Download 1201 Discussion Notes

Wednesday, December 1, 2004
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. Andrew Buxbaum
2. Rachel Lubich
3. Jaimee Motley
1. Stacey Barnett
2. Margaux Maizlish
3. Justin Wild
Please choose question one or two.
Team B:
1. Meagan Bond
2. Shin-Ae Choi
3. Thomas McCormick
1. Kristin Buda
2. Deepak Kumar
3. Bobby Lee
OUT OF DISCUSSION ASSIGNMENT: 2 BONUS pts.
The discussion after the next one is the last of the semester. If you
could only ask one question before the exam, what would it be? Please
email me this question. I should receive it by 11:59 pm on Sunday,
December 5. I will send you a responding email confirming that I got
your question. If you don’t get a response within 24 hours, you should
try sending your email again. (If all of this emailing sounds too
cumbersome, you can just put the question in my mailbox, but you
won’t get any confirmation if you do that. The same due dates apply.)
Your question should NOT be copied from the text or the
Astronomy Place website.
More (but general) Extra Credit! 5 pts.
You are planning on going online to do the course evaluation,
right? (I’m really looking forward to hearing feedback from you!)
Remember, those need to be completed by December 10. Well, while
you’re online, you might as well complete the extra credit assignment on
the astronomy place web site – the "Detecting Dark Matter in Spiral
Galaxies" tutorial and exercises. It is due the last day of classes,
December 10. Remember that you must join the online class to get
credit.
Exam Averages
Exam one:
203 – 64.453
Exam two:
203 – 72.389
204 – 60.973
204 – 69.875
One multiple choice question actually had two correct answers! The
answers to the question "What happens when a star exhausts its core
hydrogen supply?" are: "Its core contracts, but its outer layers expand
and the star becomes bigger and brighter," which is true for LOW-mass
stars, and "Its core contracts, but its outer layers expand and the star
becomes bigger but cooler and therefore remains at the same
brightness," which is true (approximately) for HIGH-mass stars. The
following people may add 2 points to their exam scores:
Megan Bond
Kristin Buda
Allison Deluca
Richard Dulsky
Thomas McCormick
Morgan Rock
James Sanders
Andrew Wehrli
All necessary modifications have been made in my grading book.
Question One: Name two ways of finding a distance to Cepheid
variable that is fairly close (closer than 50 parsecs).
3 Possible Answers:
1. Stellar parallax (aka heliocentric parallax) (d=1/p)
2. Distance-Luminosity Relation of Cepheid variables
3. Hubble’s Law
Question Two: Name the three ways of measuring dark matter in
galaxies.
Answer:
1. Use galaxy orbits (ex. rotation curve in spirals)
2. Observe the gravitational lensing predicted by Einstein
3. Examine the temperature of the hot gas in clusters
Dark Matter in Spiral and Elliptical Galaxies
A rotation curve is a graph of (linear) rotational velocity vs. distance
from a central point. On Nov. 17, we talked about rotation curves,
although we didn’t call them by name, and how they indicated that most
of the mass in our galaxy is in the halo. Remember, 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.) Well, this is the same thing as saying that the rotation curve
remains fairly flat at large distances from the galactic center.
We then made the claim that most of the mass in the halo is dark matter
because we do not detect it emitting any light.
It turns out that most spiral galaxies have flat rotation curves, indicating
that there is a lot of mass in the halo – and most spiral galaxies have a lot
of dark matter (about 90% of the total mass of the galaxy.) We know
that these spirals have dark matter because there aren’t enough lightemitting stars to account for all of the mass that must be there to make
the stars orbit about the galactic center the way they do. (Remember, we
can calculate the amount of mass in a galaxy by using Newton’s version
of Kepler’s third law.)
If we are looking at an elliptical galaxy, we can only measure the
velocities of the inner stars by looking at Doppler shifts. Can someone
remind us of how Doppler shifted lines indicate how the light source is
moving?
Remember, the orbits in an elliptical galaxy are fairly random, so even if
we focus on one area there will be a range of Doppler shifted lines from
the stars that are moving towards and away from us at different speeds.
These all combine (they sort of add together) to make a broadened line.
The broader the spectral line, the faster the stars are moving.
Quasars
Quasars are a very bright active galactic nuclei – an active galactic
nucleus is an unusually luminous galactic center. Most active galactic
nuclei are found at large distances from us, with relatively few nearby.
What does this imply? Well, remember that at large distances we are
seeing into the past. Since active galactic nuclei are far away, they are
young. The galaxies we see nearby are older. Therefore, activity is more
common among young galaxies than older ones.
Current theory holds that active galactic nuclei are powered by
supermassive black holes. How is the energy produced? The
gravitational potential energy of matter that is falling toward the
supermassive black hole (we say that this matter is being accreted onto
the black hole) is converted into kinetic energy, which in turn is
converted into thermal energy.