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Astronomy 115
Homework 6
Due May 13, 2012 at 5 p.m., either electronically or on paper.
1. Some basic galaxy information: What is a galaxy, and what holds it together? How
many stars does it contain (on average), and how many of them are there in the
universe? And (you may have to look this up outside the textbook), what structures
larger than galaxies exist in the universe? No citation needed for the last question.
2. Is the Hubble “tuning fork” model of galaxy classification also a diagram that
shows how galaxies evolve over time? Find evidence (you will have to research this
outside of the textbook) for the text’s intriguing statement on page 160: “Galaxies do not
start as ellipticals and then progress to be spirals, though there is some evidence that
the reverse is true.” For this question, I do want a full citation.
3. What is an active galaxy, or, more accurately, an active galactic nucleus?
Though the text lists many different kinds of active galactic nuclei, what is the power
source common to all of these kinds?
4. On page 179, the text recounts how the astronomer Maarten Schmidt discovered that
quasars have “large redshift” of their hydrogen lines. What does the term “redshift”
mean, and what does this tell us that quasars are doing? (Edwin Hubble already
suggested that quasars were quite far away).
5. Two useful websites for open star clusters: The Astronomical League’s Guide for
the Open Star Cluster Observing Club (scroll to the bottom for the link to the manual) at and the Dark
Horse Observatory site at
First, which type of open cluster (poor, medium or rich) would be best for our term
project? Think of practicalities like being able to distinguish individual stars and
counting. Second, how many minutes should we leave the camera shutter open
(exposure time) in order to get a good image of the cluster? The Dark Horse Observatory
site lists its camera exposure times (note that they take multiple images of the same