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
Cepheids: period-luminosity relationship, brighter
globular clusters: RR Lyrae stars as “standard candles”, short periods of < or ~1 day, horizontal branch stars ---> similar
? 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