Physics PHYS 102

General Astronomy


Problem Set #7 - Due: Tuesday, April 18th


After you finish this problem set, you should be familiar with:

Spiral galaxies

Barred-spiral galaxies



Elliptical galaxies

Irregular galaxies

Milky Way

Active Galaxies



21 cm radiation

Seyfert Falaxies


Starburst Galaxies

dark matter

BL Lacertae objects


density waves

spiral arms

Please answer the following questions using sentences and paragraphs.  When it is helpful, please use diagrams and equations.

  1.  What is a galaxy? Briefly describe the geometry of the major types of galaxies. Describe the distributions of stars, gas, and dust in each type. Draw a diagram of a spiral galaxy, labeling the important regions.
  2. The Milky Way is our home galaxy. Why do we see the Milky Way as a band of faint light around the sky? 
  3. Describe the Shapley-Curtis debate. Explain how Edwin Hubble proved that M31, the Andromeda "Nebula", is not a nebula at all. What did Hubble prove about M31?
  4. Describe the nucleus of the Milky Way galaxy.  Is there any evidence that might indicate the presence of a supermassive black hole at the core of our galaxy?
  5. Look at this simulation of spiral arm formation.  From what you know about stellar evolution, the interstellar medium, and density wave theory, as well as what you just read, explain the formation and appearance of the spiral arms of the Milky Way.  To read more about the simulation, check out this article from de l’Observatoire à Paris.  For even more, you can read the preprint of the paper describing this research.
  6. What is "dark matter"?  Why do astronomers believe it exists?
  7.  Look at these movies showing a collision between two galaxies and a collision between many galaxies.  Describe what happens when two galaxies "collide".  Look at this movie showing one large galaxy "eating" a smaller one.  What is galactic "cannibalism"?  To see more modern galatic dynamics simulations, check out the work of John Dubinski.  Note that these movie files are BIG so they take time to download.


Here are some practice "figure it out rather than memorize" type questions.  They will not be graded as part of the homework (so don't turn them in), but they may be useful as you prepare for the test.  More questions like these can be found in the test practice folder at the library.


A Cepheid variable star with a pulsation period of a few days is seen in the spiral arm of a galaxy. Its apparent brightness is measured as 104 times fainter than an equivalent star 1000 ly away from the Sun in our Galaxy. Assuming no light absorption between galaxies, what is the distance to the far Cepheid and hence to the galaxy?



107 ly (104 times farther away)



10,000 ly (10 times farther away)



10 ly (100 times closer)



100,000 ly (100 times farther away)



On the basis of the distance to the Coma cluster of galaxies and the Hubble relation (using an intermediate value of the Hubble constant H0 equal to 80 km/s per million parsecs; see Fig. 16-20, Comins and Kaufmann, Discovering the Universe, 7th Ed.), what would be the approximate wavelength shift of the Balmer H spectral line at 656.3 nm emitted by a galaxy in the cluster because of the general expansion of the universe? (See also Astronomer's Toolbox 16-1 of Comins and Kaufmann, Discovering the Universe, 7th Ed.)



1.61 nm



161.0 nm



1.61 × 106 nm



16.1 nm



One astronomer (astronomer A) claims that the Hubble constant is 84 km/s per Mpc, while another (astronomer B) claims that it is 63 km/s per Mpc. If, based on the Hubble constant, astronomer A claims that a particular galaxy is 4 billion ly away, then astronomer B would claim that it is



3 billion ly away.



5.3 billion ly away.



6 billion ly away.



2 billion ly away.