Download 1128/1130 Discussion Notes

Monday/Wednesday, November 28/30, 2005
Please come to the front of the room and take back your papers.
Concept Check
If you are one of the “chosen ones,” please sit next to the other members of your
group. You may not use your notes, book, or un-chosen members of your group.
Team A:
1. Meredith Banos
2. Talia Leeds
3. Daniel Weitz
1. Gleb Feldman
2. Jordon Helgason
3. Ariel Sims
Team B:
1. Robin Barretta
2. Erik Knapp
3. Ronnie Reyes
1. Jessica Cecil
2. Georgia Fields
3. Jonathan Russo
Please choose question one or two.
Question One: Where would you look to find a spiral density wave?
What do they cause?
Answer: Spiral density waves are located in the disks of spiral galaxies.
They cause gas clouds to crash into each other, squishing them
and causing star formation.
Question Two: What are the three major components of the Milky Way,
a spiral galaxy?
Answer: Halo, disk, bulge
<figure 14.1>
be able to reproduce these images
Where are disk stars found? What are their orbits like?
Disk Stars
Found in the disk
Spend all of time in disk
Orbit in same direction, bobbing up and down
Where are halo stars found? What are their orbits like?
Halo Stars
Found in or near the halo
Spend most of time in halo
Orbit in random directions
Bulge Stars
Found in bulge
Spend all of time in bulge
Orbit in random directions
Measuring masses – finding dark matter!
Remember how we could measure the mass of a binary system using
Newton’s version of Kepler’s third law? To do so, we needed to know
the period of the binary system and the distance between the two stars.
In a similar way, we can determine the mass of the galaxy interior to the
orbit of a star if we know the period of the star’s orbit about the galactic
center and the distance between the star and the galactic center.
Why is it that this only tells us the mass of the material within the Sun’s
orbit? Newton’s version of Kepler’s third law is based upon gravity. (If
we had derived this law from basic physical principles rather than
simply presenting it to you, this would be obvious. As it is, just trust me
on this one.) The force of gravity from the material outside of the orbit
is in all different directions, so it practically cancels out. The force of
gravity from the material within the orbit is all in the same direction, so
it adds up.
We can, in theory, do similar calculations for stars that are closer to the
edge of the disk to find the mass of a greater portion of the galaxy.
Since the bulge looks so big and bright, we expect that most of the
mass of the galaxy is located in the center. However, in order for this
to be true the orbital periods of the stars closer to the edge of the disk
would have to be larger (i.e. their velocities would have to be
smaller.) Conversely, if most of the mass is in the halo, the orbital
periods of the stars closer to the edge of the disk would have to be
about the same as the stars near the bulge (i.e. their velocities would
have to be about the same as the stars near the bulge.)
Contrary to our expectations, stars that are far from the galactic center
have velocities that are comparable to stars near the bulge. We explain
this by saying that the bulk of the mass in our galaxy is not in our bulge,
but in our halo. We call this mass “dark matter” because we do not
detect it emitting light.
We’ll return to this idea next week.
Galaxy Classification
There are a few major types of galaxies
Spiral Galaxies
 Galaxy has bulge (yellowish) and disk (white)
 What does color signify?
 Disk has visible spiral arms
Elliptical Galaxies
 Round or oval shaped and redder than spiral
Irregular Galaxies
 Any galaxy that does not fit into the above scheme is called an
irregular galaxy
Identify the following galaxies by type.
Spiral Galaxy
Elliptical Galaxy
Irregular Galaxy