar inventory almost exclusively absorption lines
can be seen here. With such spectra a so-called "Integrated spectral class" [330] can be
determined, and this way, by analogy with the open clusters, the age of the cluster can be
estimated.

In the professional field [331], these profiles are also compared with synthetic model spec-
tra and the Turn Off Point is determined, inter alia, by means of the Balmer lines. According
to [331] the integrated light-spectra are obtained here by sweeping over the cluster to pre-
vent, that the profile is determined by the stars of just one particular cluster zone. Due to
the relatively short focal lengths and the relatively inaccurate autoguiding, such an action is
not necessary at the amateur level. Anyway when recording the spectra in Table 79, the
image was slightly defocused in order to avoid any disproportionate influence of individual
stars.