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The Milky Way
More than just a candy bar.
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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?
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 emission from stars – 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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Near-Infrared stellar emission – copyright E. L. Wright and COBE
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John P. Gleason
M51 - Copyright HST
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Where are We?
• Not at the center of the
Milky Way.
• Where is the center?
• Globular Clusters point
the way.
M10 – copyright Credner and Kohle
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You Are Here
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Concept Test
•
If the Earth were located at the center of the
galaxy, we would see:
a.
b.
c.
d.
e.
Globular clusters in only one direction.
Globular clusters in all directions.
Many more supernovae than we do now.
Many more stars being formed than we do now.
None of the above.
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Galactic Distances
• How do we know the distance to stars and
clusters in our galaxy?
• Stellar parallax:
– Parallax of nearby stars relative to background stars.
– Good out to ~500 pc.
• What about the background stars?
• How far are they?
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Standard Candles
• “Standard Candles”
• If we know how bright something looks,
• And we know how bright it should be
(luminosity),
• Result  Distance
• We do this everyday
with size.
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m versus M
• 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).
• m – M give you distance.
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Example
Distance = 1000 pc
• Deneb is A2Ia star
m = 1.25
A2  Blue star
Ia  Supergiant
M = -8.8
 distance
m  M  5log 10 
 10pc
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


Standard Candles
• Other “Standard Candles”
• Variable stars.
• Stars that change in luminosity.
– 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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Variables in Clusters
M3
Copyright – K. Stanek (Harvard
)
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30 kpc
8.5kpc
500 pc
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Concept Test
•
A standard candle can be any object (or class of
object) that:
a. Always has the same luminosity.
b. Has some means of knowing its luminosity without
first needing to know its distance.
c. Can vary in brightness (as long as it always has the
same average luminosity).
d. Has a known absolute magnitude.
e. Always gives off the same amount of energy,
regardless of distance from us.
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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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The Sun’s “age”
R
V
• We define our age
by trips around the
Sun.
• How many trips of
Sun around Milky
Way?
R = 8.5 kpc
V = 220km/s
P = 2.5x108 yrs
• 20 trips.
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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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Supermassive Black Hole
• 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!
• Event Horizon ~ 0.02 AU!
• Probably in the centers of all
spiral galaxies.
Copyright Eckart & Genzel
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Homework #15
• For Wednesday:
• Read: Bennett Ch20.1 - 20.3:
• Do Ch20:
– Problems 3, 12, 13
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