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Spectroscopic Atlas for Amateur Astronomers 139

tions, also the globular clusters, allow the analysis of the brighter individual stars. In the
professional field, multichannel spectrographs allow the simultaneous recording of up to
several 100 profiles. Main objective here is mostly the determination of the metal abun-
dance ܼ "[‫݁ܨ‬/‫( ]ܪ‬sect. 4.7). This value allows direct conclusions about the age of a star
cluster. The stars of the first generation, somewhat oddly called Population II, were created
with the birth of the Milky Way 12 billion years ago, where the interstellar matter was still
dominated by hydrogen, helium and lithium. The enrichment with heavier elements took
place only later by matter from SN explosions or repelled planetary nebulae. This enriched
material generated later on the metal-rich second star generation, similarly confusing
called Population I, to which belongs also our Sun. The most efficient way for the determi-
nation of the ܼ- value, has proven the analysis of the Ca II calcium triplet in the near infra-
red at λλλ 8542, 8498 and 8662. This process is abbreviated called as CaT. The empirical
relationship between the metallicity ܼ and the summed EW- values of the mentioned Ca II
absorptions has been refined over the last 25 years and outlined in numerous publications,
such as [325 – 327].

27.5 Spectroscopic Age-Estimation of Star Clusters by Amateurs

The described CaT-method is too demanding for most amateurs. So, for example, the Ca II
calcium triplet is located in the infrared range, somewhat outside the reach of most ama-
teur equipment and frequently contaminated by blends with other absorptions. In [30] sect.
14.6, a simple method is presented, based on the correlation between the duration of stay
on the main sequence MS and the spectral class of a star. It is based on:

– The assumption, that the individual stars of such clusters have been formed at about the
same time from a common gas- and dust cloud

– The known relation: The earlier classified (or more massive) the star, the shorter the
lifetime

– The quintessence: Within a stellar sample, the duration of the stay on MS for the earli-
est spectral class determines very roughly the age of the cluster.

At the so called Turn off Point the star reaches on the MS its ear- Spectral- Stay on MS
liest possible spectral class. Then it moves within the HRD to the class [Years]
top right of the giant branch, showing thereby significantly later O7 6 Mio.
classifications. The table to the right shows, how the stay of later O9 8 Mio.
classified stars on the MS increases dramatically. B0 12 Mio.
B1 16 Mio.
The table to the right shows that for later classified stars the stay B2 26 Mio.
on the MS increases dramatically [405]. Starting from the late G- B4 43 Mio.
class it even exceeds significantly the present age of the Universe B6 95 Mio.
of 13.7 billion years. Such data are a rough guide only. Based on A0 350 Mio.
model calculations they often show (source-dependent) a consid- A5 1.1 bn.
erable spread. F2 2.7 bn.
G2 9.4 bn.
Example: If within a cluster a larger spectral sample yields an K0 23 bn.
early A-type as the earliest classification, its age can roughly be
estimated to 350 – 500 million years. Any earlier classified MS
stars of types B and O, are much more short-lived and thus either
already exploded in a SN or migrated in the HRD to the top right
of the giant branch (see [30], sect. 14.6).

In professional fields, such studies are often carried out photometrically in the ‫ ܸ –ܤ‬system.
The major disadvantage is that these measured magnitude values always appear reddened
by interstellar matter and need first of all to be adjusted with appropriate models. In the ta-
ble of sect. 38, the assignment of "de-reddened" ‫ ܸ –ܤ‬magnitudes to the corresponding