Introduction – Star Clusters I
The Gaia Satellite Mission
Launched in 2013
At Sun–Earth Lagrange
point L2 located
approximately 1.5 million
kilometres from Earth
• Positions and proper motions of objects
• Spectrophotometry of objects
• Spectra of objects
The Gaia Satellite Mission
Open Clusters
Textbook
Open Clusters
Reality
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 – Star Clusters in
the Milky Way
Open Clusters Globular Clusters
Age < 5 Gyr ≤ age of the Milky Way
Distance from
the Sun
> 45 pc > 2000 pc
Mass range of
the members
Complete range depending
on the age of the cluster
< 20 Mʘ
Total mass < 60 000 Mʘ < 1 000 000 Mʘ
Absolute
diameter
< 25 pc < 100 pc
Metallicity -1.0 to +0.6 dex compared to
the Sun
-2.5 to -0.5 dex compared to
the Sun
Star Clusters – tricky to analyze
NGC
7789
Photometry
We measure astrophysical properties in different filters
The Asiago Database on Photometric Systems: http://ulisse.pd.astro.it/Astro/ADPS/enter.html
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
Absorption = Extinction = Reddening
• AV = k1 E(B-V) = k2 E(V-R) = …
• General extinction because of the ISM characteristics
between the observer and the object
• Differential extinction within one star cluster because
of local environment
• Both types are, in general wavelength dependent
• Extinction of the earth’s atmosphere
Cardelli et al., 1989, ApJ, 345, 245
Important parameter:
RV = AV/E(B - V)
Normalization factor
Standard value used
is 3.1
Be careful, different
values used!
Depending on the
line of sight
Formation of Star Clusters
All members of an individual Star Cluster are
born within one Giant Molecular Cloud (GMC)
over a time scale of some few Myrs.
The „Initial Mass Function“ (IMF) describes the
mass distribution for a population of stars when
they are formed together:
IMF(m) ≈ m-G Salpeter law (1955)
Formation and Evolution of
Star Clusters
• Stars with all masses form in a star clusters
• Much more low mass that high mass stars
• How is such a star cluster evolving?
• High mass evolve much faster than low mass
stars
• Luminosity:
• Life time on the main sequence
Evolution of Star Clusters
Evolution of Star Clusters
From the Pre-Main Sequence to
Globular clusters
Turn-off point
Distance:V0-MV
AV
k.AV
Turn-off
Not very accurate but still useful, never done for 2MASS and NIR
Götz, 1989, Die offenen Sternhaufen unserer Galaxis
Isochrones
Isochrones - Metallicity
Gaia – before and after