Lecture 18

1. Lecture 18 Stellar populations

2. Stellar clusters • Simple stellar populations: stars were probably all born at nearly the same time; thus have the same age and composition • Open clusters: • contain 10-1000 stars • loose structure • Globular clusters: • 1000 - 1 million stars • centrally concentrated • ~5 pc

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

4. 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 Open clusters • Mostly found in the disk of the Milky Way

5. Stellar systems • Galaxy clusters: • Thousands of galaxies, trillions of stars • The largest bound structures in the Universe • Galaxy groups: • A few tens of galaxies in orbit about one another • ~5x105 pc • ~2x106 pc

6. 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

7. Isochrones and Evolutionary tracks • For a given mass, we can model how it will evolve with time • For a collection of stars with a range of masses, we can plot where they will be at a given time: these are isochrones. log10 (age/yr) Models for different masses Models for different ages

8. Single-aged populations • Nearby stars of all ages • Cluster of stars all formed at the same time.

9. 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 RGB HB MS

10. 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

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

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

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

14. 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

15. Open clusters • Example: The Hyades cluster • The colour of the brightest main sequence stars is (B-V)~0.1 • This corresponds to an A0 star.

16. Open clusters The ten nearest known open clusters • typically young, and metal-rich <1 billion years old • Mostly found in the disk of the Milky Way

17. 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

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

19. 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.

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

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

22. 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

23. Observational Difficulties

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

25. Break

26. 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.

27. 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)?

28. 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

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

30. 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?

31. 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?

32. 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

33. 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.