The Milky Way
1st observed as a cloud-like band that stretched across local sky
MPP©2004
The Milky Way
Appears year round ∴ encircles the Earth
Racetrack Playa, Death Valley
Galaxias (Greek)
Via Lactae (Latin)
MPP©2004
Structure of the Milky Way Galaxy
Side View
Main Components
Bulge, Disk, Galactic Halo (stars, gas, dark matter)
MPP©2004
1
Structure of the Milky Way
MMW = 1011 - 1012 MSuns
LMW = 1010 LSuns
Oblique View
Mass-to-Light ratio suggests that we see < 10% of the matter
MPP©2004
Galactic Bulge
Diameter = 6500 L.Y.
Mass = 1010 Msuns
~ 3 L.Y. between stars
Stars very close to the Galactic center orbit around a strong source of radio
waves (Sagittarius A*) at very high speeds (225 km/s)
MPP©2004
Galactic Center
MPP©2004
Periodically, bursts of X-Rays are observed
2
Super-massive Black Hole
M > 1,000,000 Suns confined to space smaller than 20 AU
- Gas in the accretion disk produces continuous source of radio waves
- In falling stars are torn apart producing periodic X-ray bursts
MPP©2004
Galactic Disk
Diameter = 100,000 L.Y.
Mass = 7 x 1010 MSuns (Stars & ISM)
Stellar Density ~ 10 L.Y. between stars
Thickness depends on object
- ISM: ~ 4000 L.Y.
- Sun-like stars: ~ 2500 L.Y.
- O & B stars: ~ 500 L.Y.
MPP©2004
Stellar Halo
Diameter = 200,000 L.Y.
Mass = 109 Msuns
~ 300 L.Y. between stars
1st
Location of
generation stars that lack “heavy elements”
Interstellar Medium is gone
MPP©2004
3
Rotation of the Galaxy
How does the Milky Way rotate as a whole entity?
Rotation curve of a solid disk
Rotation curve as result of a collection of
orbiting objects
MPP©2004
Milky Way Rotation Curve
Galactic rotation curve implies ~ 90% of the matter in a galaxy does not
emit or interact with light (Dark Matter)
MPP©2004
Dark Matter Halo
Halo of non-light producing matter extending well beyond the
stellar halo (600,000 L.Y.)
How much dark matter?
Nature of dark matter?
MAssive Compact Halo Objects
Vs.
Micro-lensing can give mass
Weakly Interacting Massive Particles
MPP©2004
4
Galaxies
19th century astronomers began observing tiny, cloud-like objects
(“fuzzy nebulae”)
The Great Nebula Debate (April 26, 1920)
What is the nature of the “spiral nebulae”?
Harlow Shapley – small clouds of ISM in the Galaxy
(based on “spiral nebulae” rotations & novae distances)
Heber Curtis – far-away galaxies similar to the Milky Way
(based on novae distances, zone of avoidance problem)
MPP©2004
Galaxies
1922: Edwin Hubble resolves the “Great Nebula Debate” by observing
Cepheid Variables in the Andromeda Nebula
Using P-L relationship Hubble shows its distance to be well outside
the accepted boundaries of the MW.
MPP©2004
Hubble Sequence
Hubble Sequence: classification system based on appearance
Ellipticals (E), Lenticulars (S0), Spirals (S/SB), Irregulars
MPP©2004
5
Hubble Sequence
Arranged according to the perceived evolutionary sequence
a.k.a the Hubble Tuning Fork Diagram
MPP©2004
Elliptical Galaxies
Sub-classified according to ellipticity
E(n): n = 10 (1 – b/a), where b = minor axis
a = major axis
b = 8 arcsec
a = 10 arcsec
n = 10 (1 – 8/10) = 10 (1 – 0.8) = 10*0.2 = 2
E2
E0 is perfectly circular; E7 is most elliptical
Classification is HIGHLY dependent on our viewing angle
MPP©2004
Elliptical Galaxies
Properties
• Light dominated by old stars
• Little/no ISM (no present star formation)
• Majority are dwarf ellipticals (dE) containing 107-108 stars
• Generally found in the centers of galaxy clusters
M110 (dE)
M32 (dE)
MPP©2004
6
Spiral Galaxies
Seen as either regular spirals (S) or barred spirals (SB)
Sub-classification based on bulge, spiral arms, and amount of ISM
Bulge
Spiral Arms
ISM
b
c
Large
↓
↓
Tightly wound
↓
↓
Small amounts
↓
↓
d
Small
Loosely wound
Large amounts
a
MPP©2004
Milky Way Classification
SBb
MPP©2004
Spiral Galaxies
Properties
• Star formation takes place in disk
• Light dominated by young O & B stars
• Tend to be very large (109 – 1011 stars)
• Rotation curves show evidence of dark matter
NGC 4414
M 96
MPP©2004
7
Spiral Galaxies
Viewing angle can affect classification
M104: Sombrero Galaxy
MPP©2004
Lenticular Galaxies (S0)
Properties
• Elliptical light distribution
• Disk clearly present but spiral arms
not well defined
NGC 5866
MPP©2004
Irregulars (Irr)
Fit none of the previous characterizations
Irr I have some discernable structure to them while Irr II do not
Large Magellanic Cloud
Sagittarius Dwarf
MPP©2004
8
Galactic Evolution
Universe was initially filled with gas composed only of H & He
MPP©2004
Galactic Evolution
MPP©2004
Galactic Evolution
MPP©2004
9
Spiral Galaxies
Andromeda Galaxy
MPP©2004
M 94
NGC 2903
Galaxy Clusters
Each galaxy orbits the COM where collisions can occur
Coma Cluster of Galaxies
MPP©2004
Galaxy Interactions
M 82
Rarely (if at all) will individual stars collide
MPP©2004
ISM collides which increases star formation
10
Galaxy Interactions
Mutual gravity causes tidal stripping of spiral arms
Mice Galaxy
MPP©2004
Gas Exchange
Bird’s Eye Galaxy
Whirlpool Galaxy with NGC 5195
MPP©2004
Galactic Mergers
Computer simulations show that when spirals collide:
- star formation rates increase which uses up ISM
- spiral arms torn away
Antennae Galaxy
MPP©2004
11
Galactic Mergers
MPP©2004
Elliptical Formation
Bulges of each spiral merge to form a single elliptical galaxy,
which contains old stars and no ISM
MPP©2004
12