The Milky Way
Three Major Components Bulge young and old stars Disk young stars located in spiral arms Halo oldest stars and globular clusters Components are chemically, kinematically, and spatially distinct
Chemical Composition Disk Large range in metallicity Lots of gas and dust Bulge Mixed populations Halo Inner halo has very low metallicity stars Outer halo has older stars
Kinematics
Stars orbit the center of the galaxy in circular orbits (v r ~ 0) Stars concentrated into orbital plane Vertical motions of stars give disk its 1,000 lyr thickness Near the sun, stars have orbital period of ~200 Myrs Disk
Halo and Bulge Stars not concentrated in plane Randomly oriented elliptical orbits Appear spherical in shape
Why does that matter? We can use orbital velocities to determine mass The orbital velocity of a star is entirely dependent on the mass enclosed inside its orbit M r v G 2
Rotation Curves
Rotation Curves
Rotation Curves Orbital velocity increases as more mass is enclosed Velocity should decrease once edge of visible material is reached Stays constant to large r There must be a lot of mass we can t see Dark Matter
Spatial Distribution Disk 0.3 kpc thick 15 kpc radius M ~ 8*10 10 M sun Contains spiral arms Location of Interstellar Medium (ISM) Bulge Spherical distribution 1 kpc radius Halo Slightly flattened spheroid ~45 kpc radius M ~ 10 9 M Sun M ~ 2*10 10 M Sun
Spatial Distribution Disk 0.3 kpc thick 15 kpc radius M ~ 8*10 10 M sun Contains spiral arms Location of Interstellar Medium (ISM) Bulge Spherical distribution 1 kpc radius Halo Slightly flattened spheroid ~45 kpc radius M ~ 10 9 M Sun M ~ 2*10 10 M Sun
The Star-Gas-Star Cycle Universal Recycling
Star Birth Giant molecular cloud gives birth to cluster Cloud is very cold (10-30 K) Stellar winds prevent rest of cloud from forming stars
Blowing Bubbles Stellar winds blow material off of star and return it to the ISM Large stars have very strong winds, which can blow big bubbles Smaller stars have weaker winds
Supernovae O and B stars live short lives and SN after only a few million years SN explosion creates a shock wave Shockwave sweeps up material compresses gas, heating it to about 10 6 K Shockwave loses momentum when it sweeps up other material
Superbubbles Clusters create superbubbles when SN explosions merge Cannot be contained in disk Bubble can extend up to 1 kpc above disk
Material cools and falls back to the disk T = 100-10,000 K Forms atomic H (H I ) Brings metals to disk Cooling gas allows gas to form Dust absorbs visible light Observe H I with 21 cm line H I exists throughout the Galactic disk Atomic H
Molecular Clouds Atomic H cools and combines to form H 2 T ~ 10-30 K Molecular clouds very dense, so they settle to the center of the disk Clouds too cold to emit, so we use other gasses CO, H 2 O, NH 3, C 2 H 5 OH
The Cycle Doesn t Last Mass locked up in the end products of stellar evolution WD, NS, and BH Mass also locked up in brown dwarfs Eventually Milky Way will completely run out of material necessary for the S-G-S Cycle to continue, and star formation will cease
Distribution of Gas in the Milky Way
Star Forming Regions H II Regions O and B stars in cluster heat and ionize the surrounding gas T~10 4 K Gas cools from atomic emission lines H II regions are found primarily in spiral arms Emits H α light 656 nm (red) Dust reflects blue light
Spiral Arms Where all the star formation occurs Red spots are H II Regions Blue spots are new O and B stars Dark areas have lots of dust
Spiral Arms Spiral arms caused by spiral density waves Gas is attracted to density wave Waves condense and induce star formation VIDEO
Formation of Spherical Population Old, metal-poor stars in the spherical parts of the Galaxy (Bulge and disk) are called Pop. II Formed by collapse of big cloud Actually probably several clouds
Disk Population Younger, more metalrich stars located in the disk are called Pop. I Ongoing star formation makes young stars and drives up metallicity Galactic cannibalism builds up galactic disk VIDEO
Galactic Center
Sgr A* Strong radio source indicates strong magnetic field Stars and gas swirl around it
Sgr A* Star motions and Newton s Kepler s 3 rd law indicate mass of about 4*10 6 M Sun concentrated in tiny region Must be a supermassive black hole