1. The Milky Way Galaxy • Introduction • Structure • Formation
• Neighborhood

2. Introduction
In greek mythology: Milky Way = milk of Hera feeding Heracles
1610: Galileo realizes that the milky aspect is due to the accumulation of millions of stars
Original definition: dim glowing band arching across the night sky whose individual stars cannot be distinguished by the naked eye
Disk-shaped structure of stars, gas and dust (galactic disk) in which the solar system is located → viewed from inside
360° panorama of the Milky Way as seen from Earth

3. Modern definition: the Milky Way = our galaxy
Introduction - 2
Modern definition: the Milky Way = our galaxy
1785: William Herschel tries to determine its shape by star counts
Drawbacks: • distances to stars unknown
• solar system assumed close to the center
Our galaxy’s shape as determined by Herschel
1930: Robert Trumpler studies the effect of dust (extinction) and gets a much more accurate view of our galaxy

4. Global properties • Contains ~200 billion stars + gas and dust
Introduction - 3
Global properties
• Contains ~200 billion stars + gas and dust
• `Visible´ mass ~ 1011 M
• Type: barred spiral
• Diameter ~ L.Y.
• Center in the direction of the Sagittarius constellation
• Differential rotation
• The Sun is located ~ L.Y. from the center, orbits in ~ 220 million years (already completed ~ 20 orbits)
Position of the Sun in the Milky Way

5. Structure • Galactic bulge (includes the galactic center)
• Galactic halo
globular clusters
dwarf galaxies
• Galactic disk
thin disk + thick disk
open star clusters
spiral arms

6. The galactic bulge The galactic center • Diameter ~ 7000 L.Y.
Structure - 2
The galactic bulge
• Diameter ~ 7000 L.Y.
• Contains 5% of the `visible´ mass, mostly old stars
• Hard to observe because of dust in the galactic plane
The galactic center
• Densely populated in stars
• Strong magnetic field
(synchrotron radiation – see AGN chapter)
Towards galactic bulge in Sagittarius

7. Structure - 3
The galactic center (2)
Highly energetic environment: distorted gas clouds, hot gas, arcs, plasma filaments, supernova remnants
Radio image of the galactic center
Orbits of stars around the center:
→ central mass ~ M with size < a few light-months
→ supermassive black hole very likely
IR image of the galactic center

8. The galactic disk Contains ~ 90 % of the visible matter in our galaxy
Structure - 4
The galactic disk
Contains ~ 90 % of the visible matter in our galaxy
Flat structure of diameter ~ L.Y.
Thin disk
Young stars often found in open clusters (Pop I, thickness ~ 2000 L.Y.)
Giant molecular clouds and dust (thickness ~ 200 L.Y.)
Thick disk
Progressive dispersion of think disk stars (collisions)
Atomic hydrogen extending over L.Y. diameter

9. Open clusters Clusters of ~ 100 to 1000 young stars bound by gravity
Structure - 5
Open clusters
Clusters of ~ 100 to 1000 young stars bound by gravity
Diameter ~ 100 L.Y.
Sometimes not very concentrated → hard to identify
Stars born from the collapse of a giant molecular cloud
→ same age
same chemical composition
progressively dispersed in the disk
The Pleiades open cluster

10. Structure - 6
The galactic halo
Spheroidal structure, much more extended and less dense than the bulge
Contains ~ 5 % of visible matter in our galaxy
Oldest stars in our galaxy
Orbits with any inclination with respect to the disk
Negligible global rotation
→ `high velocity stars´ with respect to the Sun
+ globular clusters and dwarf galaxies

11. Structure - 7
Globular clusters
Gravitationally bound structures, ± spherically symmetric
Formed in the very first stages of the galactic evolution
Among the oldest objects in the Universe
Contain ~105 old stars each (Pop II)
± spherical distribution
~ 100 L.Y. in diameter
~ 200 have been indentified
Contain ~1% of halo stars (the other 99% are field stars, not belonging to clusters)
Globular cluster M80

12. Structure - 8
Dwarf galaxies
Some small satellite galaxies are tidally disrupted by the gravitational field of the Milky Way
Their stars are progressively stripped away and added to the halo stars (`cannibalism´)
→ form a 2nd (younger) halo population
`Stellar streams´ of tidally stripped stars are found along the orbits of currently or recently disrupted galaxies
Stellar stream in the Milky Way halo

13. Spiral arms 2 main spiral arms identified in 1951 by William Morgan
Structure - 9
Spiral arms
2 main spiral arms identified in 1951 by William Morgan
2 additional ones identified in 1976 by Yvon and Yvonne Georgelin
Evidence for a bar extending over ~ L.Y.
Structure determined from radio/optical/UV observations of tracers
Main difficulty: determine their distance
Sketch of the Milky Way spiral structure

14. Structure - 10
Origin of spiral arms
First idea: effect of differential rotation (rejected since arms would be much more tightly wound after ~20 rotations)
Best hypothesis: density wave that propagates at a speed different
from the stars and gas
Origin of the density wave:
– gravitational instability caused by differential rotation?
– tidal effect from satellite galaxies?
– chaotic phenomenon?
Spiral galaxy similar to the Milky Way

15. Why are there tracers in the spiral arms?
Structure - 11
Why are there tracers in the spiral arms?
Density wave and matter do not propagate at the same velocity
When a molecular cloud crosses an overdensity region
→ compression
→ collapse of the cloud
→ star formation
Massive stars → HII regions
Short-lived stars → already at the end of their life when the density wave moves away → tracers disappear

16. Structure - 12
Rotation of the disk
In the outer regions, the rotation velocity is ~ constant while the predictions based on visible matter would imply a Keplerian decrease
→ two possibilities:
(1) the gravitation theory is wrong in these `extreme´ conditions (a <<)
(2) there is more mass than what we can `see´
Most popular hypothesis: dark matter halo composed of some kind of WIMPs (Weakly Interacting Massive Particles)
Rotation curve of the Milky Way

17. Structure - 13
The dark matter halo
Most specialists consider that our Galaxy is embedded in a huge dark matter halo, made of massive fundamental particles interacting only via gravitation (and maybe the weak force)
Total mass: ~ M
Luminous (ordinary, baryonic) matter: ~ 1011 M
Dark matter: ~ M
Dark halo size: ~ L.Y.
Sketch of our Galaxy’s structure

18. Formation of our Galaxy
• Giant gas cloud of primordial (Big Bang) matter: H + He +…
• Gravitational collapse with simultaneous collapse of smaller parts (→ globular clusters and field halo stars: old stars, low metallicity)
• The remaining gas continues collapsing and flattens because of conservation of angular momentum → formation of the disk
• Density waves appear in the disk and give birth to a 2nd, 3rd… stellar generations
• Successive stellar generations enrich the interstellar gas in metals (stellar winds, supernova explosions…)
→ newborn disk stars have higher and higher metallicities