1. The Milky Way and Beyond
Galaxies and the Larger Scale Structure of the Cosmos

2. The Milky Way Our Home Galaxy We live on the “fringes”
75% of the distance out from center
Our solar systems makes one orbit of Galactic center every 250 million years!
Makes Galaxy difficult to describe due to our perception

3. The Milky Way Home to some 100 billion stars
Believed to have a Spiral Structure
This is inferred from various observations
Interstellar dust thwarted early observers
Advent of Radio and IR telescopes improved model

4. What Shape is the Milky Way?
Dust initially confused observers
Thought we were in the center of the Galaxy
Stars seemed to be equally distributed
Shapley demonstrated that the 100+ Globular Clusters didn’t orbit us
They orbit a point 30,000 ly distant

5. The Milky Way Galaxy Side View
Nuclear Bulge
Disk
Sun
Halo
Globular Clusters

6. Side View Structure Disk- 100,000 ly across, 2000 ly thick
Contains Spiral Arms
Nuclear Bulge- 20,000 ly across
Contains Nucleus of Galaxy
Halo- 300,000 ly across
Contains Orbiting Globular Clusters and Dark Matter
Each part has a different population of Stars

7. Stellar Populations Blue disk stars = Population I
Red bulge and halo stars = Population II
Further study yields
Bulge- Aging Population I Stars and Pop II
Disk- Young Population I Stars
Halo- Old Population II stars

8. Stellar Populations Population I Population II lots of metals
Young and blue
Circular orbits
Population II
metal poor
Old and red
Elliptical and tilted orbits

9. Stellar Populations
Not all stars (i.e. Sun) fit easily into either category
Subdivisions include extreme and intermediate Populations, and the “old disk” category
Open clusters contain Pop I
Globular clusters contain Pop II

10. Galactic Motions Globular Clusters orbit around the nucleus randomly
Bulge stars are “semi-random”

11. Galactic Motions
Sun and other disk material orbits nucleus of Galaxy in an orderly way
Experiences Differential rotation
Observed in other Spiral Galaxies
“Rotation” occurs due to a Density Wave
It is not a rigid motion of an “arm”
Wind-up problem

12. Density Waves Material in the wave is not fixed
Material can move through the wave
Not a material wave but a disruption wave
Like a traffic jam behind a slow moving vehicle
Wave passes through ISM and triggers star formation

13. Spiral Arm Structure Number of Arms isn’t well known.
All numbers between 2-10 have been suggested
Use Spiral Arms Tracers to map the arms
Molecular Clouds (Radio)
H II regions (Optical)
Cepheid Variables (Optical)
OB Stars (Optical)

14. Variable Stars Cepheid and RR Lyrae Variables
Luminosity Varies in predictable ways
RR Lyrae vary over day
Cepheids vary days
Both on Instability Strip of HR diagram

15. Period-Luminosity Relation
Relationship of Period of Pulse and Luminosity of Star
Linear for Cepheids
Constant for RR Lyraes
Cepheid distances can then be determined
Used for large distances because they are brighter

16. Period Luminosity

17. RR Lyrae Found in Globular Clusters
Shapley used observations to establish distances to GC

18. Other Tracers Molecular Clouds emit in Radio H II regions and OB stars
Use Doppler shift to map arm structure
H II regions and OB stars
Luminosity is known
Distance obtained from Inverse Square Law
Group objects by distance, spiral structure seen

19. Nucleus Very Obscured Very crowded
Sagittarius A- powerful radio source, x-ray jets
Million M Black Hole?
Radio reveals two H arms shooting out

20. Nucleus
Jansky first looked into the heart of the Galaxy with Radio waves
Evidence of star formation ongoing with giant molecular clouds and HII regions
Cool hydrogen and a ring of molecule rich gas exist even closer to the center
As we approach the center, we use many “eyes” to see

21. The Heart of the Galaxy Swarm of stars circle the center of the Galaxy
Millions packed into a cubic light year
At the very center is a ring of dust and gas
This surrounds a very small (10 AU) but very powerful source
This is the suspected black hole

22. In the Halo
If mass were condensed in the center of the Galaxy, rotation would obey Kepler’s 3rd law (T2=R3)
More distant objects would orbit more slowly and we can calculate speeds
This relationship doesn’t hold true

23. Rotation Curve
Plotting speeds of objects based on distance from Galactic center
Appears that most of the mass is contained in the halo

