Lecture 4: Open clusters
Galactic star clusters Main points
• Also known as open star clusters
• About 400 known; ~18000 may exist in Galaxy
• Gal. latitude b ≤ 5º in most cases, very few >10º
• Open clusters are mainly young Popn I objects
• Typically they contain a few hundred stars
• The stars are coeval (of same age), at essentially same distance and of same metallicity
• There is a spread in star masses (given by IMF)
The Pleiades star cluster
Some open clusters
top l: double cluster h and χ Per top centre: the Hyades
top r: Messier 67
l: κ Crucis, the Jewel Box cluster
Young open clusters and OB associations in the galactic plane are found mainly in the spiral arms
The galactic distributions of both early B-type field stars and of open clusters closely follow the Milky Way and
are only found close to the galactic equator
Most famous young Populatio I clusters include:
• the Pleiades
• the Hyades
• h and χ Persei (the double cluster)
• Praesepe
• κ Crucis (the Jewel Box)
Old Popn I clusters are much rarer and include:
• M67
• NGC188
Galactic cluster Hertzsprung-Russell diagrams
• This is a plot of magnitude as a function of colour index (often (B–V)) or of spectral type.
• If C.I. is used, then also known as a colour- magnitude diagram (CMD)
Colour-magnitude diagrams for the Pleiades and Praesepe
Colour-magnitude diagrams for the Hyades and NGC188
Messier 67 colour-magnitude diagram
Composite HR diagram for open star clusters
Features of the HR diagram for a galactic cluster:
• Zero-age main sequence (ZAMS)
The locus of stars which have just started to shine
• Subgiant branch
Stars that have just exhausted H in their cores, and are now moving off the main sequence
• Red giants
Evolved stars in upper right-hand part of diagram with either He cores, or they are burning He to C and O in their cores.
They have a H-burning shell. These were once the more massive MS stars.
Photometric distances for galactic clusters 1. Plot colour-magnitude diagram mV vs (B-V)
2. Compare with MV vs (B-V) for Hyades (MV is known from moving cluster method)
3.Hence find mV – MV (distance modulus)
= 5 log d – 5 where d is disatnce in parsecs.
Distances of some well-known clusters Cluster distance
Hyades 44 pc
Pleiades 127 pc Praesepe 159 pc
Sco-Cen 170 pc
M67 830 pc
h Persei 2250 pc χ Persei 2400 pc
Ages of clusters
Lifetimes of main-sequence stars depend strongly on mass.
Mass (M⊙) M-S lifetime (yr) 15 10 × 106
5 66 × 106
3 22 × 107 1 10 × 109
The cluster turn-off point
• This is the (B-V) colour index of bluest main-
sequence stars, and corresponds to most massive stars still on M-S (core hydrogen-burning stage).
• Turn-off goes to redder M-S stars as cluster ages.
Ages of some well-known galactic clusters
Cluster age (yr) h and χ Persei 3 × 106
Pleiades 5 × 106
Praesepe 4 × 108 Hyades 5 × 108 M67 5 × 109 NGC188 8 × 109
