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Galactic coordinates
in celestial equator plane
NCP
in galactic plane
galactic equator tilted ~ 63 1/2 deg to CE
galactic center is toward Sagitarrius
RA~18h, Dec ~ -29deg
Makeup of Milky Way Galaxy
Stars - Disk (O, B stars, SG, young to old open clusters)
Halo & Bulge (RR Lyr, globular clusters, MACHOs)
Gas - some in disk, hot gas in halo
Dust - in disk [results in reddening E(B-V), Av]
DUST
m-M = -5 + 5 log d + A
E(B-V) = (B-V)observed - (B-V)normal
Av ~ 3 E(B-V) [~ 1 mag/kpc roughly]
Find E(B-V) from:
• spectral type of star and observed B-V
• H / H ratio [normal = 3]
• HI maps NH/E(B-V) = 5x1021 atoms/cm2/mag
• 2200Å bump
0
Extinction =
10
A()/E(B-V)
4
0
0
0
0
0
Wavelength (Å)
0
Stellar Populations
z(pc) Age (109yrs)
Z
Distr
Extreme Pop I
120
<0.1
0.04
patchy
Older Pop I
160
0.1-10
0.03
patchy
Examples
O,B,SG, open clusters
sun, A stars
*************************************************************************************
Disk Pop II
400
3-10
0.02
smooth
planetaries, RR Lyr
Intermed Pop II
700
10
0.01
smooth
long P var
Halo Pop II
2000
>10
0.003 smooth
globular clusters
Interstellar Gas
• optical absorption lines CaI, CaII, NaI
• HII regions (recombination around hot star) T~10,000K,
density ~ 5000 ions/m3
• HI gas (21 cm) T~100K, density ~106 atoms/m3
• molecular clouds (radio) H2, OH, NH3 T~10K, density ~109
mol/m3
• X-rays (hot coronal gas) T~ 106K, density < 104 particles/m3
Counting Stars D= #stars/unit volume
Local luminosity function: #stars/unit V with given Mv
total sky = 4 steradians = 41,253 sq deg
for solid angle , area =  r2
dV = 
r2
dr
dr
N(r) = D(r)dV=  Dr2dr = 1/3 Dr3 
log r = (m-M+5)/5 = 0.2 m + const (for given M)
r = 10(0.2m+c) and N(r) = 10(0.6m+c)
since 100.6 = 4, expect 4xmore at m+1 than m
not observed
r
r2
Finding the mass of the Milky Way
Kepler’s law using sun’s orbit (P=250
million yrs, v=250 km/s, a=8kpc)
mMW + msun = 42a3/GP2 ~ 1011M
Halo mass: MACHOs, high vel stars
Rotation curve: M = rv2/G
The Galactic Center (Sgr A*)
Evidence for a Supermassive BH at the center:
• stationary (located at dynamic center of MW)
• energetic X-ray source
• small size (radio shows smaller than solar system)
• no visible object at opt nor IR from Keck images
• motions of nearby stars (1000’s of km/s) imply 3 million M
How does Supermassive BH form?
• stars in center are < 1000AU apart (200,000AU near sun)
• SN chain reaction could produce many stellar BHs
• collisions between BHs cause monster supermassive BH
Galaxy Evolution
• Top Down: large concentration of matter
(1015M) fragment into galaxies of 1012M
• Bottom up: small structures merge into
galaxies, then clusters
1)
globulars formed ~ 13 billion yrs ago
2)
collapse to disk
3)
star formation continued in disk
4)
collisions with dwarf galaxies add to halo
5)
in ~ 5 billion yrs, collision with Andromeda could
cause burst of star formation, uses up gas & dust
and turns MW into an elliptical galaxy
Review of Astr 322- the Contents of the Milky Way
Content
Structure
Disk Bulge
Stars
Gas & Dust
Hot, cold; Av, E(B-V)
Dark Matter
MACHOs + ?
Pop I
Halo
Pop II Pop II
Motion
Disk - LSR
Halo - high v, elliptical
Single (sun), binary, clusters (open, globular)
Viewing geometry
Properties (d, T, L, Mv, spectra, mass, radius)
Horizon (alt, azimuth)
Celestial (RA, Dec)
Galactice (b, l)
Evolution - low mass (T Tauri, MS, giant, planetary, WD)
- high mass (MS, SG, SN, pulsar or BH)
Variables - geometric, eruptive, pulsating
Instrumentation:
Telescopes (refractors, reflectors)
CCDs, spectrographs, Space