Transcript Ch16: The Milky Way

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Chapter 16
The Milky Way
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sun
Herschel “discovered” that we live in a disk of stars
Doppler-velocity measurements tell us we live in a spiral galaxy
Components of a spiral galaxy:
• Disk
• Nuclear bulge
• Halo
halo
disk
bulge
Spiral Galaxy
Disk
Component:
stars of all
ages,
many gas
clouds
Spheroidal
Component:
bulge & halo:
old stars,
Very little gas
& dust
Halo Stars: Population II
0.02-0.2% heavy elements (O, Fe, …),
only old stars
Halo stars
formed first,
then stopped
Disk Stars: Pop. I
2% heavy elements,
stars of all ages
Disk stars
formed later,
kept forming
Stars in the disk all orbit in the same direction with a
little up-and-down motion
Orbits of stars
in the bulge
and halo have
random
orientations
Multiple
supernovae
create huge hot
bubbles that can
blow out of disk
Gas clouds
cooling in the
halo can rain
back down on
disk
Star-gas-star
cycle
Recycles gas
from old stars
into new star
systems
The orbital speed (v) and radius (r) of a star on
a circular orbit around the galaxy tells us the
total mass (Mr) contained within that orbit
Gravitational force = centripetal force
GMr m mv 2

2
r
r
2

rv
Mr 
G
Mr
v
r
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Fritz Zwicky discovered dark matter in 1933
(Coma cluster)
Sun’s orbital
motion (radius
and velocity) tells
us mass within
Sun’s orbit:
1.0 x 1011 MSun
The total amount of
light suggests ~ few
x 109 Msun
Dark matter!
Galactic Center
Swirling gas near center
Orbiting star near center
Stars appear to
be orbiting
something
massive but
invisible … a
black hole?
X-ray flares
from galactic
center suggest
that a black
hole
occasionally
tears apart
chunks of
matter as it
falls in
Galactic Center (Chandra, Hubble, Spitzer)
Best hypothesis for how galaxies form:
1. A giant cloud of H and He condensed
after the Big Bang
2. halo stars (oldest) formed first, in
clumps, cloud contracted due to gravity.
These clumps later merged.
3. Bulge stars form next
4. Remaining gas settled into spinning disk
5. Star formation continues as long as there is
material in the ISM