Designation of star clusters
• IAU:
– C aa bb ± ccd
– aah bbm ± cco.d, Coordinates (1950.0)
• Catalogues:
– IC, M(essier), NGC, and OCL
• „Discoverer“, surveys and „special names“
– Basel, Bochum, Lynga, Melotte, Stock, Trumpler
and much more
• Pleiades: C 0344+239, M45, Melotte 22
Classification of open clusters
• Trumpler, 1930, Lick Observatory
Bulletin, 420, 154, three criteria
1. Degree of Concentration
2. Range of Brightness
3. Number of Stars in the Cluster
• Janes & Adler, 1982, ApJS, 49, 425:
definition of a so-called richness class
• Open clusters can also be classified on
the basis of color-magnitude diagrams
Trumplers classification
• Degree of Concentration
– I … Detached clusters with strong central
concentration
– II … Detached clusters with little central
concentration
– III… Detached cluster with no noticeable
concentration
– IV … Clusters not well detached, but has a
strong field concentration
Trumplers classification
• Range of Brightness
– 1 … Most of the cluster stars are nearly the
same apparent brightness
– 2 … A medium range of brightness
between the stars in the cluster
– 3 … Cluster is composed of bright and faint
stars
Trumplers classification
• Number of Stars in the Cluster
– p … Poor clusters with less than 50 stars
– m … Medium rich cluster with 50 to 100
stars
– r … Rich clusters with over 100 stars
• Open clusters with any type of nebulosity
are denoted with an "n" at the end of the
classification.
• Richness Class (Janes & Adler)
– 1 … Less than 25 stars
– 2 … Between 25 and 50 stars
– 3 … Between 50 and 100 stars
– 4 … Between 100 and 250 stars
– 5 … More than 250 stars
• How “good” can the number of members
be established?
Diameters of open clusters
• How could we determine the diameter of a star
cluster?
1. The determination, for example inspection by eye,
should be no problem. Be careful, most open clusters
show no real concentration
2. Count the number of stars (members) in concentric
rings around the cluster center
3. If the derived distribution is not symmetric => go to 1.
and shift the coordinates of the center
• This procedure could be easily done via a computer
program
Pietrukowicz et al., 2006, MNRAS, 365, 110
III 2 m
II 2 m
Galactic field
van den Bergh, 2006, AJ, 131, 1559
No correlation with the age
Janes et al., 1988, AJ, 95, 771
all
young
Galactic Distribution
+- 20 degree Galactic latitude
Kharchenko et al.,
2013, A&A, 558,
A53
Blue dots: OCLs
Red triangles: GCs
Spiral arms
Projection on the
Galactic plane
Classification of Globular Clusters: Shapley H. & Sawyer H.B., 1927,
Harvard College Observatory Bulletin No. 849, pp.11-14
Definition - Radii
• Core Radius: Distance at which the apparent surface
luminosity has dropped by half
• Half-Light Radius: Distance from the core within
which half the total luminosity from the cluster is
received
• Half-Mass Radius: The radius from the core that
contains half the total mass
• Tidal Radius: Distance from the center at which the
external gravitation of the galaxy has more influence over
the stars in the cluster than does the cluster itself
Important observables
• Single stars: „all“ we can think of
• Star clusters
1. Hertzsprung-Russell-diagram
2. Kinematic data
3. Integrated spectra
4. Integrated colors
5. Polarimetric measurements
From the Pre-Main Sequence to
Globular clusters
NGC 104: d = 4500 pc, D = 31‘,
[Fe/H] = -0.76 dex, t = 12 Gyr
Harris, 2000, Space Telescope Science Institute Symposium Series, Vol. 14, p. 78
Hyades
log t = 8.90
d = 45 pc
[Fe/H] = +0.17 dex
Width of Main Sequence
about 1.8 mag in MV
NO
observational error
A typical example
One typical example from the literature:
Piatti et al., 2006, MNRAS, 367, 599: First estimates of the
fundamental parameters of the relatively bright Galactic
open cluster NGC 5288
CCD BVI Photometry, 1 Pixel = 0.4´´, 13.6x13.6´ field,
No other observations available for this open cluster
15 688 stars in the complete
field
Different „main sequences“
due to fore- and background
populations
Klein Wassink, 1927, Publications of the Kapteyn Astronomical Laboratory Groningen, Vol. 41, 1
Praesepe
d = 190 pc
Field stars
Sanner et al., 2001, A&A, 369, 511 (Hipparcos and Tycho data)
The proper motion for „distant“ star clusters is almost zero.
Only field stars with large proper motions can be sorted out.
These are almost only foreground stars.
Field stars
Field stars +
Members
Hole et al., 2009, AJ, 138, 159: NGC 6819, one of the „best“ cases,
more than three measurements for each star, 6571 radial velocities
for 1207 stars, 3.5 meter telescope
Observed sample
Hole et al., 2009, AJ, 138, 159
Field stars +
Members
Hole et al., 2009, AJ, 138, 159
members non-members
Integrated properties
• Integrated spectra and colors
• Especially interesting for distant and
extragalactic star clusters
• “Think small”
• Pleiades: 2
o
1’
Bica & Alloin, 1986, A&A, 162, 21
Age
Integrated spectra
Z
Same age
Lata et al., 2002, A&A, 388, 158
Integrated colors
“HRD diagram”