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Introducing the Milky Way Galaxy. Next two weeks. Today: The Milky Way Galaxy (material will be covered in Exam #3) Next Tues: Exam #2 Review Next Thurs: Exam #2 (60 questions). And Keep in Mind Homework #7:  Due Friday at 9am (this week only)

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Introducing the

Milky Way Galaxy


Next two weeks
Next two weeks

  • Today: The Milky Way Galaxy (material will be covered in Exam #3)

  • Next Tues: Exam #2 Review

  • Next Thurs: Exam #2 (60 questions)

  • And Keep in Mind

  • Homework #7:  Due Friday at 9am (this week only)

  • Telescopes Project F–L: Last night is tonight

  • Telescopes Project M–R: Begins next week



Different ways of seeing the milky way
Different Ways of Seeing the Milky Way

  • Interstellar Medium

    • Atomic Hydrogen – H I


Emission from h i gas
Emission from H I Gas

The lowest orbital of the hydrogen atom is not one level – it is actually 2 levels, separated by a miniscule amount of energy. Occasionally, (weak) collisions will push electrons into this state.


Emission from h i gas1
Emission from H I Gas

When the electron decays, a photon with a wavelength of 21 cm is produced, corresponding to radio wavelengths. Radio waves are not blocked by dust, so we can see the emission from H I across the entire galaxy.


Milky way at 21 cm
Milky Way at 21 cm

All-sky picture of the Galaxy at 21 cm


Different ways of seeing the milky way1
Different Ways of Seeing the Milky Way

  • Interstellar Medium

    • Atomic Hydrogen – H I


Different ways of seeing the milky way2
Different Ways of Seeing the Milky Way

  • Interstellar Medium

    • Atomic Hydrogen – H I

    • Molecular Hydrogen – H2


Molecules in the interstellar medium
Molecules in the Interstellar Medium

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 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.


Different ways of seeing the milky way3
Different Ways of Seeing the Milky Way

  • Interstellar Medium

    • Atomic Hydrogen – H I

    • Molecular Hydrogen – H2

      • Traced by the molecule CO


Different ways of seeing the milky way4
Different Ways of Seeing the Milky Way

  • Interstellar Medium

    • Atomic Hydrogen – H I

    • Molecular Hydrogen – H2

      • Traced by the molecule CO

    • Ionized Hydrogen –H II


H ii regions
H II Regions

When H I is located near an extremely hot (O or B) star, it can be ionized. You will then see emission lines from recombination.


The milky way in h ii
The Milky Way in H II

All sky picture of the Galaxy at 6563 Å


Different ways of seeing the milky way5
Different Ways of Seeing the Milky Way

  • Interstellar Medium

    • Atomic Hydrogen – H I

    • Molecular Hydrogen – H2

      • Traced by the molecule CO

    • Ionized Hydrogen –H II


Different ways of seeing the milky way6
Different Ways of Seeing the Milky Way

  • 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
The Milky Way in Optical Light

Dust blocks background light (blue more than red)


The milky way in the near infrared
The Milky Way in the Near-Infrared

At near-infrared wavelengths, the dust in more transparent, so we can see more stars in the Milky Way.


The milky way in the near infrared1
The Milky Way in the Near-Infrared

At near-infrared wavelengths, the dust in more transparent, so we can see more stars in the Milky Way.


The milky way in the far infrared
The Milky Way in the Far Infrared

At longer infrared wavelengths, interstellar dust glows (via blackbody radiation).



Different ways of seeing the milky way8
Different Ways of Seeing the Milky Way

  • Interstellar Medium

    • Atomic Hydrogen – H I

    • Molecular Hydrogen – H2

      • Traced by the molecule CO

    • Ionized Hydrogen –H II

    • Dust


Different ways of seeing the milky way9
Different Ways of Seeing the Milky Way

  • Interstellar Medium

    • Atomic Hydrogen – H I

    • Molecular Hydrogen – H2

      • Traced by the molecule CO

    • Ionized Hydrogen –H II

    • Dust

  • Stars


Star clusters
Star Clusters

  • Open Clusters

    • Hundreds or thousands of stars

    • Loosely bound; self gravity not strong enough to hold the stars together over time

    • No more than a few billion years old







Measuring the age of a star cluster
Measuring the Age of a Star Cluster

The older the cluster, the fainter the turn-off.


Ages of open clusters
Ages of Open Clusters

For open clusters, the main sequence extends to bright magnitudes. These clusters are young!


Star clusters1
Star Clusters

  • Open Clusters

    • Hundreds to thousands of stars (i.e., low mass)

    • 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., high mass)

    • Self-gravity strong enough to keep stars from wandering off

    • As old as about 13 billion years






Ages of globular clusters
Ages of Globular Clusters

In globular clusters, the main sequence turnoff is at spectral type G or K. These clusters are very old.


