Download Galaxies

M51 Whirlpool Galaxy
31 million LY distant
30,000 LY across
February 28, 2006
Chapter 24: The
Milky Way Galaxy
Astronomy 2010
1
How Do We Determine the Shape
of the Galaxy?
We are in the galaxy and can’t easily step
back to see what it looks like.
 William Herschel: measure the distance
and direction to stars and make a map.

• Dust obscures view of much of the galaxy.

Harlow Shapley: measure the distance
and direction to globular clusters
scattered above and below the galaxy
• away from the dust

Modern: use radio and infrared light
February 28, 2006
Astronomy 2010
2
Mosaic of 51 wide-angle photographs. Made over a three year period from locations in
California (USA), South Africa, and Germany, the individual pictures were digitized and
stitched together to create an apparently seamless 360 by 180 degree view.


NGC 4103, 55 million LY away
edge on spiral galaxy
dark dust band 500 LY thick

February 28, 2006
Astronomy 2010
5
side
top
center of galaxy
obscured by dust
February 28, 2006
Astronomy 2010
7
Our Milky Way Galaxy
The Milky Way Galaxy


stars grouped in galaxies
our galaxy: the Milky Way
• roughly disk shaped, 100,000 LY diameter
• 1000 LY thick


central spherical nuclear bulge,
4 major spiral arms plus smaller “spurs”
• arms: Cygnus, Perseus, Sagittarius-Carina
• fourth unnamed arm – hard to see (on other side of bulge)
• 80,000 LY long

Sun in short spur – the Orion arm – between Perseus
and Carina
• 15,000 LY long
• also contains Orion Nebula
• Sun in middle of disk, 70 LY from central plane
February 28, 2006
Astronomy 2010
9
Spiral Arms





hot blue stars
delineate spiral
structure – like Xmas
lights on a tree
arms: regions where
gas and dust more
densely concentrated
need dust to replenish
short lived blue stars
cool orange and red
stars found in and
between spiral arms
interstellar dust limits
our view in visible light
to dashed circle
Why Spiral
Arms?


stars orbit
around center of
mass of galaxy
– like planets
Kepler’s Laws:
• closer in –
faster
• farther out –
slower

differential
rotation of
stars explains
curved shape of
spiral arms

Rotation Speeds
• Inner Parts: Rise from Zero to few 100 km/sec
• Outer Parts: Nearly constant at a few 100 km/sec


Sun has Vrot=220 km/sec at R=8.5 kpc
Orbital Period: 240 Myr
Rotation Spiral Arms
Spiral pattern for billions of years?
Spiral density waves
February 28, 2006
Astronomy 2010
14
Spiral Density Waves: Traffic Jam
Spherical Structure
Nuclear Bulge
• Many RR Lyrae stars
• A little gas & dust
Galactic Halo: outer
sphere with very few
stars
•
•
•
•
Old metal-poor stars
Globular clusters
dark matter
RR Lyrae Stars
RR Lyrae stars
• pulsate like Cepheid
Variables
• distance scale
Galactic Center
Galaxy
Mapping with
Radio Band



radio is best for
mapping the
distribution of
hydrogen in the
galaxy
most of the
hydrogen gas is
not ionized
because O and B
stars are rare
radio waves pass
easily through
dust
February 28, 2006
Astronomy 2010
19
Stellar Populations
Population I: Disk Stars
 Ordered, roughly circular
orbits in a plane.
 All orbit in the same
general direction.
 Orbit speeds similar at a
given radius.
Population II: Spheroid Stars
 Disordered, elliptical orbits at
all inclinations.
 Mix of regular and retrograde
orbits
 Wide ranges of orbital
speeds.
Population I
Location: Disk and Open Clusters
 Age: Mix of young and old stars
 Composition: Metal rich (roughly solar)
 70% Hydrogen
 28% Helium
 ~2% "metals"
 Environment: Often gas rich, especially
for the young stars.

February 28, 2006
Astronomy 2010
21
Population II
Location: Spheroid and Globular Clusters
 Ages: Oldest stars, >10 Gyr
 Composition: Metal Poor (0.1-1% solar)
 75% Hydrogen
 24.99% Helium
 ~0.01% metals
 Environment: gas poor, no star formation

February 28, 2006
Astronomy 2010
22
Contrast & Compare
Population I
Population II
 Disk & Open Clusters
 Spheroid & Globular
Clusters
 Young & Old Stars
 Oldest Stars
 Metal-rich
 Metal-poor
 Blue M-S stars
 No Blue M-S stars
 Ordered, circular
orbits in a plane
 Disordered, elliptical
orbits in all directions.
 Gas-rich environment
with recent star
 Little or no Gas &
formation.
Dust, and no star
formation.
February 28, 2006
Astronomy 2010
23
Mass of the Milky Way
Observe orbital period, P, of stars or interstellar
matter vs. distance, D, from center
Kepler’s Third Law 
period of orbit determined
by mass within orbit:
D3 = (Mgalaxy + Msun)P2
earth orbit  sun’s mass
farther from the galactic
center, the more mass
within orbit
FIND: more mass than we see -- dark matter
halo

February 28, 2006
Astronomy 2010
24
Dark Matter

Two possibilities to explain the observed
rotation:
• Law of gravity is wrong for galaxies.
• There is additional matter that doesn’t emit
detectable radiation (dark matter).
No evidence that gravity behaves
differently.
 The “dark matter” hypothesis is favored.

• Could be brown dwarfs, black holes, or new
exotic particles.

Measurements indicate about 90% of the
mass in the universe is dark matter!
February 28, 2006
Astronomy 2010
25
Galaxy Formation