1. ASTR112 The Galaxy Lecture 3 Prof. John Hearnshaw 5. Stellar populations 6. Galactic (open) clusters The Pleiades open cluster

2. ASTR112 The Galaxy Lecture 3 Prof. John Hearnshaw 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. ASTR112 The Galaxy Lecture 3 Prof. John Hearnshaw Stellar populations l: Andromeda galaxy, M31 above: Walter Baade

4. ASTR112 The Galaxy Lecture 3 Prof. John Hearnshaw 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. ASTR112 The Galaxy Lecture 3 Prof. John Hearnshaw Stellar populations

6. ASTR112 The Galaxy Lecture 3 Prof. John Hearnshaw

7. ASTR112 The Galaxy Lecture 3 Prof. John Hearnshaw

8. ASTR112 The Galaxy Lecture 3 Prof. John Hearnshaw 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 Pop n 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)

9. ASTR112 The Galaxy Lecture 3 Prof. John Hearnshaw The Pleiades star cluster

10. ASTR112 The Galaxy Lecture 3 Prof. John Hearnshaw 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

11. ASTR112 The Galaxy Lecture 3 Prof. John Hearnshaw Young open clusters and OB associations in the galactic plane are found mainly in the spiral arms

12. ASTR112 The Galaxy Lecture 3 Prof. John Hearnshaw 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

13. ASTR112 The Galaxy Lecture 3 Prof. John Hearnshaw Most famous young Pop n I clusters include: the Pleiades the Hyades h and χ Persei (the double cluster) Praesepe κ Crucis (the Jewel Box) Old Pop n I clusters are much rarer and include: M67 NGC188

14. ASTR112 The Galaxy Lecture 3 Prof. John Hearnshaw 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)

15. ASTR112 The Galaxy Lecture 3 Prof. John Hearnshaw Colour-magnitude diagrams for the Pleiades and Praesepe

16. ASTR112 The Galaxy Lecture 3 Prof. John Hearnshaw Colour-magnitude diagrams for the Hyades and NGC188

17. ASTR112 The Galaxy Lecture 3 Prof. John Hearnshaw Messier 67 colour-magnitude diagram

18. ASTR112 The Galaxy Lecture 3 Prof. John Hearnshaw Composite HR diagram for open star clusters

19. ASTR112 The Galaxy Lecture 3 Prof. John Hearnshaw 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.

20. ASTR112 The Galaxy Lecture 3 Prof. John Hearnshaw 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.

21. ASTR112 The Galaxy Lecture 3 Prof. John Hearnshaw Proper motions of stars in the Hyades cluster, showing the convergent point located in the sky but several degrees away from the cluster itself.

22. ASTR112 The Galaxy Lecture 3 Prof. John Hearnshaw Measure θ, the angle between each star and convergent point Radial velocity of stars V R from the Doppler shift of spectral lines Proper motion of each star, μ, in arc seconds/yr VRVR μ θ θ to convergent point Earth star V

23. ASTR112 The Galaxy Lecture 3 Prof. John Hearnshaw The basic equations for the moving cluster method. V R radial velocity (km/s) μ proper motion (arc s/yr) d distance (pc) p parallax (=1/d) (arc s)

24. ASTR112 The Galaxy Lecture 3 Prof. John Hearnshaw For the Hyades the moving cluster method gives m V – M V (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.

25. ASTR112 The Galaxy Lecture 3 Prof. John Hearnshaw Photometric distances for galactic clusters 1. Plot colour-magnitude diagram m V vs (B-V) 2. Compare with M V vs (B-V) for Hyades (M V is known from moving cluster method) 3.Hence find m V – M V (distance modulus) = 5 log d – 5 where d is disatnce in parsecs.

26. ASTR112 The Galaxy Lecture 3 Prof. John Hearnshaw Distances of some well-known clusters Cluster distance Hyades 44 pc Pleiades127 pc Praesepe159 pc Sco-Cen170 pc M67830 pc h Persei 2250 pc χ Persei 2400 pc

27. ASTR112 The Galaxy Lecture 3 Prof. John Hearnshaw Ages of clusters Lifetimes of main-sequence stars depend strongly on mass. Mass (M ⊙ ) M-S lifetime (yr) 15 10 × 10 6 5 66 × 10 6 3 22 × 10 7 1 10 × 10 9

28. ASTR112 The Galaxy Lecture 3 Prof. John Hearnshaw 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.

29. ASTR112 The Galaxy Lecture 3 Prof. John Hearnshaw Ages of some well-known galactic clusters Cluster age (yr) h and χ Persei 3 × 10 6 Pleiades 5 × 10 6 Praesepe 4 × 10 8 Hyades 5 × 10 8 M67 5 × 10 9 NGC188 8 × 10 9

30. ASTR112 The Galaxy Lecture 3 Prof. John Hearnshaw End of lecture 3