Lecture 18 Stellar populations

Stellar clusters Open clusters: contain stars loose structure Globular clusters: million stars centrally concentrated S imple stellar populations: stars were probably all born at nearly the same time; thus have the same age and composition ~5 pc

Galaxies contains billions of stars stars generally have a variety of ages, compositions ~20,000 pc

Globular clusters Mostly found in the halo of the Milky Way  Concentrated around the Galactic centre  In fact their spatial distribution was first used to identify the centre of the Galaxy Globular clusters: Open clusters Mostly found in the disk of the Milky Way Open clusters

Stellar systems Galaxy groups: A few tens of galaxies in orbit about one another Galaxy clusters: Thousands of galaxies, trillions of stars The largest bound structures in the Universe ~5x10 5 pc ~2x10 6 pc

Review: Stellar Evolution Main sequence: Core hydrogen burning Red Giant branch: Shell-hydrogen burning Horizontal branch: Core helium burning Asymptotic Red Giant branch: Shell helium (and hydrogen) burning, around a CO, electron degenerate core

Isochrones and Evolutionary tracks For a collection of stars with a range of masses, we can plot where they will be at a given time: these are isochrones. Models for different ages For a given mass, we can model how it will evolve with time Models for different masses log 10 (age/yr)

Single-aged populations Nearby stars of all agesCluster of stars all formed at the same time.

Star clusters The colour-magnitude diagram of a cluster contains information about the age and composition of a cluster. Evolutionary tracks for stars of different masses HB RGB MS

Star clusters The colour-magnitude diagram of a cluster contains information about the age and composition of a cluster. Isochrones for stars of a fixed age

Theoretical Isochrones Age The main sequence turnoff is a good indicator of cluster age.

Theoretical Isochrones Stars with more heavy elements (metal-rich) tend to be redder. Metallicity Distance The magnitude of the turnoff depends on distance The colour depends on metallicity

Theoretical Isochrones Stars with more heavy elements (metal-rich) tend to be redder. Metallicity Distance Oxygen abundanceAge The magnitude of the turnoff depends on distance The colour depends on metallicity The main sequence turnoff is a good indicator of cluster age.

Colour-magnitude diagrams A young cluster:  The main sequence is the most prominent structure.  There has not been enough time for stars to leave the main sequence

Open clusters Example: The Hyades cluster Spectral type B-VAge (10 9 yr) O-0.4<0.001 B A F0.54 G0.710 K1.060 M1.6>100 The colour of the brightest main sequence stars is (B-V)~0.1 This corresponds to an A0 star.

Open clusters typically young, and metal-rich <1 billion years old Mostly found in the disk of the Milky Way NameAge (Myr) Distance (pc) [Fe/H] Collinder Melotte n Melotte Mamajek Melotte Platais Melotte IC Platais Platais The ten nearest known open clusters

Globular clusters 47 Tucanae Old clusters:  Only the faintest (low-mass) stars are still on the main sequence.  Most of the stars on the CMD are in post-main sequence phases of evolution

NGC2419 In old clusters, the bright blue stars are horizontal branch stars, while the yellow-red stars are giants

Globular clusters For a given composition and distance, find the model age that gives the best fit to the data. Here, isochrones are shown for ages of 8,10,12,14,16,18 Gy.

Globular clusters Example: M92  Best fit model:  age=14 Gyr.  [Fe/H]=-2.31

Globular clusters Isochrones for 8,10,12,14,16,18 Gyr ages in each panel, shown for different compositions and distances.

Cluster ages Model isochrone fits to various different open and globular clusters Shows the range of ages and HR-diagram morphologies spanned by these objects

Observational Difficulties

Observational difficulties Finite width of the main sequence and turn-off Presence of blue- stragglers  Probably binary mergers

Break

Other galaxies The Milky Way and Andromeda are the largest members of the Local Group of Galaxies  There are about ~30 smaller galaxies, with distances of up to about 1 Mpc away.

Local Group galaxies For some galaxies in the Local Group, it is possible to measure the colours and magnitudes of individual stars Consider an intermediate age stellar population, 4 Gyr old. Assuming a solar metallicity, what is the absolute magnitude of the main-sequence turnoff? What would be the apparent magnitude of the turnoff, in the Andromeda galaxy (~800 kpc away)?

Local Group galaxies Most main sequence stars are too faint to be seen, so the colour-magnitude diagrams are dominated by evolved stars It is not usually a good approximation that all stars formed at the same time

Composite stellar populations Need a range of model ages, metallicities to match the width of the main- sequence turnoff.

Outside the Local Group For more distant galaxies, we can only measure the integrated luminosity and colour of all stars. How will the colour and luminosity of a single burst of star formation changes with time?

Outside the Local Group For more distant galaxies, we can only measure the integrated luminosity and colour of all stars. How will the colour and luminosity of a single burst of star formation changes with time?

Elliptical galaxies The easiest ones to model Pretty well modeled by single age, metallicity Models which use high-resolution spectra of stars do a good job of reproducing features in the galaxy spectrum These models show elliptical galaxies tend to be old  Have formed most of their stars at least ~10 billion years ago  Metallicities are about solar or a bit less

Spiral galaxies Generally have stars with a wide range of ages and metallicites  Usually modeled with continuous star formation (the rate may increase or decrease with time).  Different components (bulge, disk, halo) have different stellar populations.