How we know the shape of the Galaxy and where its center is:
Imagining a picture of our galaxy is like imagining a picture of
your
house, if you have been sitting in a chair in the living-room all your life
but have never been outside.
Our view from the inside of our Galaxy
We would see a band because the Galaxy has a bright disk that we are inside
of
schematic
Our view of neighbor galaxy - Andromeda
LMC and SMC picture from APOD
Imagined views of our galaxy from the outside:
Side view
Top-ish view
Shapley’s globular clusters and 21cm line
Composition of the Galaxy:
Regions: Disk, Bulge, and Halo
Recent & upcoming additions: Sagittarius dwarf galaxy, Canis Major Dwarf Galaxy , other dwarf galaxies
Constituents: ~100 billion stars, Gas between the stars, and Dark matter
Chemical composition (by mass) of stars and gas:
70% H 28%
He 2% metals
Dark Matter, How we know that it exists, and How much there is:
Newton's version of Kepler's 3rd Law: p2 = (4 pi2 a3) / G (M1 + M2) thus Minterior = r x v2 / G Higher mass -> faster orbits Our Galaxy's rotation curve , other galaxies' rotation curves Plots' shapes mean that the mass is spread out (not concentrated at center) Solar system vs. Milky Way Why doesn’t the matter "quit"? Dark matter
Introduction to Formation of the Milky Way
Dark matter "clouds" draw gas into them, gas starts to form stars
"Clouds" come together, form more stars, gravitationally collapse, "spins-up", forms disk
Combination of
older model of contracting cloud that spins up
+
colliding material
This explains why halo stars have low metalicities compared with disk stars
and why metalicities vary between globular clusters
The Star -> Gas -> Star -> Gas Cycle:
Hot young stars also heat surrounding gas, ex: Orion Nebula HII region Stars fuse atoms -> make heavier atoms Winds (stellar wind bubble, planetary nebula) disperse the "new" atoms New elements show up in emission Supernova explosions make and disperse elements, too (Ex: Cygnus Loop): Shocks -> hot gas (see X-rays), cosmic rays ( see radio synchrotron) Superbubbles (we are inside one, the Local Bubble) Fountains As time goes by, the hot gas cools -> cool gas Cool hydrogen = "neutral" or "atomic" hydrogen, emission line: 21 cm Slowly, neutral atoms combine -> H2 molecules Giant Molecular Clouds Dust "Dust lanes" in another galaxy A summary in all wavelengths and an Account sheet Star->Gas->Star Cycle Diagram
Star Orbits and Spiral Arms:
The Solar neighborhood and the Sun's motion
Sun's velocity = 220 km/sec, orbital period = 230 million years
The circles we move in
Spiral arms
in our Galaxy
See spiral pattern in hot young stars and in gas:
M83 galaxy
M51 galaxy,
Spiral arms are like ripples in a whirlpool
tiny image,
painting
Spiral density waves are like
back-ups behind slow vehicles
Piles up matter
into the spiral arms
New supernova explosion, in spiral arm of M95 (galaxy 38 million light years away)
M95 movie
M95's location
The Black Hole in the Center of our Galaxy:
Views
wide angle
view of the galactic center in visible light
wide
angle view and close ups of the galactic center
A similar series, but with more steps
IR and radio,
respectively
radio
(image ~ 400 ly), IR (image ~ 200 ly), IR (image ~2.5 ly)
How we know SgrA* is a supermassive black hole
YouTube Video of orbits of stars
Recap: The neighborhoods in the Milky Way and who lives there:
The disk:
Stars like the sun, Population I (high metal) stars
Gas, including:
HII
regions due to energy emitted by bright stars
Supernova remnants
Giant Molecular Clouds
Spiral Arms:
O, B stars, young stars
Concentrated gas
(HI map)
The halo:
Population II (low metal) stars
Globular
clusters, note metal concentration varies between glob. clusters
Density of gas is lower than in disk
