1. Galaxies Read Your Textbook: Foundations of Astronomy
Chapter 16, 17
Homework Problems Chapter 16
Review Questions: 1, 2, 5-7, 10
Review Problems: 1, 5, 9, 10
Web Inquiries:
Homework Problems Chapter 17
Review Questions: 2, 4, 7, 8-10

2. Galaxy Types Ellipticals (Triaxial ellipsoids)
Lacking significant star formation

3. Ellipticals E0 through E9 E0 more spherical,
E9 more elliptical (cigar)

4. Galaxy Types
Spirals (Disks)
Significant star formation, gas, dust

5. Spiral Galaxies S0 through S9 S0 spiral arms not well defined
S9 spiral arms very well defined

6. Spiral Galaxies Sa, Sb, Sc Sa tightly wound spiral arms Sb less so
Sc barely wrapped spiral arms

7. Barred Galaxies
Barred Spirals

8. Barred Galaxies
SBa, SBb, SBc
(Spiral Arm winding)

9. Barred Galaxies
Barred Spirals

10. Galaxy Types Irregulars (includes those that are interacting)
Lots of star formation, gas and dust

11. Interacting
Interacting (Irregular)
Mergers

12. Hubble “Tuning Fork” Classification

13. Milky Way Map
Our view from within our own galaxy home.

14. Hydrogen 21-cm Radio Image

15. The Galactic Center
Picture (c): Home of a supermassive black hole?

16. You Are Here

17. Disk and Halo Stars

18. Population I and II Stars
Population II Stars (Halo objects):
Older stars
“First or Second” generation
Metal poor chemical composition (Anything else but H, He)
Large space velocities relative to the sun
High inclination orbits
Population I Stars (Disk objects):
Younger stars (solar-type)
“> Second” generation
Metal rich chemical composition (Formed from enriched ISM)
Small space velocities relative to the sun
Low inclination orbits

19. Galaxy Formation Population II stars: formed first
spherically distributed
globular clusters
Population I stars:
formed later
disk distribution
open clusters

20. Galaxy Schematic

21. Solar Galactic Orbit

22. Spiral Arm Structure Spiral structure viewed as the precession of
elliptical galactic
orbits creating
density waves.

23. Spiral Density Waves

24. Galaxy Rotation
Invariably, it is found that the stellar rotational velocities remain
constant, or "flat", with increasing distance away from the
galactic center.
This result is highly counterintuitive since, based on
Newton's law of gravity, the rotational velocity would steadily decrease
for stars further away from the galactic center. Analogously, inner
planets within the Solar System travel more quickly about the Sun than
do the outer planets (e.g. the Earth travels around the sun at about
100,000 km/hr while Saturn, which is further out, travels at only one
third this speed). Kepler’s Third Law: P2 ~ a3
One way to speed up the outer planets would be to
add more mass to the solar system, between the planets.

25. Galactic Rotation Curve

26. Galaxy Rotation Curves

27. Dark Matter There is a LOT of non-luminous matter.
Gravitationally, our observations show that the universe is almost 90% non-luminous matter!
90% of the universe is made up of stuff we can not see!
Solar System Mass to Light Ratio ~ 1
The Sun has 99.85% of the mass and 100% of the light
This changes to ~100 on galactic and extra-galactic scales
There is a LOT of mass inferred by gravity, but not much light

28. Dark Matter What is this stuff? Normal Stuff (protons, neutrons)
very small faint objects
brown dwarfs (large “jupiter” planets, star duds)
gas and dust
stellar remnants
white dwarfs
neutron stars
black holes
known or exotic yet undiscovered particles with mass
nutrinos

29. Galaxy Red Shifts

30. “Redshift”
Hubble wanted to know the distance to the faint nebulosities
He took galaxy spectra to obtain their radial velocities.
Armed with distance determinations to local galaxies utilizing pulsating Cepheid stars, Hubble discovered that galaxies were in general moving away from the Milky Way.
The recessional velocity of galaxies increases with increasing distance. (Redshift-Distance relation, a.k.a. the Hubble Law)

31. Hubble’s Redshift-Distance Relation

32. Vr = radial velocity (a.k.a. redshift,doppler shift) D = Distance
Hubble Law
Vr = H0D
Vr = radial velocity (a.k.a. redshift,doppler shift)
D = Distance
H0 = Constant (a.k.a. Hubble constant) Vr D

33. Hubble Flow Galaxies A, B, and C separated by distance d
After some time (Dt) this distance has doubled (2d)
Distance between A,B and B,C is now 2d, and A,C is 4d
Recession velocity of B from A is v = d/Dt, and B from C from A is v = 2d/Dt
The farther away you are, the faster you appear to recede!

34. Local Group Galaxies

35. Hubble Law

36. Galaxy Clusters

37. Hubble Constant
Vr = H0D
H0 = Hubble constant
= km/s/Mpc
Units for 1/H0 = seconds
This is an age estimate for our universe.
The true age should be less than this
maximum age estimate because matter has
surely caused some amount of deceleration.

38. Age Estimation
H0 = km/s/Mpc
1/H0 ago, all matter was piled up onto each other
if there has been no acceleration or deceleration.
1/H0 ~ Billion years (Gigayears)

39. H = H(t) Has expansion remained constant throughout time?
Has expansion been slowing down due to the matter in the
universe gravitationally attracting? OR
Is the universe accelerating?
Therefore, H0 = H(tnow)
The Hubble constant is not a fundamental constant,
it is variable with time.

40. How Far?