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Take a Giant Step Outside the Milky Way
Artist's Conception
Example
(not to
scale)
Another galaxy: NGC 4414. The Milky Way roughly resembles it.
The Three Main Structural Components of the Milky Way
1. Disk
- 30,000 pc diameter (or 30 kpc)
- contains young and old stars, gas, dust. Has spiral structure
- vertical thickness roughly 100 pc - 2 kpc (depending on component.
Most gas and dust in thinner layer, most stars in thicker layer)
2. Halo
- at least 30 kpc across
- contains globular clusters, old stars, little gas and dust,
much "dark matter"
- roughly spherical
3. Bulge
- About 4 kpc across
- old stars, some gas, dust
- central black hole of 3 x 106 solar masses
- Spherical or football shaped (as seen from side)
Measuring the Milky Way
We have already encountered variable stars – novae,
supernovae, and related phenomena – which are
called cataclysmic variables.
There are other stars whose luminosity varies in a
regular way, but much more subtly. These are called
intrinsic variables.
Two types of intrinsic variables have been found: RR
Lyrae stars and Cepheids.
Measuring the Milky Way
The upper plot is an
RR Lyrae star.
All such stars have
essentially the same
luminosity curve, with
periods from 0.5 to 1 day.
The lower plot is a
Cepheid variable;
Cepheid periods range from
about 1 to 100 days.
Measuring the Milky Way
The variability of these
stars comes from a
dynamic balance between
gravity and pressure –
they have large
oscillations around
stability.
Measuring the Milky Way
The usefulness of these stars comes from their
period–luminosity relationship.
Measuring the Milky Way
This allows us to measure the distances to these
stars.
• RR Lyrae stars all have about the same luminosity;
knowing their apparent magnitude allows us to
calculate the distance.
• Cepheids have a luminosity that is strongly
correlated with the period of their oscillations; once
the period is measured, the luminosity is known and
we can proceed as above.
Shapley (1917) found that Sun was not at center of Milky Way
Shapley used distances to variable “RR Lyrae” stars (a kind of Horizontal
Branch star) in Globular Clusters to determine that Sun was 16 kpc from
center of Milky Way. Modern value 8 kpc.
Measuring the Milky Way
We have now
expanded our cosmic
distance ladder one
more step.
Precise Distance to Galactic Center
Distance = 7.94 +/- 0.42 kpc
SgrA*
Eisenhauer et al. 2003
Orbital motion 6.37 mas/yr
Clicker Question:
Where is our solar system located?
A: near the center of the Milky Way Galaxy in the
bulge.
B: 4 kpc from the center of the Milky Way in the halo.
C: 8 kpc from the center of the Milky Way in the disk.
D: 20 kpc from the center of the Milky Way in the disk.
Clicker Question:
What lurks at the center of our galaxy?
A: A 3 million solar mass black hole.
B: A giant star cluster.
C: A 30 solar mass black hole.
D: Darth Vader
Galactic Structure
This infrared view of our Galaxy shows much more
detail of the galactic center than the visible-light view
does, as infrared is not as much absorbed by gas and
dust.
Stellar Orbits
Halo: stars and globular clusters
swarm around center of Milky
Way. Very elliptical orbits with
random orientations. They also
cross the disk.
Bulge: similar to halo.
Disk: Stars and gas rotate in
circular orbits.
Stellar orbits in the disk are in a
plane and in the same direction;
orbits in the halo and bulge are
much more random.
The Formation of the Milky Way
Any theory of galaxy formation should be able to
account for all the properties below.
The Formation of the Milky Way
The formation of
the galaxy is
believed to be
similar to the
formation of the
solar system, but
on a much larger
scale.
Rotation of the Disk
Sun moves at 225 km/sec around center. An orbit takes 240 million years.
Stars closer to center take less time to orbit. Stars further from center take
longer.
=> rotation not rigid like a phonograph record or a merry-go-round. Rather,
"differential rotation".
Over most of disk, rotation velocity is roughly constant.
The "rotation
curve" of the
Milky Way
Spiral Structure of Disk
Spiral arms best traced by:
Young stars and clusters
Emission Nebulae
HI
Molecular Clouds
(old stars to a lesser extent)
Disk not empty between arms,
just less material there.
Problem: How do spiral arms survive?
Given differential rotation, arms should be stretched and smeared out after
a few revolutions (Sun has made 20 already):
The Winding Dilemma
The spiral should end up like this:
Real structure of Milky
Way (and other spiral
galaxies) is more loosely
wrapped.
Proposed solution:
Arms are not material moving together, but mark peak of a
compressional wave circling the disk:
A Spiral Density Wave
Traffic-jam analogy:
Traffic jam on a loop caused by merging
Now replace cars by stars
and gas clouds. The traffic
jams are actually due to the
stars' collective gravity.
