Download The Milky Way

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
r

v
• M 
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*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
• Blue spots are new O
formation occurs
and B stars
• Red spots are HII Regions • 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
• Galactic cannibalism
builds up galactic disk
• VIDEO
• Ongoing star formation
makes young stars and
drives up metallicity
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