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 × 106 566 ×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.