Download The Milky Way

The Milky Way
Our Galaxy
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The Milky Way
Almost everything we see in the night
sky belongs to the Milky Way.
We see the Milky Way as a faint band of
light across the sky.
Based on this (our only) view of the
Milky Way, what would you say the
Milky Way would look like from a
position far outside the Milky Way?
1. A flat, round disk.
2. A flat, elliptical disk
3. A sphere.
4. A spiral with one arm wound
around multiple times
5. A spiral with multiple arms.
The Structure of the Milky Way
Galactic Plane
Galactic Center
The actual structure of our Milky Way is
very hard to determine because:
1) We are inside.
2) Distance measurements are difficult.
3) Our view towards the center is
obscured by gas and dust.
In order to get a good outside view of the Milky Way, you are
trying to send a spacecraft high above the plane of the Milky
Way, to a height equal to our distance from the Galactic
center. If you had a spacecraft that could travel at almost the
speed of light, how long would it take it to get there?
Sun
1) 2 months
2) 5 years
3) 250 years
4) 30,000 years
5) 5 million years
Galactic Center
The Sun is about
Sun
8.5 kpc = 8,500 pc
≈ 30,000 light years
from the Galactic
center.
Galactic Center
=> No spacecraft will
ever travel a
significant distance
through or even out
of the Milky Way.
The Voyager Spacecraft
(launched in the 1970s)
have just reached the
“edge” of the solar system!
The Extended Solar Neighborhood
Approx. 1,700 light years
The Milky Way Galaxy
Diameter of the Milky Way:
Approx. 75,000 light years
Viable ways to explore our Milky Way:
1. Select bright objects that you can see throughout
the Milky Way; trace their directions and distances.
2. Observe objects at wavelengths other than visible
(not absorbed by gas and dust); note their directions
and distances.
3. Trace the orbital velocities of objects in different
directions.
To trace out the structure of our
Milky Way, you want to select the
brightest stars, which are …
1.
2.
3.
4.
5.
A stars.
G stars.
O stars.
M stars.
White dwarfs.
Remember: O and B
stars are the most
massive, most luminous
stars (unfortunately, also
the shortest-lived ones).
=> Look for very young
clusters or associations
containing O and B stars:
O/B Associations!
Which method might astronomers
use to measure the distances to
star clusters across the Milky Way?
1.
2.
3.
4.
5.
Light travel time measurements
Cepheid Variables
Trigonometric Parallax
Cosmological Redshift
Gravitational Redshift
Exploring the structure of the Milky
Way with bright objects
O/B Associations
Sun
O/B Associations trace out
3 spiral arms near the Sun.
Distances to O/B Associations
determined using Cepheid Variables
Star Formation in Spiral Arms
Shock waves from
supernovae, ionization
fronts initiated by O and
B stars, and the shock
fronts forming spiral
arms trigger star
formation
Spiral arms are
stationary shock waves,
called Spiral Density
Waves, initiating star
formation
Star Formation in
Spiral Arms
Spiral arms are basically
stationary shock waves
Stars and gas clouds orbit
around the Galactic center and
cross spiral arms
Shocks initiate star formation
Star formation self-sustaining
through O and B ionization fronts
and supernova shock waves
The Nature of Spiral Arms
Spiral arms are also bright in
infrared, from dust.
Spiral arms appear bright in visible light,
from newly formed, massive stars,
Self-Sustained Star Formation in
Spiral Arms
Star forming regions get elongated due
to differential rotation
Star formation is self-sustaining due to
ionization fronts and supernova shocks
The Whirlpool Galaxy
Self-sustaining
star forming
regions along
spiral arm
patterns are
clearly visible
M 51 (Whirlpool Galaxy)
Why do the star-forming regions in this
image of the Whirlpool Galaxy appear red?
1.
2.
3.
4.
5.
This is the red color of Hydrogen
Balmer (Ha) emission from Hydrogen
that has been ionized by young,
massive, hot stars.
Their light is highly red-shifted because
especially the star-forming regions are
moving away from us at high speed.
This is the red color of interstellar dust
that is present in the molecular clouds
out of which stars are formed.
Star forming regions are red from the
dominant red light of cool, low-mass
stars.
They are red from the intensive
Hydrogen burning that goes on in the
newly formed stars.
Exploring the structure of
the Milky Way (II)
Globular Clusters
Globular Cluster M80
• Dense clusters of 50,000 – a million stars
• Old (~ 11 billion years), lower-main-sequence stars
• Approx. 200 globular clusters in our Milky Way
Globular Cluster M53
How do we know that globular
clusters are old?
1.
2.
3.
4.
5.
They contain many O and B stars.
Their light is highly red-shifted.
They are at very large distances, so they must have
formed a long time ago.
The turn-off point in their Hertzsprung-Russell
diagram is very low on the main sequence.
Stellar surfaces appear wrinkled.
Example:
HR diagram of the star cluster M 55
High-mass stars
evolved onto the
giant branch
Turn-off point
Low-mass stars
still on the main
sequence
The Structure of the Milky Way
Revealed
75,000 light years
Disk
Nuclear Bulge
Sun
Halo
Open Clusters,
O/B Associations
Globular Clusters
Infrared View of the Milky Way
Interstellar dust (absorbing optical light)
emits mostly infrared light.
