Definition of Star Clusters
Star clusters are physically related
groups of stars held together by mutual
gravitational attraction.
The number of all star clusters in the Milky Way is
about 10 000 but only 3000 in catalogues. From
these, about 170 Globular Clusters (“old”,
Population II).
Working Hypothesis
All members of an individual Star Cluster
are born within one Giant Molecular Cloud
(GMC) over a time scale of some few Myrs.
What are the immediate conclusions?
All members of an individual star cluster have:
• Identical distance from the Sun: +- The volume
expansion of the cluster (diameters < 25 pc)
• Identical age: +- Time scale of star formation (a few
Myrs)
• Identical metallicity: +- Inhomogeneities of the
initial GMC and the chemical evolution of the giant
branch
• Identical kinematical characteristics: +Intrinsic
spread
– Radial velocity
– Proper motion
Characteristics – Open Clusters
• Age: 1 Myr – 5 Gyr (Population I)
• Metallicity: -1.0 to +0.6 dex (factor 10 to
4) compared to the Sun
• Distance from the Sun: > 45 pc
• Mass range of the members: 0.08 to 100
M(sun)
• Total masses: up to 40000 M(sun)
• Absolute linear diameter: 2 to 25 pc
The cluster parameters
1. Reddening
2. Distance modulus
3. Age
4. Metallicity
Determination in the order: Reddening, age,
distance modulus simultaneously, metallicity with
possible iterations
Distance:V0-MV
AV
k.AV
Turn-off
Glushkova et al., 2013, MNRAS, 429, 1102
Color – Magnitude - Diagram
Different CMDs for one open cluster
Grocholski & Sarajedini, 2003, MNRAS, 345, 1015
Maeder & Mermilliod, 1981, A&A, 93, 136
Maeder & Mermilliod, 1981, A&A, 93, 136
Different treatment
of convection
von Hippel, 1998, AJ, 115, 1536
Single Multiple
In total, 41 WDs until 1998 found, no firm
improvement after that
Fellhauer et al., 2003, ApJ, 595, L53: The White
Dwarf Deficit in Open Clusters: Dynamical Processes