Lecture 18Lecture 18

Stellar populations

Stellar clustersStellar clusters

Open clusters:• contain 10-1000 stars• loose structure

Globular clusters:• 1000 - 1 million stars• centrally concentrated

• Simple stellar populations: stars were probably all born at nearly the same time; thus have the same age and composition

~5 pc

GalaxiesGalaxies

• contains billions of stars • stars generally have a variety of ages, compositions

~20,000 pc

Globular clustersGlobular 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 clustersOpen clusters

Stellar systemsStellar 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

~5x105 pc ~2x106 pc

Review: Stellar EvolutionReview: Stellar Evolution

Main sequence: Core hydrogen burningRed Giant branch: Shell-hydrogen

burningHorizontal branch: Core helium burningAsymptotic Red Giant branch: Shell

helium (and hydrogen) burning, around a CO, electron degenerate core

Isochrones and Evolutionary tracksIsochrones 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

log10 (age/yr)

Single-aged populationsSingle-aged populations

Nearby stars of all ages Cluster of stars all formed at the same time.

Star clustersStar 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 clustersStar 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 IsochronesTheoretical Isochrones

Age

• The main sequence turnoff is a good indicator of cluster age.

Theoretical IsochronesTheoretical 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 IsochronesTheoretical Isochrones• Stars with

more heavy elements (metal-rich) tend to be redder.

Metallicity Distance

Oxygen abundance Age

• 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 diagramsColour-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 clustersOpen clusters

Example: The Hyades cluster

Spectral type

B-V Age (109 yr)

O -0.4 <0.001

B -0.2 0.03

A 0.2 0.4

F 0.5 4

G 0.7 10

K 1.0 60

M 1.6 >100

•The colour of the brightest main sequence stars is (B-V)~0.1

•This corresponds to an A0 star.

Open clustersOpen clusters

•typically young, and metal-rich <1 billion years old

•Mostly found in the disk of the Milky Way

Name Age (Myr)

Distance (pc)

[Fe/H]

Collinder 285 199 25 0

Melotte n25 787 45 +0.17

Melotte 111 449 96 0

Mamajek 1 7.9 97 0

Melotte 227 135 120 0

Platais 8 60.2 132 0

Melotte 22 135.2 150 0

IC 2602 32.1 161 -0.09

Platais 3 398 161 0

Platais 9 100 174 0

The ten nearest known open clusters

Globular clustersGlobular 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

NGC2419NGC2419

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

Globular clustersGlobular 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 clustersGlobular clusters

Example: M92 Best fit model:

age=14 Gyr. [Fe/H]=-2.31

Globular clustersGlobular clusters

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

Cluster agesCluster 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 DifficultiesObservational Difficulties

Observational difficultiesObservational difficulties

•Finite width of the main sequence and turn-off

•Presence of blue-stragglers

Probably binary mergers

BreakBreak

Other galaxiesOther 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 galaxiesLocal 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 galaxiesLocal 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 populationsComposite stellar populations

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

Outside the Local GroupOutside 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 GroupOutside 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 galaxiesElliptical 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 galaxiesSpiral 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.