Wednesday, Nov. 5
Syllabus, class notes, and homeworks are at:
www.as.utexas.edu  courses  AST 301, Lacy
Reading for this week: chapter 12
The Wednesday help session is in GRG 424 at 5:00 (for
the entire semester).
Topics for this week
Describe the Milky Way Galaxy
Describe the Standard Candle method of determining
distances and how Cepheid variable stars are used as
standard candles.
Describe how astronomers measure the distribution of mass
in the Milky Way and what they find.
Explain why we might expect the spiral arms in the Milky
Way to become more tightly wrapped and how density
wave theory solves this problem.
Describe the various types of galaxies, both normal and
active, and how they differ from the Milky Way
Our place in the Milky Way
Harlow Shapley (about 1920)
Determined the constant of proportionality between
period of variation and luminosity, so the luminosity of
a Cepheid variable star could be determined from its
period.
Then he could use Cepheid variables as standard
candles.
He mapped out the distribution of globular clusters.
He found that they formed a distribution centered some
distance from us in the direction of Sagittarius.
He concluded that we are not at the center of the Milky
Way and that the Milky Way is more than 10,000 ly
across.
We now know it is more than 100,000 ly across.
Components of the Milky Way Galaxy
disk – the stars in the band of light we see
a flat arrangement of stars orbiting on nearly circular
paths
bulge – an elliptical distribution of stars near the center
halo – a large diffuse distribution of stars surrounding and
overlapping the disk
bar – an elongated ridge of stars toward the centers of
some galaxies (including the Milky Way)
ring – a ring of stars around a bar
Distribution of Mass in the Milky Way
At least for other galaxies, we can measure the distribution
of luminous matter by simply taking a picture.
But is that a fair sample of all of the matter in a galaxy?
Some stars emit much more light than others.
We can measure the distribution of mass in a galaxy by
observing how fast stars orbit in the galaxy.
Near the center, the stars are orbiting around the black
hole.
Farther out, they orbit around whatever is inside of their
orbits – black holes, stars, gas, dust, ???
We can use Kepler’s 3rd law to determine the mass of
whatever is inside of their orbits.
Orbital speeds and mass
All stars seem to orbit around the center of the Milky Way
with speeds of 200-250 km/s.
Kepler’s 3rd law then tells us
vorbital =
GM inside _ orbit
rorbit
or
2
vorbital
rorbit
M inside _ orbit =
G
so if the orbital speed is approximately constant,
M inside _ orbit ! rorbit
There is mass where we see almost no stars
We conclude that Minside of radius r α r.
This means that there is as much mass between 11 and 12
kiloparsecs from the center as there is between 1 and 2
kpc.
This is surprising because there is much more light coming
from the stars between 1 and 2 kpc from the center.
The amount of mass we calculate is also surprising.
It is several times more than the mass we calculate of the
stars we can see.
Most of the mass in the Milky Way must be dark.
Spiral arms
The brightest stars, and most young stars, in spiral galaxies
are arranged in a spiral pattern in their disks.
Could the stars in spiral arms be moving along the arms
toward the centers of their galaxies?
Or could stars stay where they are in spiral arms as a spiral
galaxy rotates?
The Doppler shifts of the stars give us information about
their motions.
Doppler shifts show that stars in disks of spiral galaxies
follow nearly circular orbits – they do not spiral in along
the arms.
Rotation of spiral arms
Doppler shifts also show that stars in a spiral galaxy all
orbit with about the same speed.
Would this allow stars to remain in spiral arms, and the
galaxy to rotate like a pinwheel?
If a galaxy rotated like a pinwheel, how would the speeds of
stars as they orbit depend on distance from the center of
the galaxy?
Think it over and discuss the question with your neighbors.
The density wave explanation for spiral arms
Since stars near the center of a galaxy take less time to orbit
the galaxy than stars farther out, the arms would wrap up if
stars stayed in the spiral arms.
Stars near the center orbit faster than the pattern rotates.
Stars far from the center orbit slower than the pattern rotates.
Lin & Shu explained how stars could move through the spiral
pattern.
If stars slow down as they move through the arms, they
would spend more time there, so we would see more stars
there at any one time.
In addition, shock waves cause new stars to form as
molecular clouds move through spiral arms.