Download The Milky Way Galaxy

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 ??)
WHAT DO YOU THINK?
1.
How big is the Milky Way Galaxy? “Hella big”
2.
Where is our solar system located?
25555 Hesperian Blvd, Hayward, CA, USA, Earth
3.
Is there really a HUGE black hole at the center
(and why weren’t we sucked into it in 2012 ??)
Yes, and how do you know we weren’t??
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
Other Galaxies we see –
do we look like one of them??
A spiral galaxy “edge on”
Is this how we look to the rest of the Universe?
A spiral galaxy “edge on”
Is this how we look to the rest of the Universe?
Some other spirals…
M51 (enhanced)
M51 (enhanced)
Infer we probably look like
other galaxies
 See
new, young stars, gas, dust
 See
“pancake” shape across sky
indicating a disk.
 Use

other wavelengths to confirm spiral.
=> Milky Way is a Spiral Galaxy!
Drawings – not photos – of our galaxy!
Milky Way
Structure
•Disk
•Bulge
•Halo
Structure of Milky Way Galaxy

Disk
open clusters
 younger generation of
stars, gas, dust
 Population I stars

Structure of Milky Way Galaxy
Disk
 Bulge

mixture of both
young & old stars
 Population II Stars

Structure of Milky Way Galaxy
Disk
 Bulge
 Halo

oldest generation of
stars, no gas/dust
 globular clusters

Regions of the Milky Way Galaxy
diameter = 100,000 l.y
radius = 50,000 l.y.
thickness ~ 1,000 l.y.
number of stars = ~200 Billion
Sun is in disk,
28,000 l.y. out
from center
Regions of the Milky Way Galaxy
diameter = 100,000 l.y
radius = 50,000 l.y.
thickness ~ 1,000 l.y.
number of stars = ~200 Billion
Sun is in disk,
28,000 l.y. out
from center
Mapping the Galaxy

Using Optical light

Using Radio Waves

Using IR light

Using X-rays & Gamma
Views of the Milky Way
Mapping the Galaxy- Optical

Observation:
Cepheid variables in
“globular clusters”
Cepheid Variable Stars in Globular Clusters
Mapping the Galaxy- Optical

Observation:
Cepheid variables in
“globular clusters”

Hypothesis:
Distribution around
galactic center defines
location of our solar
system in Milky Way!
Cepheids in Globular Clusters
All we need to find is their distance!
Henrietta Leavitt, along with Harlow
Shapely, used Cepheid variable stars to
determine direction & distance to center
Shapely Curtis Debate - 1920!
Another example of “good science”!

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

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
•

gravitational pull from disk
Stellar Orbits in the Galaxy
Stars in the bulge and halo all
orbit the Galactic center:
•
•
•
in different directions
various angles to the disk
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 as “Waves”
M 51

Caused by compression waves
which move around the disk

Increased density of matter at
“crests”

Density waves revolve at
different speed than individual
stars orbit Galactic center
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
The Star–Gas–Star Cycle
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*
Missing Mass?

Observation: Stars in edges orbit center
“too fast” to stay attached using gravity …
from mass we “see”.

Conclusion: So there must be more mass
we *don’t* see pulling as well?

Hypothesis: Dark Matter (Missing Mass)
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!