The higher gravity of the
jams keeps stars in them
for longer. Calculations
and computer simulations
show this situation can be
maintained for a long time.
Video Links:
Density Waves &
Spiral Arms
Density Waves &
Traffic Jams
Spiral Galaxy Simulation
Molecular gas clouds pushed together in arms too => high density of
clouds => high concentration of dust => dust lanes.
Also, squeezing of clouds initiates collapse within them => star formation.
Bright young massive stars live and die in spiral arms. Emission nebulae
mostly in spiral arms.
So arms always contain same types of objects, but individual objects come and go.
Galactic Spiral Arms
As clouds of gas and dust move through the spiral arms,
the increased density triggers star formation. This may
contribute to propagation of the arms. The origin of the
spiral arms is not yet understood.
The Mass of the Milky Way Galaxy
The orbital speed of an object depends only on the
amount of mass between it and the galactic center.
90% of Matter in Milky Way is Dark Matter
Gives off no detectable radiation. Evidence is from rotation curve:
10
Rotation
Velocity
(AU/yr) 5
Solar System Rotation Curve: when
almost all mass at center, velocity
decreases with radius ("Keplerian")
1
1
10
20
30
R (AU)
observed curve
Milky Way
Rotation
Curve
Curve if Milky
Way ended
where visible
matter pretty
much runs out.
What is “dark matter”?
What could this “dark matter” be? It is dark at all
wavelengths, not just the visible. Accounts for 90% of mass
• Stellar-mass black holes?
Probably contributes
but no way enough could have been created
• Brown dwarfs, faint white dwarfs, and red dwarfs?
Currently the best star-like option
• Weird subatomic particles, mostly in the halo?
Could be, although no evidence so far
What is “dark matter”?
The bending of spacetime can allow a large mass to
act as a gravitational lens:
Observation of such events suggests that low-mass
white dwarfs could account for about
half of the
mass needed.
The rest is
mystery.
still a
The Galactic Center
This is a view toward the
galactic center, in visible light.
The two arrows in the inset
indicate the location
of the center; it is entirely
obscured by dust.
The Galactic Center
These images, in
infrared, radio,
and X ray, offer a
different view of
the galactic center.
The Galactic Center
The galactic center appears to have
• a stellar density a million times higher
than near Earth
• a ring of molecular gas 400 pc across
• strong magnetic fields
• a rotating ring or disk of matter
a few parsecs across
• a strong X-ray source at the center
The Galactic Center
Apparently, there is an enormous black hole at the
center of the galaxy, which is the source of these
phenomena.
An accretion disk surrounding the black hole emits
enormous amounts of radiation.
The Galactic Center
These objects are very close to the galactic center. The
orbit on the right is the best fit; it assumes a central
black hole of 3.7 million solar masses.
Clicker Question:
How long does it take our solar system to
orbit once around the Milky Way?
A: 1 year
B: 2 million years
C: 250 million years
D: 250 billion years (longer than the age of the universe)
Clicker Question:
What makes up most of the mass (90%)
of the Milky Way Galaxy?
A: hydrogen gas
B: stars
C: dead stars (white dwarfs, neutron stars, and black
holes)
D: we don’t know
Hubble’s Galaxy Classification
Hubble’s “tuning fork” is a convenient way to
remember the galaxy classifications, although it has
no deeper meaning.
Hubble’s Galaxy Classification
Type Sa has the largest central bulge, Type Sb is
smaller, and Type Sc is the smallest.
Type Sa tends to have the most tightly bound spiral
arms, with Types Sb and Sc progressively less tight,
although the correlation is not perfect.
The components of spiral galaxies are the same as in
our own Galaxy: disk, core, halo, bulge, spiral arms.
Hubble’s Galaxy Classification
Spiral galaxies are classified according to the size of
their central bulge.
Hubble’s Galaxy Classification
Similar to the spiral galaxies are the barred spirals.
Elliptical galaxies have no spiral arms and no disk.
They come in many sizes, from giant ellipticals of
trillions of stars, down to dwarf ellipticals of fewer
than a million stars.
Ellipticals also contain very little, if any, cool gas and
dust, and show no evidence of ongoing star
formation.
Many do, however, have large clouds of hot gas,
extending far beyond the visible boundaries of the
galaxy.
Ellipticals are classified according to their shape,
from E0 (almost spherical) to E7 (the most
elongated).
S0 (lenticular) and SB0 galaxies have a disk and bulge,
but no spiral arms and no interstellar gas.
The irregular galaxies have a wide variety of shapes.
These galaxies appear to be undergoing interactions with
other galaxies.
Irregular galaxies are the most common type of galaxy
Hubble’s Galaxy Classification
A summary of galaxy properties by type