1. Galaxies
Island Universes

2. Copyright – FORS1 VLTI, European Southern Observatory

3. Ellipticals

4. Ellipticals Huge No gas. No dust. No young stars.
Nothing but old stars.
Random orbits.

5. Spirals Like Milky Way. Disks and bulge. Young stars and old.
Gas and dust.
Stars forming.
Stars dying.
M81 and M82 – Copyright R. Gendler

6. M63 Copyright – S. Miyazaki, Suburu

7. NGC1365 Copyright – VLT

8. M33 – Copyright NOAO

9. Irregulars

10. Distance to Near Galaxies
Need a standard candle.
If there is something in a galaxy which has a known luminosity we can determine a distance.
Several candidates:
Variable stars
Supernovae
“Brightest” stars

11. Variables in Clusters M3
Copyright – K. Stanek (Harvard)

12. Variable Stars For RR Lyrae stars: For Cepheid variables:
Average luminosity is a standard candle
Always ~ 100 x Sun
For Cepheid variables:
Pulsation period is proportional to average luminosity
Observe the period  find the luminosity
Good to 15 Mpc!

13. Nearby Galaxies
Cepheids
Period
Luminosity  Mv
Know mv
Get Distance

14. The Local Group

15. Groups

16. The Virgo Cluster

17. Clusters

18. Concept Test
A standard candle can be any object (or class of object) that:
Always has the same luminosity.
Has some means of knowing its luminosity without first needing to know its distance.
Can vary in brightness (as long as it always has the same average luminosity).
Has a known absolute magnitude.
Always gives off the same amount of energy, regardless of distance from us.

19. Distant Galaxies Can’t see individual stars. Supernovae rare.
Can use nearby galaxies to get distances to further galaxies.
Distance ladder:
Parallax  nearby stars
Nearby stars  H-R diagram
H-R diagram  distant stars (variables)
Variable stars  nearby galaxies
Nearby galaxies  Hubble’s Law

20. 21cm Radiation Neutral hydrogen (HI) gives off light, l = 21cm.
Milky Way HI emission – Copyright J. Dickey

21. Extragalactic HI Observe HI in other galaxies.
Measure wavelength of 21 cm radiation.
Doppler Shift: Get velocity away from us.

22. Hubble’s Law Measure the velocity of every galaxy.
Ho = 71 km/s/Mpc
Measure the velocity of every galaxy.
Nearly all are redshifted.
Use Cepheids to measure distances to nearby galaxies.
Result: The faster it’s moving, the farther away it is.

23. Map the Universe v = HoD If you know Ho: 71 km/s/Mpc Measure v Get D
Find:
Voids
Walls
Clusters

24. 140 Mpc
70 Mpc

25. Concept Test
Imagine that Cepheid variables were not as luminous as previously thought. As a result, Hubble’s constant would be:
Smaller than previously thought.
Larger than previously thought.
Unchanged since we aren’t changing either the velocity or position of the galaxy.
None of the above.

26. Limits to Hubble’s Law Negative velocity? Galaxy pairs? Clusters?
Orbits?

27. Homework #13
Read: Bennett Ch 22.1 – 22.4
Do Ch22:
Problems: 9, 12