1. This is NOT the Milky Way galaxy! It’s a similar one: NGC 4414.

2. New distance unit: the parsec (pc).
Using Earth-orbit parallax, if a star has a parallactic angle of 1",
it is 1 pc away.
If the angle is 0.5", the distance is 2 pc. 1
Parallactic angle (arcsec)
Distance (pc) =
Closest star to Sun is Proxima Centauri. Parallactic angle is 0.7”, so distance is 1.3 pc.
1 pc = 3.3 light years
= 3.1 x 1018 cm
= 206,000 AU
1 kiloparsec (kpc) = 1000 pc
1 Megaparsec (Mpc) = 10 6 pc

3. The Milky Way Galaxy

5. Take a Giant Step Outside the Milky Way
Artist's Conception
Example (not to scale)

6. . from above ("face-on") see disk, with spiral and
bar structure, and bulge (halo too dim) .
Sun
from the side ("edge-on")

7. The Three Main Structural Components of the Milky Way
1. Disk
- 30 kpc diameter
- contains young and old stars, gas, dust. Has spiral structure
- vertical thickness roughly 100 pc - 2 kpc (depending on component. Most gas and dust in thinner layer, most stars in thicker layer)

8. 2. Halo 3. Bulge - at least 30 kpc across
- contains globular clusters, old stars, little gas and dust, much "dark matter"
- roughly spherical
3. Bulge
- About 4 kpc across
- old stars, some gas, dust
- central black hole of 3 x 106 solar masses
- spherical

9. Globular Clusters
- few x 10 5 or 10 6 stars
- size about 50 pc
- very tightly packed, roughly spherical shape
- billions of years old
Clusters are crucial for stellar evolution studies because:
1) All stars in a cluster formed at about same time (so all have same age)
2) All stars are at about the same distance
3) All stars have same chemical composition

10. . How was Milky Way size and our location determined?
Herschel (late 18th century): first map of Milky Way. Sun near
center.
Herschel’s Milky Way drawing .
Sun
3 kpc

11. Shapley (1917) found that Sun was not at center of Milky Way
Shapley used distances to Globular Clusters to determine that Sun was 16 kpc from center of Milky Way. Modern value 8 kpc.

12. Stellar Orbits
Halo: stars and globular clusters swarm around center of Milky Way. Very elliptical orbits with random orientations. They also cross the disk.
Bulge: similar to halo.
Disk: rotates.

13. How Does the Disk Rotate?
Sun moves at 225 km/sec around center. An orbit takes 240 million years.
Stars closer to center take less time to orbit. Stars further from center take longer.
=> rotation not rigid like a phonograph record. Rather, "differential rotation".
Over most of disk, rotation velocity is roughly constant.
The "rotation curve" of the Milky Way

15. Spiral Structure of Disk
Spiral arms best traced by:
Young stars and clusters
Emission Nebulae
Atomic gas
Molecular Clouds
(old stars to a lesser extent)
Disk not empty between arms, just less material there.

16. Problem: How do spiral arms survive?
Given differential rotation, arms should be stretched and smeared out after a few revolutions (Sun has made 20 already):
The Winding Dilemma

17. So if spiral arms always contain same stars, the spiral should end up like this:
Real structure of Milky Way (and other spiral galaxies) is more loosely wrapped.

18. A Spiral Density Wave Proposed solution:
Arms are not material moving together, but mark peak of a compressional wave circling the disk:
A Spiral Density Wave
Traffic-jam analogy:

19. Now replace cars by stars
Now replace cars by stars. The traffic jams are due to the stars' collective gravity. The higher gravity of the jams keeps stars in them for longer. Calculations and computer simulations show this situation can be maintained for a long time.
Traffic jam on a loop caused by merging
Not shown – whole pattern rotates

20. Gas clouds pushed together in arms too => high density of clouds => high concentration of dust => dust lanes.
Also, squeezing of molecular clouds initiates collapse within them => star formation. Bright young massive stars live and die in spiral arms. Emission nebulae mostly in spiral arms.
So arms always contain same types of objects, but individual objects come and go.

