r
/3)=NElmegreen, Ch. 12: 3, 5, 7
Elmegreen, Ch. 11: 1, 2*, 4
Elmegreen, Ch. 12: 1
*On problem 11.2, there are typos in both the problem and the formula given in the text; be sure to correct these from the errata page.
Homework #6 - Due Friday, 4/9
Elmegreen, Ch. 7: 2, 3
Elmegreen, Ch. 9: 7*, 8
*use Figure 9.11 for this problem, and comment on the answer.
Be sure to make the correction to the exponent of 10 in problem 7.2. It should be 2, not 5. Almost all of you got burned on the previous homework by not correcting the text according to the errata sheet. In the problem calculating the K correction for the elliptical galaxy the uncorrected entry in Table 4.2 gave a negative answer which is impossible.
A. Elmegreen Ch. 4: 1, 2*, 4, 5**
*For problem 2, please write on a sheet of paper (i.e., I don't want a photocopy of what you wrote in the table in the book). I recommend using NED, the National Extragalactic Database. Click on "Catalogs" and you will see a web form. Click "RC3" and unclick "UGC" to get information from only the Third Reference Catalog of Bright Galaxies. The headings are given in successive rows before the entries begin. Click here for a description of the headings and catalog entries.
Also for problem 2, when you calculate distances using the Hubble Law and the radial velocity given in the catalog, use 75 km/sec/Mpc for the Hubble Constant.
To calculate the surface brightness, use the equation on page 82 in the text, remembering that it's -0.26, not +0.26. You can use either B(T) or B(T)0 in your calculation of surface brightness, but be consistent, and include a sentence about why you think the one you chose is more valid or meaningful than the other one.
** For problem 5, the numerical Hubble Type, T, for an E0 galaxy is -5 and for an Sc galaxy it's 5.
B. Elmegreen Ch. 5: 5, 6
Homework #4 - Due Friday, 3/5
A. Elmegreen Ch. 6:1, 2, 3, 6, 7
B. Consider a pure hydrogen cloud with unform density, surrounding a hot star. This star emits N ultraviolet photons per second beyond the Lyman limit, and thus capable of ionizing hydrogen fron the ground state. Assume that each photon ionizies one and only one hydrogen atom.
Let R be the number of recombination per second per volume per unit time. In a steady state, the number of recombinations will equal the number of ionizations inside the resulting spherical ionized region, called a Stromgren sphere, whose radius is r:
r/3)=NThe recombination rate R involves a two-body process: the two
bodies are the electron and the proton. The rate must be proportional to
the product of their number densities
(in cm
), n
and n
. Overall charge neutrality requires
that n = n. So we have the total recombination
rate per second per unit volume:
n
is the
"recombination coefficient," representing the constant of
proportionality in the recombination equation.
a. Now solve for the Stromgren radius, r.
For temperatures characteristic of H II regions,
b. Compute r when N = 3 x 10
e. Convert these answers to light-years and
parsecs. What kinds of main-sequence stars create significant H II
regions around them?
Homework #3 - Due Tuesday, 2/23
1. Write one paragraph each summarizing the
post-main-sequence evolution of a 1 solar-mass single star and a 20
solar-mass single star, respectively.
2. Draw and label a schematic HR Diagram for a
typical open cluster and a typical globular cluster. Describe the
galactic distribution of these clusters and the brightest stars in
each.
3. Using the Mass-Luminosity relation for
main sequence stars, calculate the relative luminosity (in units of
solar luminosity) of the following kinds of stars in a hypothetical
open star cluster containing 5 20-solar-mass O stars, 75 1-solar-mass G
stars, and 200 0.5-solar-mass M stars. Comment.
Homework #2 - due SUNDAY, FEBRUARY 14 via
email: is approximately
3 x 10
cm sec. Assume n = 10 cm.
sec (an O5 V star).
c. Compute r when N = 4 x 10
sec (a BO V star).
d. Compute r when N = 1 x 10 sec (a G2 V star).
1. Go to the Princeton Catalog and classify the galaxies in the following list, according to Hubble type and then, if possible, according to de Vaucouleurs type. Click on the thumbnail image of each galaxy to get a selection of images. Use the "color JPEG" option (the last one) for your classifications. E-mail me your answers by 5 PM Sunday, 2/14 so I can tabulate them for class on Tuesday.
Due Tuesday, 2/16 2.
3. From your Astronomy 111 or other text (or p. 221 in Elmegreen), write down the expression for the Jeans density, the minimum density an interstellar cloud of a given radius and mass must achieve in order to be unstable against gravitational collapse. (You may find the Jeans mass or Jeans length; it is straightforward to convert to Jeans density since density = mass/volume.) All other things being equal, how does the value of the Jeans density depend on the mass involved? How does this behavior explain the formation of stars in clusters?
Homework #1- due Tuesday, 2/9
To get a handle on the motivation and practice of galaxy classification, read pages 1-25 of Sandage's Hubble Atlas of Galaxies and pages 1-58 of Sandage and Bedke's Carnegie Atlas of Galaxies. Both are in the reading room of the Physics & Astronomy Library.
As you think about the Hubble Tuning Fork Diagram, here's a "real" one
from the Princeton on-line
catalog:
.
Also, the following derivation a la Herschel, where
r is the distance, and B is the observed brightness.
For uniformly distributed stars, all of luminosity L, knowing the
inverse square law:
.