1. Lecture 32: Galaxies
We see that scattered through space out to infinite distances, there exist similar systems of stars, and that all of creation, in the whole extent of its infinite grandeur, is everywhere organized into systems whose members are in relation with one another…
-- Immanuel Kant, 1755

2. Topics: Different types of galaxies
Finding the distances to far away galaxies
Hubble’s Law and the expansion of the Universe

3. Dwarf Irregular Galaxies
Large Magellanic Cloud
Small Magellanic Cloud

4. The Sagittarius Dwarf galaxy: our nearest companion

5. dwarf ellipticals
Antlia
Leo I

6. M31 – the Andromeda Galaxy

7. The Local Group

8. Spiral Galaxies

9. A Grand Design Spiral

10. Edge-on Spirals

11. Edge on spirals
“Sombrero” Galaxy

12. A Lenticular galaxy

13. Elliptical Galaxies

14. Galaxies like to live in groups

15. The Virgo Cluster

16. The Virgo Cluster
M87
giant elliptical
galaxy

17. Hubble Sequence

18. Trends along the Hubble Sequence
Lenticular
Spiral
Elliptical
Irregular
red blue
old stars young stars
gas poor gas rich
no star formation lots of star formation

19. Measuring distances to far away galaxies
many techniques for measuring distances in astronomy rely on finding standard candles
these are objects that always have the same luminosity
if we know the luminosity, we can measure the flux and find the distance

20. Example: main sequence fitting
an example of a standard candle is a star of a particular spectral type – for example, a G-type star like the Sun
we plot the main sequence for a nearby cluster of stars, for which we can find the distance by parallax
then we assume that all main sequence stars of the same spectral type have the same luminosity.

21. main sequence fitting

22. other ways to measure distances
Cepheid variable stars (period-luminosity relation)
white-dwarf supernovae used as standard candles
Tully-Fisher relation

24. Tully-Fisher Relation
for spiral galaxies
relationship between rotation velocity and luminosity

25. The Cosmic Distance Ladder

26. measuring recession velocity

27. reminder: Doppler formula
redshift = z = (lobserved-lrest)/lrest

28. Hubble’s Law
recession velocity
distance

29. Hubble’s Law (modern version)

30. v = H0 r Hubble’s Law Formula
recession velocity = constant times distance
units of H0: km/s/Mpc

31. The expanding Universe
Hubble’s Law implies that all galaxies are moving away from us
the farther away they are the faster they are moving away from us
this makes sense if the whole fabric of space-time is expanding

32. Please finish reading Ch. 19 over break!