1. Galaxies
Island Universes

2. Copyright – FORS1 VLTI, European Southern Observatory

3. Galactic Distances How do we know the distance to objects in space?
Stellar parallax:
Parallax of nearby stars relative to background stars.
Good out to ~500 pc.
What about farther than that?

4. Standard Candles “Standard Candles”
If we know how bright something looks,
And we know how bright it should be
(luminosity),
Result  Distance
We do this everyday
with size.

5. m versus M
If you know how luminous a star REALLY is and how bright it looks from Earth, you can determine how far away it must be to look that faint.
m – M give you distance.

6. Example Deneb is A2Ia star m = 1.25 A2  Blue star Ia  Supergiant
Distance = 1000 pc

7. Standard Candles Other “Standard Candles” Variable stars.
Stars that change in luminosity.
RR Lyra stars
Cepheid variables
Supernovae

8. 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!

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

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

11. How it works For Cepheid in M100 P = 20 days.
From P-L: L = x Sun
Msun = 5, so MCep = -5
m = 20
m – M = 25
So 25/5 = 5 = log(d/10pc)
How log works:
What is 100 = 10x?
Same as saying 2 = log(100)
So 5 = log(d/10pc)
d/10pc =
D = 1,000,000 pc

12. Supernovae
A special type of Supernova (Type 1a) seems to be a good standard candle.
If all Type 1a supernovae have same maximum luminosity then look to see the maximum apparent brightness from Earth and get distance.

13. M95 supernova – Copyright Adam Block, Mt. Lemmon SkyCenter

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.

20. 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  distant galaxies?

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

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

23. 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.

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

25. 140 Mpc
70 Mpc

26. Concept Test
Imagine that Cepheid variables were more luminous than 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.

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

28. Homework #17
For Monday Read: Bennett Ch 13
Do Chapter 13 Quiz