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The Milky Way:
How do we know what it looks like?
Bryan Hill
What is this?
What does our galaxy look like?
Edge-On View of the Milky Way Galaxy
The thin disk (z_thin = 350) is a younger star population
with a higher density of stars.
The thick disk (z_thick = 1000pc) is an older star population
with lower density of stars.
Thin/Thick Disks
Thin and thick disks not just determined by scale heights and
stellar number densities.
Disk are also distinguished by metal concentrations in stars.
Population I- metal-rich
Z~0.02
Population II- metal-poor
Z~0.001
Population III- no metal
Z~0
Thin/Thick Disks
Population I- metal-rich
Stars similar to the Sun
Population II- metal-poor
Stars that exploded to form Population I stars
Population III- no metal
Stars that exploded to form Population II stars
Population II
Population I
Population III
Metallicity
Metallicity is a measure of the iron
to hydrogen ratio in the atmosphere
of a star compared to the Sun.
[Fe/H]≡ log10[((NFe/NH)star)/((NFe/NH))]
The older the star, the more negative the value of [Fe/H].
The younger the star, the more positive the value of [Fe/H].
For stars with the same metal abundance as the Sun, [Fe/H]=0.
How do we know
there are spirals?
How do stars orbit in our
galaxy?
Stars in the disk all orbit in the same direction with a
little up-and-down motion
Orbits of stars
in the bulge
and halo have
random
orientations
Thought Question
Why do orbits of bulge stars bob up and down?
A. They’re stuck to interstellar medium
B. Gravity of disk stars pulls toward disk
C. Halo stars knock them back into disk
Why do orbits of bulge stars bob up and down?
A. They’re stuck to interstellar medium
B. Gravity of disk stars pulls toward disk
C. Halo stars knock them back into disk
Star Halo
Halo Stars:
0.02-0.2% heavy elements (O, Fe, …),
only old stars
Halo stars
formed first,
then stopped
Disk Stars:
2% heavy elements,
stars of all ages
Disk stars
formed later,
kept forming
What is the galaxy made of?
Matter inside stars: ~ 0.6%
Matter outside stars: ~ 3.8%
What’s wrong with these numbers?
Where did all the Matter Go?
The “Missing Matter” caused scientist to
hypostasize the existence of Dark Matter
Dark Matter: An undetected form of mass that emits little or
no light but whose existence we infer from its gravitational
influence
Dark Energy: An unknown form of energy that seems to be
the source of a repulsive force causing the expansion of the
universe to accelerate
Why do we think it is there?
• “Normal” Matter:
~ 4.4%
– Normal Matter inside stars:
– Normal Matter outside stars:
• Dark Matter:
• Dark Energy
~ 0.6%
~ 3.8%
~ 25%
~ 71%
The visible
portion of a
galaxy lies
deep in the
heart of a
large halo of
dark matter.
Does dark matter really exist?
Our Options
1. Dark matter really exists, and we are observing
the effects of its gravitational attraction
2. Something is wrong with our understanding of
gravity, causing us to mistakenly infer the
existence of dark matter
Our Options
1. Dark matter really exists, and we are observing
the effects of its gravitational attraction
2. Something is wrong with our understanding of
gravity, causing us to mistakenly infer the
existence of dark matter
Because gravity is so well tested, most astronomers
prefer option #1
The First Option
MACHOs- Massive Compact Halo Objects
act as a Gravitational Lens
The Milky Way:
How do we know what it looks like?
Bryan Hill