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Modern Astronomy
Stars & Galaxies
Lecture 7
The Milky Way Galaxy:
Putting it all together
Geraint F. Lewis
University of Sydney 2005
Outline
Galactic Structure
 Where are the stars?
 Rotation and spiral structure
 Into the middle
 Formation of the Galaxy
 Milky Way and friends
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The Discovery of the Milky Way
Views are obscured by dust
 Discovering the true structure of the
galaxy took a lot of detective work
 Eventually the mystery was solved
when Hubble discovered Island
Universes
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The Andromeda Galaxy, the Milky Way’s sister
Galactic Makeup
100-400 billion stars
 Disk, bulge & halo
 25kpc (80,000 lyrs) across
 8kpc (30,000 lyrs) from the centre
 300pc (1,000 lyrs) thick at the Sun
 5kpc (16,000 lyrs) thick at the centre
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Monty Python
Our galaxy itself contains 100 billion stars.
It’s a 100 thousand light years side to side.
It bulges in the middle, 16 thousand light years thick,
But out by us, it’s just 3000 light years wide.
We’re 30,000 light years from galactic central point.
We go ‘round every 200 million years,
And our galaxy is only one of millions of billions
In this amazing and expanding Universe
From the Meaning of Life
In context…
Imagine each star in the Milky Way is
the size of pea
 The 200 billion stars in the Galaxy
would be a cube of peas 34m aside
 Scaling to the Milky Way, the typical
distance between peas is 220km
 The Milky Way would stretch
between the Earth and the Moon
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The Galactic Disk: Stars
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82% of stars
Star on circular orbits
Many young stars (active
star formation)
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Metal rich stars (Pop I)
Pronounced spiral arms
Thin & thick disks
The Galactic Disk: Gas & Dust
• ISM is ~10% mass of stars
• 1-2% of the ISM is dust
• Dust is visible in the infrared
• Dust made in evolved stars
• Made in supernovae
How do we see hydrogen?
The Galactic Disk: Hydrogen
• Atoms start in a low energy
state, with proton and electron
unaligned.
•When atoms bump, the atoms
can become excited, with the
proton and electron aligned.
•The electron can return to the
low energy state by emitting
radio waves at 21cm.
•If we had radio eyes sensitive
to this wavelength, the disk of
the galaxy would glow brightly.
Seeing hydrogen
Hydrogen: Stellar raw material
Dominant gas in the Galaxy
 Neutral hydrogen clouds (HI)
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Molecular complexes (H2)
Spiral arms in hydrogen
Stellar Life Cycle
Star Birth
Star Death
The Bulge
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18% of star
Very little gas & dust
Little star formation
Metal poor stars (Pop II)
Something energetic in centre
Stars on random orbits
Bulge Orbits
Unlike the Earth orbiting the
Sun, the stars in the Bulge are
not orbiting a single, massive
object.
It is the gravitational
attraction of all the stars
together that keeps the bulge
as a single object.
In effect, they are orbiting
nothing!
Out into the halo
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2% of the star
Virtually no gas or dust
No star formation
Larger, emptier version of
the bulge
~200 globular clusters
Very metal poor stars
Appears to be a very
boring place!
Metal poor stars
Stars must have formed a long time
ago out of almost pristine material.
Stars must have small masses to have
lived this long!
Velocity of the Sun
The Sun orbits the Galaxy at 220km/s
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At this velocity, it takes 240 million years
to complete a single orbit
It also bobs up and down
When the Sun was last at this location, it
was the Early Triassic, a world with the
first dinosaurs, but no birds, grass or
flowering plants!
What about other stars?
How does the Milky Way rotate?
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Is like a wheel?
Like the Planets
We can tell by
looking at the
Rotation Curve
Using the Doppler Shift
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As objects move towards
or away from us, their
spectrum of light is shifted
via the Doppler effect
Towards red for things
moving away
Towards blue for things
coming toward
Using this we can map the
radial velocities of stars in
the galaxy and work out
how the Milky Way is
rotating.
Rotation curve
http://www-astronomy.mps.ohio-state.edu/~pogge
A flat rotation curve?
Given the distribution of stars and gas
in the Milky Way, we would not
expect a flat rotation curve.
Either:
 We have problems with physics
 More matter is present than seen
A lot (10-100x) more than seen!!!
The dark matter halo
The identification of a massive dark matter halo
enveloping the Milky Way has some important
implications
 Stars, gas & dust are made from protons &
neutrons (baryons).
 Dark matter cannot be dark baryonic matter
 So you, me, all the stars, gas and dust,
everything you have seen in a NASA press
release, all of it, are a minor (tiny) player in a
Milky Way dominated by dark matter!
The dark matter halo
Spiral structure
Where does spiral structure come
from?
We have seen the disk doesn’t
rotate like a solid wheel and so it
would appear the spirals cannot be
“fixed”!
The spiral structure is seem in hot,
young blue stars and gas, but not
in old red stars.
Do the spirals just rotate with the
stars?
Overwinding
So spirals should rapidly become over wound!
Density waves
Density waves are enhancements in
the surface density of the galaxy
 Density waves rotate slowly as
coherent structure
 Stars pass through density waves,
but slow down as they do so.
Density waves
Density waves
Unlike stars, clouds of gas do not pass
through density waves unscathed
 Gas clouds are compressed as they
slow down
 Gas clouds rear-end one another
Both cause cloud collapse & forming
new stars, especially hot, luminous
(OB) stars. These light up the spiral
arms.
Density waves
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OB stars evolve quickly, reaching the end
of their lives in a few million years.
These stars explode before they leave the
spiral arms, promoting even more star
formation.
The cooler, longer lived stars leave the
spiral arms and mix with the galactic
population.
This is why spiral arms are obvious in OB
stars, but not older stars.
Seeding density waves
What causes density waves?
 Smaller perturbations can be induced
in galactic disks via interactions with
other systems
 These grow to become pronounced
waves.
 Seen in all kings of disks, including
protostellar disks!
Journey to the centre
So, what is occurring
in the Galactic
centre?
The region is clearly
very energetic!
Right in the middle
What is this nothing which the gas and stars are
rapidly orbiting?
Only real possibility is a Black Hole!
Must have a mass of 2.7£106M¯.
Not a black hole from a single stellar collapse, but
must be built up over time.
We shall meet these again when we come to look at
quasars, but we have to ask “Just how active was
the young Milky Way”?
The formation of the Milky Way
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The first stars formed in
clouds of mainly hydrogen
and helium (metal poor)
Merge into a rotating
protogalaxy of dark matter,
stars, gas & dust
Gas cools and collapses
into a disk, taking the dust
with it
Stars in the halo & bulge
just age, while in the disk
they continually forming
Nice picture, but it does not explain everything!!!
The Milky Way & friends
The Magellanic Clouds
The Local Group
Conclusions
The Milky Way does not live alone!
 We live in the Local Group,
dominated by little Dwarf Galaxies
 The closest ones are being disrupted,
but where is this material going?
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The End