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

Disk• Stars orbit the center

of the galaxy in circular orbits (vr ~ 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

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

• G

vrM

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*1010 Msun

– Contains spiral arms– Location of Interstellar

Medium (ISM)

• Bulge– Spherical distribution– 1 kpc radius– M ~ 2*1010 MSun

• Halo– Slightly flattened spheroid– ~45 kpc radius– M ~ 109 MSun

Spatial Distribution

• Disk– 0.3 kpc thick– 15 kpc radius– M ~ 8*1010 Msun

– Contains spiral arms– Location of Interstellar

Medium (ISM)

• Bulge– Spherical distribution– 1 kpc radius– M ~ 2*1010 MSun

• Halo– Slightly flattened spheroid– ~45 kpc radius– M ~ 109 MSun

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 106 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

Atomic H

• Material cools and falls back to the disk– T = 100-10,000 K– Forms atomic H (HI)– Brings metals to disk

• Cooling gas allows gas to form– Dust absorbs visible light

• Observe HI with 21 cm line

• HI exists throughout the Galactic disk

Molecular Clouds

• Atomic H cools and combines to form H2

– 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, H2O, NH3, C2H5OH

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 – HII Regions

• O and B stars in cluster heat and ionize the surrounding gas

• T~104 K• Gas cools from atomic

emission lines• HII regions are found

primarily in spiral arms• Emits Hα light – 656 nm

(red)• Dust reflects blue light

Spiral Arms

• Where all the star formation occurs

• Red spots are HII 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 metal-rich 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 3rd law indicate mass of about 4*106 MSun concentrated in tiny region

• Must be a supermassive black hole