Lecture 13: The H-R Diagram of Star Clusters Parameters of two types of stellar clusters Star Clusters: Types and Distances open clusters: ~10^3 stars, in Galactic disk, Pop I stars globular clusters: ~10^5 stars, Galactic halo, Pop II stars which open f2 cluster is closer? f 1 f /f (r /r )2 2 1 ∝ 1 2 Evolution of Star Clusters if cluster forms stars of different masses, but same chemical composition, all stars at zero-age MS at t_0 after ~10^7 years, M > 10 solar mass stars evolve off diagram after ~10^9 years, M > 2 solar mass stars move off after ~10^10 years, even 1 solar mass star begins to ascend giant branch Age of Star Clusters successive snapshots: make comparison of different open clusters correct first for relative distances by sliding diagram up, down until MS’s line up open clusters all standardized to Hyades N2362 youngest (~10^7 years old) N188 oldest (~6-10 x 10^9 years old Distance to Hyades nearest stars: parallax Hyades open cluster: moving cluster method open clusters: MS-fitting to Hyades Trigonometric Parallax r = tanπ π[rad] π = 206265[arcsec/rad]π[rad] d ≈ defines parsec π d r Distance to Hyades: Moving Cluster Method Other Distances Cepheids: period-luminosity relationship, brighter <L>, longer P, supergiants visible from far away, calibrated from Cepheids in open clusters Other Distances Cepheids: period-luminosity relationship, brighter <L>, longer P, supergiants visible from far away, calibrated from Cepheids in open clusters globular clusters: RR Lyrae stars as “standard candles”, short periods of < or ~1 day, horizontal branch stars ---> similar <L>’s RR Lyrae ? H-R Diagram of Globular Clusters differs from even old open cluster like NGC 188 long horizontal branch (with RR Lyrae stars): consequence of low heavy-element abundance “turn-off point of MS also low: even low mass stars (~0.8 solar mass) have evolved ---> GC stars are old! ~12-13 billion years “blue straggler” population: low mass stars that accreted mass from a companion? open clusters globular clusters.