Homework 10, the last one...
Please answer at least 4 of the following 5 questions correctly for full
credit:
Question 1
By spectral classification: What kind of stars are found in the galactic
halo? O and B stars or K and M stars?
Answer
The halo contains old star: K and M, red stars with long lifetimes.
Question 2
Which spectral classes of stars give a spiral's galactic disk its bluish
hue?
Answer
The O and B stars. They are much fewer than the K and M stars for example,
but they are so much more luminous that they outshine the more numerous K and
M stars - at least for as long as they last...
Question 3
Which two hydrogen lines are used to map the matter distribution in the
galactic disk?
Answer
H-I (21cm radiation from atomic Hydrogen) and H-II (ionized Hydrogen)
measurements provide a map of where the Hydrogen is in the disk. See
lecture 31 (April 7, pages 5,6) for maps that were reconstructed using redshift
expectations from model calculations to match against the redshift information
in the measurements.
Question 4
What do we think is making the spiral arms in a spiral galaxy and
why can the orbits of the stars in the disk of a spiral not be responsible
for the beautiful spiral arms?
Answer
Pressure waves are thought to be responsible for the spiral arms.
From measured rotation curves of spiral galaxies we know that their
differential rotation would erase the spiral structure in relatively short
time.
Question 5 (the conceptual one)
How do we know how much dark matter is hidden inside a galaxy?
Answer
From the rotation curves. The speed of stars on their (circular) orbits
around the galactic center measures the mass inside of this orbit.
The deadline for submitting solutions to this homework has passed.
Last modified: Fri Apr 18 07:39:20 MDT 2008