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Spectroscopic Atlas for Amateur Astronomers 138

27 Star Clusters

27.1 Short Introduction and Overview

Star clusters are formed by compacted stellar accumulations within or in the vicinity of gal-
axies. Most of the members of such clusters have evolved from a common gas cloud, and
have therefore about the same age. The individual clusters show a wide dispersion with re-
spect to density, age, and number of stars. Basically, star clusters can be divided in to the
following two main categories with completely different properties.

27.2 Open Clusters

Such clusters show an irregular shape and usually contain a few hundred (eg Pleiades) up
to at most a few thousand stars (eg, H and Χ Persei). The internal gravitational forces of
such clusters are usually too weak to hold together their stars for more than at most a few
hundred million years. One of the few exceptions is the very dense M67, whose age is es-
timated to about 4 billion years. The typical diameters of open star clusters are relatively
similar: eg Hyades: 15 ly, M67: 26 ly and Pleiades: 14 ly. Very small accumulations, just
consisting of a few stars and showing a common direction of movement, are referred as
"associations". Today, more than 1000 Open clusters are registered, which are distributed
over the entire visible region of the Milky Way. The observation of extragalactic open clus-
ters, eg in M31, remains reserved to large professional telescopes.

27.3 Globular Clusters

They differ from the open clusters in almost all aspects:

– they are strikingly spherical
– they are "packed" much denser
– they are all significantly older
– they contain up to several hundred thousand stars
– their typical diameter is more than 10 times larger

The typical distance between the stars in the outskirts of globular clusters is about 1 ly. In
the core area these interspaces can even shrink roughly to the diameter of the solar sys-
tem! Typical diameters of the comparatively much larger globular clusters are at
M2: 182 ly, M3: 180 ly, M5: 178 ly, M13: 145 ly.

In the center of individual globular clusters (eg, M15) intermediate-mass black holes of a
few thousand solar masses have been detected. This shows a certain similarity with dwarf
galaxies. Currently in the Milky Way about 150 globular clusters are known. They all orbit
the galactic center at a distance of about 130,000 light years, forming a halo this way.
Thus, these objects could even be referred as "extra-galactic" if they would not be tightly,
gravitationally bound to the Milky Way. However, it seems certain that their enormous high
age of about 12 billion years, is about the same as that of the entire Milky Way, ie slightly
younger than the universe with 13.7 billion. In contrast to the open clusters the high den-
sity and internal gravitational forces, allow the structure of globular clusters virtually an ar-
bitrarily long life. The observation of extragalactic globular clusters, eg in M31, remains re-
served to large professional telescopes.

27.4 Spectroscopic Analysis of Star Clusters

In contrast to the galaxies, in which amateurs are not able to record individual stars but
only composite spectra in the integrated light, here both, the open- and, with some restric-