Constituents of the
Milky Way Galaxy
Star Formation and Interstellar Matter
Stars are made from the
gas and dust in the
interstellar medium.
The gas and dust in the
interstellar medium
comes from stars.
Material is constantly
being recycled.
Constituents of the Galaxy
• Interstellar Medium
 Atomic Hydrogen – H I
Emission from H I Gas
The lowest orbital of the hydrogen atom is not one level – it is
actually two, separated by a miniscule amount of energy.
Occasionally, (weak) collisions will push electrons into the
higher state.
Emission from H I Gas
When the electron decays, a 21 cm photon (in the radio) is
produced. Radio waves are unaffected by dust, so we can
see the emission from H I across the galaxy.
Milky Way at 21 cm
All sky picture of the Galaxy at 21 cm
Constituents of the Galaxy
• Interstellar Medium
 Atomic Hydrogen – H I
Constituents of the Galaxy
• Interstellar Medium
 Atomic Hydrogen – H I
 Molecular Hydrogen – H2
Molecules in the ISM
In the coldest, densest
regions of the ISM,
atoms can stick
together to form
molecules, such as
H2O, NH3, CO2, HCN,
CH2O, CH3OH, CH2O,
CH3CH2CN, HC5N,
H2C2O, CH3CH2CN,
NH2CH2COOH, and
many others.
Many of these are
organic molecules!
The Milky Way in CO
The most common molecule is H2, but it has no easily observable
emission lines. Fortunately, the molecule CO is relatively
common, and has strong transitions at some radio wavelengths.
Constituents of the Galaxy
• Interstellar Medium
 Atomic Hydrogen – H I
 Molecular Hydrogen – H2
Traced by the molecule CO
Constituents of the Galaxy
• Interstellar Medium
 Atomic Hydrogen – H I
 Molecular Hydrogen – H2
Traced by the molecule CO
 Ionized Hydrogen – H II
H II Regions
When H I is located near an extremely hot (O or B) star, it
will be ionized. You will then see emission lines from the
electrons’ recombination. This is called an H II region.
H II Regions
When H I is located near an extremely hot (O or B) star, it
will be ionized. You will then see emission lines from the
electrons’ recombination. This is called an H II region.
H II Regions
When H I is located near an extremely hot (O or B) star, it
will be ionized. You will then see emission lines from the
electrons’ recombination. This is called an H II region.
H II Regions
When H I is located near an extremely hot (O or B) star, it
will be ionized. You will then see emission lines from the
electrons’ recombination. This is called an H II region.
The Milky Way in H II
All sky picture of the Galaxy at 6563 Å
Constituents of the Galaxy
• Interstellar Medium
 Atomic Hydrogen – H I
 Molecular Hydrogen – H2
Traced by the molecule CO
 Ionized Hydrogen – H II
Constituents of the Galaxy
• Interstellar Medium
 Atomic Hydrogen – H I
 Molecular Hydrogen – H2
Traced by the molecule CO
 Ionized Hydrogen – H II
 Dust
The Milky Way in Optical Light
Dust extincts background light (blue more than red)
The Milky Way in the Far Infrared
Also, since the dust is above absolute zero, it glows (via
blackbody radiation) in the far infrared.
Constituents of the Galaxy
• Interstellar Medium
 Atomic Hydrogen – H I
 Molecular Hydrogen – H2
Traced by the molecule CO
 Ionized Hydrogen – H II
 Dust
Constituents of the Galaxy
• Interstellar Medium
 Atomic Hydrogen – H I
 Molecular Hydrogen – H2
Traced by the molecule CO
 Ionized Hydrogen – H II
 Dust
• Stars
Digression: Stellar Lifetimes
The length of time a star
lives on the main
sequence depends on
1) The amount of fuel
the star has (its mass)
2) How fast the star
burns its fuel (its
luminosity)
Low mass stars live a lot
longer than high mass
stars!
Main Sequence Lifetimes
Type
Mass
(M)
Luminosity
Temperature
(L)
Lifetime
(Million yrs)
O
25
80,000
35,000
3
B
15
10,000
30,000
15
A
3
60
11,000
500
F
1.5
5
7,000
3,000
G
1
1
6,000
10,000
K
0.75
0.5
5,000
15,000
M
0.5
0.03
4,000
200,000
Digression: Star Clusters
Assume that all stars are born in clusters and that all
the stars of a star cluster
 are the same distance from us
 are born at the same time
 are born with a range of masses
All three of these assumptions are good. You can
therefore tell the age of a star cluster by looking
at its main sequence turn-off.