Different ways of seeing the milky way10
Different Ways of Seeing the Milky Way

  • Interstellar Medium

    • Atomic Hydrogen – H I

    • Molecular Hydrogen – H2

      • Traced by the molecule CO

    • Ionized Hydrogen –H II

    • Dust

  • Stars

    • Open clusters

    • Globular clusters



Structure of the milky way
Structure of the Milky Way

  • We would like to determine the structure of the Milky Way: How large is it? What is its shape? Where are we in the Milky Way?

  • To do this, we need to construct a 3-D map of the Milky Way, and for this we need to measure distances to many, many (very distant) stars

  • It will help if we can distinguish old stars from young stars, so we would like to measure ages of stars as well


Stars with different ages stellar populations
Stars with different Ages: Stellar Populations

  • Astronomers divide the stars in our the Galaxy into two populations according to age:

    • Population I: young stars

    • Population II: old stars


Measuring ages
Measuring Ages

For star clusters, measuring the age is easy.


Measuring ages1
Measuring Ages

But for individual stars that aren’t in clusters (like the Sun), we can’t use the cluster turnoff method to measure an age. For instance, a lone G star might be young, or it might be 10 billion years old. How do we tell the age?






Measuring ages2
Measuring Ages

But for individual stars that aren’t in clusters (like the Sun), we can’t use the cluster turnoff method to measure an age. For instance, a lone G star might be young, or it might be 10 billion years old. How do we tell the age?

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.

high metallicity = young

low metallicity = old


Determining a star s metallicity
Determining a Star’s Metallicity

The metallicity of a star can be determined through its spectrum.

The hydrogen 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!


Measuring distances
Measuring Distances

Parallaxes can be used to measure distances for stars only within ~300 pc; parallaxes of more distant stars are too small to measure. So we need another way to measure distances.

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 luminosity you know is astandard candle.


Spectroscopic parallax main sequence fitting
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 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


Another way to measure distances
Another Way to Measure Distances

Not all stars are stable. There is a narrow region in the HR diagram where stars cannot maintain a constant brightness. Instead, they pulsate, getting bigger and smaller (i.e., brighter and dimmer) over time.

This area is called the Instability Strip.


Rr lyrae variables
RR Lyrae Variables

Recall that after a low-mass star goes up the giant branch for the first time, it ignites helium in its core and briefly becomes more like a main-sequence star.

Some of these stars fall in the instability strip. These are called RR Lyrae stars.

Note that RR Lyrae stars are significantly brighter than globular cluster main sequence stars.


Rr lyrae pulsations
RR Lyrae Pulsations

RR Lyrae stars can double their brightness within a day. The period of this variation is constant.


Finding rr lyrae stars
Finding RR Lyrae Stars

Globular clusters contain many RR Lyrae stars. They are very easy to find.

Within a cluster, all RR Lyrae stars have the same apparent (average) brightness. This suggests they are standard candles.


Building a distance ladder
Building a Distance Ladder

There is no globular cluster close enough for a parallax measurement. So…

  • Identify a nearby globular cluster

  • Derive its distance via spectroscopic parallax

  • Using its distance, derive the absolute brightness of all its RR Lyrae stars. These stars are now standard candles.

  • Observe a more distant globular cluster (whose main sequence stars are too faint to see). Measure the brightness of its RR Lyrae stars.

  • Derive its distance assuming RR Lyrae stars are standard candles.


The distance ladder
The Distance Ladder

RR Lyrae Stars

Spectroscopic Parallax

Trigonometric Parallax


The distribution of globular clusters
The Distribution of Globular Clusters

Globular Clusters are not confined to the galactic plane.


The distribution of globular clusters1
The Distribution of Globular Clusters

The center of the globular cluster system is not the Sun. Instead, the globular clusters are scattered about a point 8000 pc from us. This is the Galactic center.


Our galaxy the population ii component old
Our Galaxy: The Population II Component (old)

Population II objects, such as old stars and globular clusters, are distributed in a roughly spherical bulge or halo. Population II objects orbit in random directions, and their density decreases rapidly as one goes to larger and larger radius.


Our galaxy the population i component young
Our Galaxy: The Population I Component (young)

When we look through the dust of the Galaxy using the 21 cm emission of atomic hydrogen, we see a flat disk of gas rotating about the Galactic center. The very youngest objects are located within several spiral arms.


Our galaxy the population i component young1
Our Galaxy: The Population I Component (young)

Our Sun is located in the disk, between two spiral arms, about half of the way out. It takes the Sun about 200,000,000 yrs to complete one orbit about the Galactic center.




Summary structure of the milky way
Summary: Structure of the Milky Way

  • Ages of stars

    • Population I = young, Population II = old

    • Measure ages with the main sequence turnoff

    • Otherwise measure ages with metallicity:

      • low metallicity = old, high metallicity = young

  • Distances of stars with standard candles

    • Spectroscopic parallax

    • RR Lyrae stars

  • Structure of the Milky Way

    • Population I (young) stars are in a spiral disk

    • Population II (old) stars have a spherical distribution

    • The Sun is in the disk, about halfway out from the center


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