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Stellar Populations in Galactic Clusters: Insights from The Pleiades and Beyond

Explore the concept of stellar populations in galaxies and open clusters, including insights from Walter Baade's observations and the characteristics of different cluster types. Learn about the Pleiades cluster and the methods used to determine distances and ages of galactic clusters.

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Stellar Populations in Galactic Clusters: Insights from The Pleiades and Beyond

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  1. 5. Stellar populations 6. Galactic (open) clusters The Pleiades open cluster

  2. Stellar populations The concept of stellar populations was introduced by Walter Baade (German-American astronomer in California) in 1944 from observations of the spiral galaxy M31 in Andromeda. Spiral arms: bluer stars – population I Nuclear bulge: redder stars – population II The halo stars are also assigned to population II

  3. Stellar populations l: Andromeda galaxy, M31 above: Walter Baade

  4. Baade’s 1944 colour- magnitude diagram of stars in M31 showing two populations. The brightest pop II stars are redder than the brightest pop I stars.

  5. Stellar populations

  6. 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)

  7. The Pleiades star cluster

  8. 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

  9. Young open clusters and OB associations in the galactic plane are found mainly in the spiral arms

  10. 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

  11. Most famous young Popn 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

  12. 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)

  13. Colour-magnitude diagrams for the Pleiades and Praesepe

  14. Colour-magnitude diagrams for the Hyades and NGC188

  15. Messier 67 colour-magnitude diagram

  16. Composite HR diagram for open star clusters

  17. 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.

  18. Moving cluster distances Fundamental distance method applicable to Hyades (the nearest cluster) and Sco-Cen association Stars in cluster have common space motion. But because of the perspective effect, the proper motions appear to converge on a given point in sky – the convergent point.

  19. Proper motions of stars in the Hyades cluster, showing the convergent point located in the sky but several degrees away from the cluster itself.

  20. Measure • θ, the angle between each star and convergent point • Radial velocity of stars VR from the Doppler shift of • spectral lines • Proper motion of each star, μ, in arc seconds/yr VR μ θ star V θ Earth to convergent point

  21. The basic equations for the moving cluster method. VR radial velocity (km/s) μ proper motion (arc s/yr) d distance (pc) p parallax (=1/d) (arc s)

  22. For the Hyades the moving cluster method gives mV – MV (distance modulus) = 3.25 Hence d = 44.3 pc. This is a fundamental distance determination in astronomy, relative to which distances to other more distant objects are measured.

  23. Photometric distances for galactic clusters • Plot colour-magnitude diagram mV vs (B-V) • Compare with MV vs (B-V) for Hyades • (MV is known from moving cluster method) • Hence find mV – MV (distance modulus) • = 5 log d – 5 where d is disatnce in parsecs.

  24. 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

  25. 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

  26. 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.

  27. 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

  28. End of lecture 3

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