The Aging of a Star Cluster
QuickTime™ and a
Cinepak decompressor
are needed to see this picture.
The older the cluster, the fainter the turn-off.
Star Clusters
• Open Clusters
 Few stars -- maybe a few hundred or a few thousand
(i.e., the total mass is low)
 Loosely bound; self gravity not strong enough to hold
the stars together over time
 No more than a few billion years old
The Random Motions of Clusters
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Cinepak decompressor
are needed to see this picture.
Open Cluster Turnoffs
For open clusters, the main
sequence extends to bright
magnitudes. These clusters
are young!
Open Clusters
Open Clusters
Open Clusters
Open Clusters
Star Clusters
• Open Clusters
 Few stars -- maybe a few hundred or a few thousand
(i.e., the total mass is low)
 Loosely bound; self gravity not strong enough to hold
the stars together over time
 No more than a few billion years old
• Globular Clusters
 Tens of thousands to a million stars (i.e., the total
mass of the cluster is large)
 Self-gravity strong enough to keep stars from
wandering off
 About 13 billion years old, maybe older
Globular Clusters
Globular Clusters
Globular Clusters
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TIFF (Uncompressed) decompressor
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Globular Clusters
Globular Cluster Turnoffs
In globular clusters, the main sequence turnoff is at spectral
type G or K. These clusters are very old.
Stellar Populations
• To help interpret observations of our Galaxy (and others),
let’s divide the constituents of the Galaxy into two
populations:
 Population I: objects associated with recent star
formation
 Population II: objects with no connection to recent
star formation
Thus, short-lived stars, open clusters, dark clouds, H II
regions, etc., are all Population I. Conversely, groups
of old stars, globular clusters, etc., are Population II.
How Does One Recognize Old Stars?
For star clusters, getting
the age is easy. However,
if a G-star is alone, it could
be old or young!
In these cases, one can
examine the metallicity of
the star.
Because supernovae have
created metals over time,
stars being formed today
have more metals than
old stars.
The Astronomer’s Periodic Table
The Astronomer’s Periodic Table
The Astronomer’s Periodic Table
The Astronomer’s Periodic Table
Determining a Star’s Metallicity
The metallicity of a star can be determined through its spectrum.
The hydrogen and helium absorption lines indicate that the
temperatures of the two stars are similar. But in the second star,
all the “metal” lines are much, much weaker!
The Structure of the Galaxy
Because we are within the Galaxy, it is difficult to map out its
structure. This is especially true when looking in the Galactic
plane, because of all the dust. We are not near the center of the
Galaxy, but where is the center? How far away is it?
Measuring Distances in the Galaxy
Although parallax
measurements are
reliable, most stars
are too distant for
any parallax
measurement!
Reliable parallaxes
only exist out to
~1000 pc.
Standard Candles
Suppose you knew the absolute luminosity (or magnitude) of
some object. If you did, you could determine the distance to
the object simply from the inverse square law of light.
l = L / r2
where
l is the apparent luminosity,
L is the absolute luminosity, and
r is the distance.
Any object whose absolute luminosity
you know is a standard candle.
Ideally, you would like a standard candle to be bright, so that
it can be seen far away.
Spectroscopic Parallax/Main Sequence Fitting
Consider: for main sequence stars, there is a relationship
between the temperature (or color or spectral type) of a star
and its absolute luminosity (or magnitude).
For these types of
stars, you can tell their
absolute luminosity
just by observing their
color (or spectral
type). They can
therefore be used as
standard candles.
Spectroscopic Parallax/Main Sequence Fitting
Consider: for main sequence stars, there is a relationship
between the temperature (or color or spectral type) of a star
and its absolute luminosity (or magnitude).
For these types of
stars, you can tell their
absolute luminosity
just by observing their
color (or spectral
type). They can
therefore be used as
standard candles.
Main Sequence Fitting
and Open Clusters
Main Sequence Fitting (or spectroscopic
parallax) works well for open clusters,
since the main sequence stars are bright.
O
B
A
F
G
K
M
O
B
A
F
G
K
M