24. Rotation Curve

25. Rotation Curve

26. Formation of the Galaxy
Similar to Star Formation
Everything is on a much larger scale
Halo objects form first
Globular Clusters
Halo Stars
Disk and Nucleus collapse next
Collapse generates star formation

27. A Universe of Galaxies Normal Galaxies come in 3 types
Spirals
Ellipticals
Irregulars
Each galaxy has a different morphology
Also different stellar populations
Classified on Hubble Tuning Fork Diagram

28. Hubble Tuning Fork Diagram

29. Spiral Galaxies S0 (Sa?)

30. Spiral Galaxies (Sb)

31. Spiral Galaxies (Sc)

32. Spiral Galaxies (Sc)

33. Elliptical Galaxies (E1)

34. Elliptical Galaxies (E2)

35. Elliptical Galaxies

36. Barred Spirals

37. Barred Spiral (w/ Star Formation)

38. Barred Spiral

39. Irregular Galaxies (LMC)

48. Comparisons Ellipticals have a wide range of sizes
Giants can contain trillions of stars
Dwarfs contain millions of stars
Spirals are more consistent in size
100’s of Billions of stars
Irregulars smaller than Spirals
100 million to 10 billion stars
Smaller Irregulars are more common

49. Comparisons Spiral Galaxies contain a mix of stars
Much ISM present
Ellipticals primarily contain old stars
Very little ISM present
Irregulars contain many young stars

50. Causes of Shapes
Perhaps the circumstances of collapse determines galaxy type
Motion within pre-galactic gas cloud determines organization and star formation rate
Small motions=Spiral
Large motions=Elliptical

51. Groups of Galaxies Galaxies tend to cluster into groups
Small Groups contain 10’s
Large groups can contain 1000’s or millions
Our group = Local Group
20 or so galaxies
Virgo Cluster contains galaxies
Clusters cluster forming superclusters!

52. Distribution of Clusters
Superclusters and voids
Great Wall- large distant supercluster

53. Galactic Cannibalism
In clusters, galaxies can get trapped in a gravity war
The galaxies can merge, pass through one another, or get eaten by a larger one
Often such activities trigger large amounts of star formation
Could explain the presence of the giant galaxies in some clusters

54. Selection Effects Spirals generally more luminous
OB stars and H II regions
Easier to see than Irr and E’s
Count more of them, under count Irr and E’s
Bias data to Spirals (appear to be most common)
Irregulars and Ellipticals are much more common

55. Galaxy Comparison Level the playing field!
Need to know distance to Galaxies
No HR diagram for Galaxies
Brightness= closeness, smallness=farness
To find distance, use distance indicators

56. Distance Indicators Aka Standard Candles Assumptions
Physics is Universal
Stellar Evolution is Universal
Many methods use to find distance

57. Distance Indicators Find a familiar object
Star, H II region, SN, etc
Know the object’s luminosity
Determine distance using Inverse Square law for Light
Example: Cephied Variables
Know L from P-L relation
Find distance

58. Distance Indicators Not every galaxy has Cepheid
Other objects can be used
Some work for nearby galaxies, others for more distant objects
Error in luminosity can cause error in distance

59. Distance Indicators Cepheids (near) OB stars (near) Novae (near)
Globular Clusters (mid)
Planetary Nebula (near)
HII regions (mid)
Type I SN (Far)
Tully-Fisher relation (Far)
Galaxy Luminosity (Far)

60. What’s the Point? Hubble determined there were other Galaxies (1924)
Determined the Universe was expanding
Red-shift of light from Galaxies
Red-shift= moving away
Distant galaxies are receding faster!
Leads to an important law…

61. Hubble’s Law d=v/H H varies from 15-30 km/s/Mly (50-90km/s/Mpc)
d=distance
v=radial (recessional) velocity
H=Hubble Constant
d=v/H
H varies from km/s/Mly (50-90km/s/Mpc)
H is a measure of age of Universe!

62. Hubble Constant Consternation!
Depending on what you use as a distance indicator, errors arise
Different groups give different values of H
Gives a radically different value for age of the Universe!
Lower value =older universe
Higher value= younger universe

63. Flat, Closed or Open
The ultimate fate of the universe determined by it’s “shape”
Shape is determined by mass
Too much mass-Big Crunch, Oscillating Universe (closed)
Too little mass-never ending expansion (open)
Just right mass-Flat, expansion ceases, no collapse