21. 90% of Matter in Milky Way is Dark Matter
Gives off no detectable radiation. Evidence is from rotation curve: 10
Rotation
Velocity
(AU/yr)
Solar System Rotation Curve: when almost all mass at center, velocity decreases with radius ("Keplerian") 5 1 1 10 20 30
R (AU)
Curve if Milky Way ended where radiating matter pretty much runs out.
observed curve
Milky Way Rotation Curve

22. Not enough radiating matter at large R to explain rotation curve => "dark" matter!
Dark matter must be about 90% of the mass!
Composition unknown. Probably mostly exotic particles that hardly interact with ordinary matter at all (except gravity). Small fraction may be brown dwarfs, dead white dwarfs.
Most likely it's a dark halo surrounding the Milky Way.
Mass of Milky Way
6 x 1011 solar masses within 40 kpc of center.

23. from above ("face-on")
see disk and bulge (halo
too dim)
Perseus arm
Orion spur
Sun
Cygnus arm
Sagittarius arm
Carina arm
from the side ("edge-on")

24. Galaxies

25. Early drawings of
nebulae by Herschel (1811). Stars, gas or
both? Distances?
First “spiral nebula” found in 1845 by the Earl of Rosse. Speculated it was beyond our Galaxy.
"Great Debate" between Shapley and Curtis on whether spiral nebulae were galaxies beyond our own. Settled in 1924 when Hubble observed individual stars in spiral nebulae.

26. The Variety of Galaxy Morphologies

27. More on bars…
Milky Way schematic
showing bar
Another barred galaxy
A bar is a pattern too,
like a spiral.

28. Galaxy Classification
Hubble’s "tuning fork diagram"
bulge less prominent,
arms more loosely wrapped
Irr
disk and large bulge, but no spiral
increasing apparent flatness
Spirals Ellipticals Irregulars
barred unbarred E0 - E Irr I Irr II
SBa-SBc Sa-Sc "misshapen truly
spirals" irregular

29. Still used today. We talk of a galaxy's "Hubble type"
bulge less prominent,
arms more loosely wrapped
Irr
disk and large bulge, but no spiral
increasing apparent flatness
Still used today. We talk of a galaxy's "Hubble type"
Milky Way is an SBbc, between SBb and SBc.
What the current structure says about a galaxy’s evolution is
still active research area.
Ignores some notable features, e.g. viewing angle for ellipticals, number of spiral arms for spirals.

30. Sa vs. Sc galaxies
Messier 81 – Sa galaxy
Messier 101 – Sc galaxy

31. Irr I vs. Irr II Irr I (“misshapen spirals”) Irr II (truly irregular)
bar
poor beginnings
of spiral arms
Large Magellanic Cloud
Small Magellanic Cloud
These are both companion galaxies of the Milky Way.

32. Ellipticals
Similar to halos of spirals, but generally larger, with many more stars. Stellar orbits are like halo star orbits in spirals.
Stars in ellipticals also very old, like halo stars.
An elliptical
Orbits in a spiral

33. A further distinction for ellipticals and irregulars:
Giant vs Dwarf
stars stars
10's of kpc across few kpc across
Dwarf Elliptical NGC 205
Spiral M31
Dwarf Elliptical M32

34. In giant galaxies, the average elliptical has more stars than the average spiral, which has more than the average irregular.
What kind of giant galaxy is most common?
Spirals - about 77%
Ellipticals %
Irregulars %
But dwarfs are much more common than giants.

35. Local solar neighborhood, typical stellar distances ~ 1pc (3.26 ly)
ASTROPHYSICAL “DEMOGRAPHICS”
Local solar neighborhood, typical stellar distances ~ 1pc (3.26 ly)
-- 50% are double/multiple systems
-- Most in near neighborhood (local arm) are cooler/fainter
Most of Galaxy within ~ 20 kpc ( stars)
800 kpc to M31
Local group (about 20), radius ~ 1 Mpc
Local supercluster (about 2500), distance ~ 15 Mpc to Virgo cluster
Several other galaxy clusters to ~75 Mpc
Increasing numbers of AGNs out to radius ~ 100 Mpc
QSOs to extent of observable universe, ~10000 Mpc

36. SIZE SCALES MILKY WAY Jupiter’s Orbit Radius 5.2AU LOCAL GROUP
DISTANCES TO:
GALACTIC CENTER Kpc
LARGE MAG. CLOUD 55 Kpc
M31 ~ 800Kpc
LOCAL ISM