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The Milky Way
Our Galactic Home
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Goals
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Structure of our Galaxy.
Its size and shape.
How do stars and things move through it?
Mass and Dark Matter.
The Galactic Center.
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The Milky Way
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Stars
Dust
Gaseous Nebulae
Open Clusters
Globular Clusters
Pulsars
Black Holes
How do they all fit together to make our galaxy?
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Optical emission from stars and nebulae
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Near-Infrared stellar emission – copyright E. L. Wright and COBE
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Far-Infrared dust emission – copyright E. L. Wright and COBE
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Radio emission from neutral hydrogen – copyright J. Dickey
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X-ray emission from hot gas – copyright S. Digel and ROSAT
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Gamma-ray emission from pulsars and black holes – copyright NASA
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Where are We?
• We aren’t at the center of
the Milky Way.
• Where is the center then?
• Globular Clusters point
the way.
M10 – copyright Credner and Kohle
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You Are Here
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Near-Infrared stellar emission – copyright E. L. Wright and COBE
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Galactic Distances
• How do we know the distance to stars and
clusters in our galaxy?
• Trigonometric parallax good out to 100 pc.
• We believe galaxy is ~30 kpc wide.
• How do we know?
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Spectroscopic Parallax
• If you know how luminous a star REALLY is and
how bright it looks from Earth, you can determine
how far away it must be to look that faint.
• For any star in the sky, we KNOW:
– Apparent Magnitude (m)
– Spectral Type (O, B, A, F, G, K, M)
– Luminosity Class (Main Sequence, Giant, etc…). These
are denoted by a roman numeral (V, III, I,…).
• Combine spectral type and luminosity class to get
absolute magnitude (M).
• From Lecture 7B: m – M give you distance.
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• Deneb is A2Ia star
Example
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m = 1.25
A2  Blue star
Ia  Supergiant
M = -8.8
 distance
m  M  5log 10 
 10pc



Distance = 1000 pc
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Standard Candles
• “Standard Candles”
• If we know how bright something should be, and we
know how bright it looks  Distance
• Variable stars.
– RR Lyra stars
– Cepheid variables
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Variable Stars
• For RR Lyrae stars:
– Average luminosity is a
standard candle
– Always ~ 100 x Sun
• For Cepheid variables:
– Pulsation period is
proportional to average
luminosity
– Observe the period 
find the luminosity
• Good to 15 Mpc!
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Rotation …
• Objects in the disk,
rotate in the disk.
– Nebulae
– Open clusters
– Young stars
• Objects in the
halo, swarm in a
halo.
– Old stars
– Globular clusters
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… and Formation
• Picture the formation of
the Sun:
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Spherical cloud
Condenses to disk
Planets in a plane
Oort cloud sphere.
• Perhaps the same with
the galaxy?
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Missing Mass
• From variable stars we know distances.
• From Doppler shift we know rotation velocity.
• Use Kepler’s Third Law (again) to get mass of the
Milky Way.
• M = 1011 x Msun
P a
2
3
4 3
P 
a
GM
2
2
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Dark Matter
• What causes the mass to keep on increasing?
• Don’t see anything there. Thus  “dark” matter.
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Brown dwarfs
Planets
White dwarfs
Strange matter?
• Use gravitational lensing (last lecture) to look for
these “dark” objects.
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The Heart of the Galaxy
• Because of all the
dust in the Galaxy,
we can’t see its
center in visible
light.
• Can use IR and
radio to pierce the
dust.
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200 pc
5 pc
Sagittarius A* - Sgr A*
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Stellar Motion
• Infrared images of stars in the
Galactic Center over 8 years.
• The “+” is the radio source
Sgr A*
• Conclusion: Must be over
one million solar masses
within less than 1/5 of a light
year!
• Supermassive Black Hole!
• Event Horizon < 0.05 AU!
• Probably in the centers of all
spiral galaxies.
Copyright Eckart & Genzel
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