Download Constituents of the Milky Way

The Milky Way
The Milky Way: Our Home Galaxy
What are the different
components of the Milky Way?
How do we see those
components?
What does a map of each
component look like from our
point of view?
Stars in the Milky Way
At visible wavelengths, we see mostly light from stars. Some of
that starlight is blocked by huge clouds of gas and dust.
Stars in the Milky Way
At the shortest infrared wavelengths (slightly redder than the
visible spectrum), dust becomes transparent, so light from
distant stars reaches us more easily.
Dust in the Milky Way
At longer infrared wavelengths, we see thermal emission
from interstellar dust rather than light from stars.
Radio emission from Hydrogen
The lowest orbital of the hydrogen atom is not one level – it is
actually 2 levels separated by a miniscule amount of energy. A
transition from the upper level to lower level produces a
photon at very low energies (radio wavelengths).
Hydrogen in the Milky Way
Note: the colors in this image are not real. Different colors represent different radio brightnesses
Radio waves are not blocked by dust, so we can see the
emission from hydrogen in space across the entire galaxy.
The Milky Way across the Electromagnetic Spectrum
Star Clusters in the Milky Way
• Open Clusters
 Hundreds to thousands of stars
 Gravitational attraction between the
stars is not strong enough to hold them
in the same area of space, so the stars
escape and the cluster disappears
 Relatively young (<2 billion years)
• Globular Clusters
 10,000 to millions of stars
 Because there are so many stars,
gravity is strong enough to keep stars
from wandering away
 Relatively old (up to 13 billion years)
Open Clusters
Open Clusters
Open Clusters
Open Clusters
Globular Clusters
Globular Clusters
Globular Clusters
Globular Clusters
Motions of Stars in Clusters
Measuring the Age of a Star Cluster
The main sequence turn-off is fainter for older clusters.
Ages of Open Clusters
Open clusters contain stars high on the main sequence. Since
those stars don’t live long, these clusters must be fairly young.
Ages of Globular Clusters
In globular clusters, only the low-mass stars are still on the
main sequence, so these clusters must be very old (>10 billion
years).
Structure of the Milky Way
• How large is the Milky Way? What is its shape?
Where are we in the Milky Way?
• To answer these questions, we need to construct
a 3-D map of the Milky Way, and for this we need
to measure distances to lots of stars
• It also will help if we can distinguish old stars from
young stars, so we need to measure ages of stars
Metals = elements heavier than H and He
Measuring Ages of Individual Stars
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 measure its age?
The universe contained only hydrogen, helium, and one other
element (lithium) when it was born. Stars have created heavier
elements, or “metals”, over time through fusion and supernovae.
Some of these metals are sent into space when stars die. The
cloud of gas and dust enriched by those metals can then form a
new generation of stars. As a result, a star born more recently
has a higher fraction of metals, or a higher metallicity, than a star
born long ago. So we can estimate the ages of stars by measuring
their metallicities.
Measuring a Star’s Metallicity
If the absorption lines from metals in the spectrum of a star are
strong, then the star has a high metallicity, and it must be
young.
If the metal lines are weak, then the metallicity is low, and the
star must be old.
Measuring Distances in the Milky Way
Parallaxes can be used to measure distances for stars within ~1000
light years from the Sun; parallaxes of more distant stars are too
small to measure, even with modern telescopes. Most stars in the
Milky are farther than 1000 light years, so we need another way to
determine the diameter of the Milky Way and our location within it.
Measuring Distances in the Milky Way
Imagine that you find a star with an unknown distance, but you
notice that it has a distinctive characteristic that tells you that it
is a specific type of star. And let’s imagine that all stars of that
type have the same luminosity, and you happen to know the
value of that luminosity. You can then estimate the distance for
the star you found from the inverse square law of light:
b = L / d2 where
b is the brightness seen from Earth
L is the luminosity that all stars of
this type are known to have
d is the distance.
Objects that are distinctive and
have known luminosities are
standard candles.
Pulsating Stars as Standard Candles
There is a narrow region in
the HR diagram where
stars pulsate, getting
bigger and smaller (i.e.,
brighter and dimmer) over
time:
There are 2 types of
pulsating stars: RR Lyrae
Finding RR Lyrae Stars
Pulsating stars are good standard candles because they have
a distinctive signature (pulsating light) that makes it easy to
identify them. Globular clusters contain many RR Lyrae stars.
Since we know that all RR Lyrae stars have a specific luminosity,
we can measure the distance to a RR Lyrae star (and hence the
cluster containing it) with the inverse square law of light.
The Center of the Milky Way
When we measure distances to
globular clusters with RR Lyrae
stars and map their distribution,
we find that they are not
centered around the Sun. Instead,
the globular clusters are scattered
about a point 25,000 light years
from us, which we assume is the
center of the Milky Way.
25,000 light years
100,000 light years
Shape of the Milky Way
Gas and dust clouds, open clusters, and most stars are
concentrated in a narrow band wrapping around the sky.
So these parts of the galaxy must form a flattened disk.
Stars/gas/
dust/open
clusters
However, globular clusters are found all across the sky, not
just in that narrow band, so they must have a spherical
distribution surrounding the disk, called a halo.
Size of the Milky Way
The Sun is in the disk between 2 spiral arms about halfway
from the Galactic center to the edge of the galaxy. The Milky
Way contains 200 billion stars and is 100,000 light years in
diameter.
Rotation of the Milky Way
Objects that are gravitationally bound to each other orbit
around a point called the center of mass, which is the average
position of those 2 objects. If they have equal masses, the
center of mass is the point halfway between the objects.
x
center of mass
Rotation of the Milky Way
If the objects have different masses, the center of mass is
closer to the more massive object.
x
center of mass
Rotation of the Milky Way
In the Milky Way, stars orbit around its center, which is the
center of mass of all of its stars, gas, and dust. The Sun
completes one orbit in roughly 250 million years.
x
center of mass
Rotation of the Milky Way
An alternative way of visualizing the rotation of the Milky
Way is in terms of its gravity well. Just as a planet orbits in the
gravity well of a star, the Sun and other stars orbit in the
gravity well produced by the sum of all matter in the galaxy.
Slides beyond this point contain
extra material that you might
find interesting but is not
covered on homeworks and
exams
The Milky Way’s Spiral Structure
The Milky Way has a distinctive spiral pattern. Each spiral arm
is a concentration of stars (especially young ones) and clouds
of gas and dust. Why does the Milky Way have this spiral
structure?
The Milky Way’s Spiral Structure
If the Milky Way’s spiral arms were rigid concentrations of stars,
the arms would rotate as the stars orbited the galaxy’s center.
Also, the outer stars would move faster than the inner stars, but
that’s not what we observe (and it would violate with Kepler’s
2nd Law). So the arms are not rigid concentrations of stars.
The Milky Way’s Spiral Structure
If the spiral arms were concentrations of stars that always stay
together, and if outer stars orbit slower than inner stars like we
expect, then the arms would become more tightly wound, and
would dissolve after a few rotations. But this seems unlikely since
it would mean we got lucky and lived at just the right time to see
spiral arms.
The Milky Way’s Spiral Structure
In reality, the spiral arms are produced by a density wave that
compresses gas/dust clouds and stars as it sweeps around the
disk, analogous to a concentration of cars in congested traffic.
When the gas/dust clouds are
compressed, many stars are born,
which is why young stars are
often found in the arms. Those
stars eventually drift behind the
density wave. Meanwhile, the
wave sweeps up new stars and
clouds to compress into spiral
arms.