The Milky Way Galaxy
The infinitude of creation is great enough
to make a world, or a Milky Way of worlds,
look in comparison with it what a flower or
an insect does in comparison with the
Earth.
Immanuel Kant (1724 – 1804)
German philosopher
WHAT DO YOU THINK?
1.
How big is the Milky Way Galaxy?
2.
Where is our solar system located?
3.
Is there really a HUGE black hole at the center
(and why weren’t we sucked into it in 2012 ??)
Our View of the Milky Way…
Our view

Clearly a “disk” shaped, with us inside

Optical view blocked by dust & cold gas

Infer we probably look like other galaxies
Spiral?
 Elliptical?
 Neither?

M51 (enhanced)
A spiral galaxy “edge on”
Some other spirals…
Our view

Infer we probably look like other galaxies
=> Milky Way is a Spiral Galaxy!
 We
see new, young stars, gas, dust
|
 We see the “pancake” shape across our sky
indicating a disk.
 We
can use other wavelengths to confirm structure
of spiral arms.
Drawings – not photos – of our galaxy!
Milky Way
Structure
•Disk
•Bulge
•Halo
Regions of the Milky Way Galaxy
diameter of disk = 100,000 l.y. (30,000 pc)
radius of disk = 50,000 l.y. (15,000 pc)
thickness of disk = 1,000 l.y. (300 pc)
number of stars = ~200 Billion
Sun is in disk,
28,000 l.y. out
from center
Mapping the Galaxy

Using Optical light
Cepheid variables in “globular clusters”
 Distribution around galactic center defines
location of our solar system


Using Radio Waves
Cool Hydrogen Gas emits 21-cm light
 Map out spiral arms

Mapping the Galaxy

Using IR light


Warm clouds of gas/dust locates star
formation sites in spiral arms
Using X-rays
Highest energy emissions from center of
galaxy
 Supermassive Black Hole

Views of the Milky Way
Mapping the Galaxy- Optical

Observation: Cepheid
variables in “globular
clusters”

Hypothesis: Distribution
around galactic center
defines location of our
solar system
Cepheid Variable Stars in Globular Clusters

Henrietta Leavitt, along with Harlow
Shapely, used Cepheid variable stars to
determine direction & distance to center
Mapping the Milky Way!

Observation: Cepheid variables in
“globular clusters”

Hypothesis: Distribution around
galactic center defines location of
our solar system

Critical Test: Distances average to ~
30,000 light years, toward Sagittarius
Globular Clusters
Our
Sun
Not centered
around Sun
Towards the Center…
Mapping the Galaxy
Using Radio Waves
Cool Hydrogen Gas emits 21-cm light
 Map out spiral arms

Structure of Milky Way Galaxy

Disk
younger generation of stars
 contains gas and dust
 location of open clusters


Bulge


mixture of both young & old stars
Halo
older generation of stars
 contains no gas or dust
 location of globular clusters

The Star–Gas–Star Cycle
Stellar Orbits in the Galaxy
Stars in the disk all orbit the
Galactic center:
•
•
•
in the same direction
in the same plane (like planets
orbit our sun)
they “bobble” up and down


due to gravitational pull from
the disk
this gives the disk its thickness
Stellar Orbits in the Galaxy
Stars in the bulge and halo all
orbit the Galactic center:
•
•
•
in different directions
at various angles to the disk
they have higher velocities

not slowed by disk as they
plunge through it
Spiral Arms
M 51

Galactic disk does not
appear solid
•
Spiral arms are not fixed
strings of stars which revolve
like the fins of a fan.
Spiral Arms

Caused by compression waves
which move around the disk.
• Increase density of matter at
“crests”
•

M 51
density waves revolve at
different speed than individual
stars orbit Galactic center
Note how the spiral arms appear
bluer compared to the bulge or the
gaps between the arms.
Does the Milky Way have a Bar?
Mapping the Galaxy

Using IR light

Warm clouds of gas/dust locates star
formation sites in spiral arms
Where the new stars are….
Where the new stars are….
Views of the Center!
Infrared
Visual
The Center in Radio & X-Rays
Although dark in visual light,
there are bright radio, IR, and
X-ray sources at the center of
the Galaxy, known as Sgr A*.
Radio Image of Center of Milky Way
“SNR” = Supernova Remnant
Sgr A = brightest radio source in Sagittarius
11 years of
observation
in IR
6 stars
orbiting
unseen
central
mass ~ 4
Million x
Mass of Sun
Mapping the Center - IR
Mapping the Center in IR

Use IR telescopes to measure orbits of fastmoving stars near the Galactic center.



One star passed within 1 light-day of Sgr A*
using Kepler’s Laws, mass = 2.6 million M
What can be so small, yet be so massive?
Mapping the Galaxy in X-Rays
Using X-rays

Highest energy
emissions from
center of galaxy

Confirms
Supermassive
Black Hole
Chandra image of Sgr A*
Mapping the Galaxy in X-Rays
Chandra image of Sgr A*

Rapid flare rise/drop time (< 10 min)

Tiny emission region only 20 times the
size of event horizon of 2.6 million M
black hole.

Observations consistent w/
supermassive black hole at the center
of our Galaxy.

Energy from flare probably came from
a comet-sized lump of matter…torn
apart before falling beneath the event
horizon!
Missing Mass?

The edges of the galaxy orbit center “too
fast” to stay attached using gravity from
mass we “see”.

So there must be more mass we *don’t*
see pulling as well?

Dark Matter (Missing Mass)
Key Terms
dark matter (missing mass)
disk (of a galaxy)
distance modulus
galactic cannibalism
galactic nucleus
galaxy
halo (of a galaxy)
microlensing
Milky Way Galaxy
missing mass
nebula (plural nebulae)
nuclear bulge
rotation curve (of a galaxy)
Sagittarius A
Shapley–Curtis debate
spin (of an electron or proton)
spiral arm
synchrotron radiation
21-cm radio radiation