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

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

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

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.

ASTR112 The Galaxy Lecture 3 Prof. John Hearnshaw Stellar populations

ASTR112 The Galaxy Lecture 3 Prof. John Hearnshaw

ASTR112 The Galaxy Lecture 3 Prof. John Hearnshaw

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)

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

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

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

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

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

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)

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

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

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

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

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.

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.

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.

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

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)

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.

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.

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

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) × × × × 10 9

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.

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

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