Astro 1050     Mon., Nov. 4, 2002
   Today: Chapter 12, The Milky Way

Chapter 12   The Milky Way Galaxy
Band of light running around sky in a “great circle”
Name from Greeks and Romans:  Milky Circle, Milky Road
Galileo saw it was made of thousands of faint stars
Great Circle suggests a plane of material with us in plane (like ecliptic)

The Milky Way (during the Leonid Meteor Shower)
Milky Way made of many faint stars
Bands of dark dust visible too
Many types of objects (eg. O, B stars, Hydrogen clouds) concentrated along plane of Milky Way

Where are we within the plane?
Great Circle suggests a plane of material with us in plane (like ecliptic)
Similar brightness in all directions in plane.
Does that really mean we are located in the center?

To sort out location will need:
Bright objects visible at large distances
Objects above or below the plane – so not as obscured
Ways to measure distances to those objects
Ways to see material other than stars
Gas, dust, ???, mass distribution
Ways to map motion of objects in our Galaxy
Examples of other Galaxies
The Shapley-Curtis debate:
Are spiral nebulae external galaxies or a type of object within our own galaxy

Distances to the farther stars
Parallax only works for nearby stars
Spectroscopic “Parallax” works somewhat further out
Measure spectra and get Spectral Type and Luminosity Class
From those get Luminosity and then use m-M to find distance
Limited because need relatively bright stars to get high resolution spectra
Need another way to find M, then use m-M to get distances
Variable stars and the Period-Luminosity relationship:
Some stars vary in intrinsic brightness with time
The larger, more massive,  and brighter stars vary more slowly
Like the relative pitch of a large vs. small organ pipe

Example of a Variable
Cepheid Variables named after prototype Delta Cephei
RR Lyrae Variables names after prototype RR Lyrae
Related to presence of partially ionized He at right level of star
Partially ionized material can act as a local energy source or sink

The Instability Strip
If T too low partially ionized He too deep to cause instability
If T too high partially ionized He too high to cause instability
Larger stars oscillate with longer periods

The Period-Luminosity Relationship
Relationship discovered by Henrietta Leavitt in 1912
stars in Small Magellanic Cloud
all at roughly same distance
but didn’t have absolute M, just apparent M
need absolute M to get distances
Calibrated by Harlow Shapley
If you can find distance (so M-m) for just one nearby Cepheid, you can convert Leavitt’s “m” scale to the “M” you want.

Calibrating the Period-Luminosity Relationship using Proper Motion
Suppose all planes fly at 500 MPH = 733 ft/sec
Observe the angle that a plane shifts in 1 second of time
A plane that moves 10 in 1 sec  (900 in 90 sec) is at 42,000 ft
A plane that moves 20 in 1 sec  (900 in 45 sec) is at 21,000 ft
Works for stars too:  closer stars have faster “proper motion”
Can only get average distances using average proper motion, since any given star might be moving faster or slower than average
Harlow Shapley found 11 Cepheids with proper motion
Used average proper motion, and average distance, to find average (M-m)
Let him replace Leavitt’s relative “m” axis with absolute “M”
Now period Þ M then M-m Þ d

Globular vs. open clusters
Open Clusters
Typically a few thousand stars
Not gravitationally bound
Often contain young stars
Concentrated in plane of Milky Way
Distributed “randomly” around the circle of the Milky Way
Globular Clusters
Typically >hundred thousand stars
Only contain older stars
Are gravitationally bound
Not strongly concentrated in plane of Milky Way
Not randomly distributed around the circle of the Milky Way – more towards Sagittarius

The Distribution of Globular Clusters
Assume Globular Clusters orbit around center of galaxy
Center of Globular Cluster distribution is 8.5 kpc in direction of Sagittarius
We are about 2/3 of the way out to one side – so “diameter” is approx. 25 kpc or 75,000 ly.
Dust within the galactic plane fools us with respect to distribution of ordinary stars

The Shapley-Curtis Debate:  1920
Are spiral nebulae really other galaxies, or just swirling clouds of gas and dust within our own galaxy?
Many spiral galaxies had much larger radial velocities than other objects within our own galaxy

The Andromeda Galaxy

M51:  The Whirlpool Galaxy

Sensing Hydrogen Gas
Radio emission at 21 cm wavelength
Penetrates gas and dust
Requires little energy to excite

The Structure of our Galaxy
The Disk Component
Stars, gas, and dust
The spiral arms
Size:
Luminous Diameter ~ 25 kpc
Thickness 300 pc – 1 kpc
O stars and dust 30 pc
Sunlike stars greater
The Spherical Component
Old Stars, but little gas or dust
The Halo
Globular clusters
Isolated old stars
red dwarfs, giants, white dwarfs
The Nuclear Bulge

