1. Galaxies – Island Universes
A historic tour of the discovery of the dwindling significance of humans in the universe:
From the center of the universe towards the edge of an average galaxy amongst 100 billion others

2. How do we know where we are?
“Obviously” we are living on a flat Earth at the center of the universe, as a quick look tells us:
The stars, Sun, Moon and planets rotate us
There is no apparent curvature of the ground
The Milky Way is a band that surrounds us
There are no signs for any movement of the Earth (like wind, or forces throwing us off)

3. Logic to the Rescue How do we avoid these wrong conclusions?
Sound data
Flawed interpretation/reasoning
Further observations are necessary to decide!
Do we have to question everything?
Yes, in principle.
The signature of genius is to ask the right question, not necessarily to answer them.

4. Exploring our own Island Universe: The Milky Way
A galaxy is a huge collection of stars, gas, dust, neutron stars, and black holes, isolated from others and held together by gravity
Andromeda

5. Our view of the Milky Way
Appears as a milky band of light across the sky
A small telescope reveals that it is composed of many stars (Galileo again!)
Our knowledge of the Milky Way comes from a combination of observation and comparison to other galaxies

6. How do we know?
Obviously a bogus picture of our milky way!
Question: How can we say anything about our Milky Way, if we cannot see it from outside?

7. Enter: the Genius William Herschel (XVIII century) Simple model:
Assumed all stars have the same absolute brightness
Counts stars as a function of apparent magnitude
Brighter stars closer to us; fainter stars further away
Cut off in brightness corresponds to a cut off at a certain distance.
Conclusion: there are no stars beyond a certain distance

8. Herschel’s Findings
Stars thinned out very fast at right angles to Milky Way
In the plane of the Milky Way the thinning was slower and depended upon the direction in which he looked
Flaws:
Observations made only in visible spectrum
Did not take into account absorption by interstellar gas and dust

9. Discovering other Island Universes
Data: Lots of nebulous spots known in the nightsky
Questions: What are they? All the same? Different things?
Need more observations!
 Build bigger telescopes

10. The first nebula discovered to have spiral structure: M51

11. M99 is a spiral, too! Q: do we live in a spiral?
Q: Are we in the center of the spiral?
Most probable answer: No!

12. Enter: next genius
Harlow Shapley used variable stars, e.g. RR Lyrae stars, to map the distribution of globular clusters in the galaxy
Found a spherical distribution about 30 kpc (30,000 pc) across
This is the true size of the galaxy
Sun is (naturally!) not at the center – it’s about 26,000 ly out

13. Standing on the shoulders of Giants
Shapley used methods developed by others to measure the distance to globulars
Cepheid variables show luminosity-period correlations discovered by Henrietta Leavitt
Shapley single-handedly increase the size of the universe tenfold!

14. Structure of the Galaxy
the halo
roughly spherical, possibly flattened along the galactic equator
contains little gas or dust
globular clusters
few blue stars
halo stars are poor in heavy elements (W. Baade, 1940's)
galactic disk: concentration of stars in the galactic plane
open clusters, star forming regions (emission nebulae, etc.)
most of the blue stars of the galaxy are confined to the disk
disk has spiral structure
[Conclusion: the halo stars are old (1st generation), and little star formation occurs there. Most of the formation of new stars takes place in the galactic disk.]
galactic bulge: a thickening of the galactic disk near the center of the Galaxy
galactic center: a large mass is concentrated at the center of the Galaxy

15. Intra-galactic Dynamics
Three main parts of a galaxy:
Bulge (center of galaxy)
Disk (rotating around center)
Halo (orbiting around bulge with randomly inclined orbits)

16. Properties of Bulge, Disk and Halo
Disk Halo Bulge
Highly flattened spherical football-shaped
young and old stars only old stars young and old stars
has Gas and dust none lots in center
Star formation none since 10 billion yrs in inner regions
White colored, reddish yellow-white
blue spiral arms

17. An up-to-date “Reconstruction”

18. Other Galaxies: Hubble supersedes Shapley
Edwin Hubble identified single stars in the Andromeda nebula (“turning” it into a galaxy)
Measured the distance to Andromeda to be 1 million Ly (modern value: 2.2 mill. Ly)
Conclusion: it is 20 times more distant than the milky way’s radius  Extragalacticity!
 Shapley’s theory falsified!

19. Q: How many galaxies are there?
Hubble Deep Field Project
100 hour exposures over 10 days
Covered an area of the sky about 1/100 the size of the full moon
Probably about 100 billion galaxies visible to us!

21. About 1,500 galaxies in this patch alone
Angular size ~ 2 minutes of arc

22. Other Galaxies
there are ~ 100 billion galaxies in the observable Universe
measure distances to other galaxies using the period-luminosity relationship for Cepheid variables
Type I supernovae also used to measure distances
Predictable luminosity – a standard candle
Other galaxies are quite distant
Andromeda (M31), a nearby (spiral) galaxy, is 2 million light-years away and comparable in size to Milky Way
“Island universes” in their own right

23. Q: How does our galaxy look like from the outside?
Probably like others, so observe them!

24. Hubble Classification Scheme
Edwin Hubble (~1924) grouped galaxies into four basic types:
Spiral
Barred spiral
Elliptical
Irregular
There are sub-categories as well

25. Spirals (S) All have disks, bulges, and halos
Type Sa: large bulge, tightly wrapped, almost circular spiral arms
Type Sb: smaller bulge, more open spiral arms
Type Sc: smallest bulge, loose, poorly defined spiral arms

26. Barred Spirals (SB)
Possess an elongated “bar” of stars and interstellar mater passing through the center

27. Elliptical (E) No spiral arms or clear internal structure
Essentially all halo
Vary in size from “giant” to “dwarf”
Further classified according to how circular they are (E0–E7)
Left E0, right E3

28. S0/SB0 Intermediate between E7 and Sa
Ellipticals with a bulge and thin disk, but no spiral arms
Not an evolutionary diagram!