Small and Large Magellanic Clouds:
Small satellite galaxies of our Milky
Way, 160,000 LY and 180,000 from
the center of the Milky Way.
Infrared View of the Milky Way
Radio View of the Milky Way
Interstellar dust does not absorb radio waves
=> We can observe any direction throughout the
Milky Way at radio waves.
Radio map at a wavelength of 21 cm, tracing
neutral hydrogen
The Milky Way Across the
Electromagnetic Spectrum
Radio
Waves
Visible
light
g-rays
Infrared
X-rays
The Structure of the
Milky Way Revealed
Distribution of stars
and neutral hydrogen
Distribution of dust
Sun
Bar
Ring
When the Milky Way was formed, the gas
contained almost exclusively H and He; the gas
is enriched by heavier elements (“metals”) only
through supernovae. For this reason, …
1.
2.
3.
4.
5.
Older stars should be more metal-rich than
younger ones.
There should be no difference in the metal
content of old and young stars.
Younger stars should be more metal-rich than
older ones.
The metal content of a star should depend
primarily on its mass.
The metal content of a star should depend
primarily on its temperature.
Stellar Populations
Younger stars are more
metal-rich than older ones.
Population I: Young stars
(less than 5 billion years):
metal-rich; located in spiral
arms and disk
Population II: Old stars (more
than 5 billion years): metal-poor;
located in the halo (globular
clusters) and nuclear bulge
Orbits of Stars in the Milky Way
Population I
(disk stars)
Population II
(halo stars)
Orbital Motions in the Milky Way (II)
Differential Rotation
Sun orbits around
Galactic center
with 220 km/s
1 orbit takes approx.
240 million years.
What can we infer from the sun’s orbital
period around the Galactic center
(knowing its distance from the GC)?
1.
2.
3.
4.
5.
The mass of the sun.
The diameter of the Milky Way.
The angular momentum of the Milky Way.
The shape of the Milky Way’s spiral arm
structure.
The mass of the Milky Way contained inside
the sun’s orbit.
Mass determination
from orbital velocity:
Method similar to mass
determination in binary systems
(variation of Kepler’s 3rd law):
The more mass there is inside the
orbit, the faster the sun has to orbit
around the Galactic center.
Combined mass:
4 billion
25
MMsun
M = 400
11
100
billion
billion
M
sun
sun
sun
The Mass of the Milky Way
Total mass in the disk
of the Milky Way:
Approx. 200 billion
solar masses
Additional mass in an
extended halo:
Total: Approx. 1 trillion
solar masses
Most of the mass is not
emitting any radiation:
Dark Matter!
Why do we not have a direct view
(in visible light) to the Center of the
Milky Way?
1.
2.
3.
4.
5.
The Galactic Center does not emit visible light.
There are too many stars in the way that block
our view to the Galactic Center.
The Galactic Center is always hidden behind
the sun.
The Milky Way does not have a center.
There is too much dust and gas in the way that
blocks our view to the Galactic Center.
The Galactic Center
Our view (in visible light) towards the Galactic
center (GC) is heavily obscured by gas and dust:
Extinction by 30 magnitudes
 Only 1 out of 1012 optical photons makes its
way from the GC towards Earth!
Galactic center
Wide-angle optical view of the GC region
Radio View of the Galactic Center
Many supernova remnants;
shells and filaments
Arc
Sgr A
Sgr A
Sgr A*: The Center of our Galaxy
The Galactic Center contains a supermassive
black hole of approx. 4 million solar masses.
The Black Hole in the
Center of the Milky Way
By following the orbits of
individual stars near the
center of the Milky Way,
the mass of the central
black hole could be
determined to ~ 4 million
solar masses.
The Black Hole in the Galactic Center
Black Hole with
4 million Msun
What keeps the stars in the disk of the
Milky Way on their orbits around the
Galactic center?
1.
2.
3.
4.
5.
The angular momentum of the Milky Way.
The gravitational attraction of the stars near the
Galactic center.
The gravitational attraction of the supermassive
black hole in the Galactic center.
The gravitational attraction of the stars in the halo
of the Milky Way.
The gravitational attraction of all mass (stars, gas,
dark matter) in and around the Galactic center.
The gravitational attraction of all mass (stars,
gas, dark matter) inside the respective orbit
keeps the stars in their orbits.
Compare 4 million solar masses
(supermassive BH in the Galactic center) to
the 1 trillion masses of the entire Galaxy!
The mass of the supermassive BH in the
Galactic center makes up only a tiny fraction
of the total mass of the Milky Way!
X-Ray View of the Galactic Center
Galactic center region contains many black-hole
and neutron-star X-ray binaries.
Supermassive black hole in the Galactic
center is unusually faint in X-rays,
compared to those in other galaxies.
Chandra X-ray image of Sgr A*
History of the
Milky Way
The traditional theory:
Quasi-spherical gas cloud fragments
into smaller pieces, forming the first,
metal-poor stars (pop. II);
Rotating cloud collapses into a disklike structure
Later populations of stars (pop. I) are
restricted to the disk of the Galaxy