lecture 18 stellar populations. stellar clusters open clusters: contain 10-1000 stars loose...
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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.