Disk vs. Halo Orbits
The Disk Component
Stars, gas, and dust
The spiral arms
Size:
Luminous Diameter ~ 25 kpc
Thickness 300 pc – 1 kpc
O stars and dust smaller
Sun-like stars greater
The Spherical Component
Old Stars, but little gas or dust
The Halo
Globular clusters
Isolated old stars
red dwarfs, giants, white dwarfs
The Nuclear Bulge

Differential Galactic Rotation
Stars far from the center take longer to orbit galaxy
If all mass is at the center get Keplerian Rotation:
If M is distributed, Meffective grows with distance, so velocity does not drop in same way

The Galactic Rotation Curve
Keplerian fall-off near center indicates compact mass at center
Flat curve throughout disk indicates much distributed mass
Lack of fall-off beyond visible “edge” indicates “dark matter”

Stellar Population and Galaxy Evolution
“Metal” abundance during time
“Metals” are elements heavier than He
A given star’s atmospheric abundance is approx. fixed at birth
Interstellar metal abundance grows with each new generation of stars
Red giants and supernova eject new heavy elements into interstellar gas
Orbits during time
A given star’s orbit is approx. fixed at birth – just plows through gas
Orbits of gas clouds evolve with time since they can collide
Orbits get more circular and disk-like with time

Traditional Model of Galaxy Evolution
Stars are stuck with their original orbits
They plow through gas like bullets
Orbits of gas can evolve
Gas clouds collide and only average motion (rotation) survives
Metal abundance grows with time
System starts out with little organized motion,few metals
Halo stars form at this time
It contracts, velocities average out leaving only rotation
Gas collapses to form the disk
Disk stars form after this has happened
Some problems with traditional model
Globular clusters not all same age
Gap in ages between halo and disk objects
Presence of some metals in even in oldest stars
System may have formed by merger of smaller galaxies
Galactic Cannibalism

M51:  The Whirlpool Galaxy

Possible Origin of Spiral Arms
Differential rotation smears features out into spiral patterns
But can’t be whole story:
Number of times Sun has orbited the galaxy:
10 billion yr/200 million yr
= 50 times
Spiral arms would have been wound up very tightly
Something must continuously rebuild them

Degree of Organization
 of the Spiral Arms
Different degrees of organization
Grand Design
Spirals: M51
Flocculent (“wooly”) Spirals

Degree of Organization
 of the Spiral Arms
Different degrees of organization
Grand Design
Spirals: M51
Flocculent (“wooly”) Spirals

Tracing the Spiral Arms
Arms NOT  obvious if you look at:
Old objects like the sun
Arms ARE obvious if you look at:
Maps of gas clouds
21 cm Hydrogen
Radio maps of CO
Far infrared observations of dust
Young stars
O, B stars
“HII” ionized hydrogen regions surrounding O,B stars
Clouds somehow form in arms , then dissipate between them
Short lived objects only get a short distance from their places of birth
O stars, Lifetime = few million years,  at 250 km/s Þ500 pc

M51:  The Whirlpool Galaxy

Density Wave Theory
SPIRAL WAVE rotates with galaxy, but slower than individual stars
Like moving traffic jam after an accident has been cleared
Gas (and stars) catch up with wave, move through it, eventually reach front
Just like cars catching up with moving traffic jam, eventually get through it
Gas is more crowded in wave – clouds collapse to form new stars
More collisions in the traffic jam
There are slightly more old stars in the arm too, because they speed up slightly coming into it and slow down slightly moving out of it.
But the best tracers are the things that mark recent cloud collapses:  O,B stars, etc.

M51:  The Whirlpool Galaxy

Self Sustaining Star Formation
Cloud collapse Þ New stars
New stars Þ Supernova after few million years
Supernova Þ Shock Waves
Shock Waves Þ Nearby clouds collapse
Differential Rotation twists pattern into spiral

Two limiting cases of spirals
Grand Design: Density Wave
Flocculent: Self Sust. Star Form. + Diff. Rot.
In most Galaxies you have some combination of the two

The Nucleus of the Galaxy
Likely Black hole
High velocities
Large energy generation
At  a=275 AU  P=2.8 yr Þ 2.7 million solar masses
Radio image of Sgr A
about 3 pc across, with model of surrounding disk

A movie of stars at the core
www.mpe.mpg.de/www_ir/GC
Very cool, brand new, and worth a look!
This is the best evidence to date for a massive black hole at the Galactic core.  Now essentially “proven.”