Download Physics 127 Descriptive Astronomy Homework #20 Key

Physics 127 Descriptive Astronomy
Homework #20 Key (Chapter 14)
Winter 2015
14-3. How did Edwin Hubble prove that the Andromeda “Nebula” is not a nebula within our Milky
Way Galaxy?
Hubble was able to detect Cepheid variable stars within that “Nebula.” Then by observing their light curves and using
the known period- luminosity relation for Cepheids, he obtained and compared the absolute magnitudes of these
Cepheids with his observed apparent magnitudes, yielding a distance for the Andromeda “Nebula” which put it far
outside of our galaxy.
14-5. Which is more likely to have a blue color, a spiral galaxy or an elliptical galaxy? Explain why.
A spiral galaxy. The blue light comes primarily from O- and B-type stars in a spiral galaxy. Elliptical galaxies contain
little gas and dust and therefore little, if any, star formation occurs within them. Therefore they contain few or no blue
O- and B-type stars.
14-6. Why are Type Ia supernovae useful for finding the distances to very remote galaxies? Can they
be used to find the distance to any galaxy you might choose? Explain your answer.
Because the peak luminosity of a Type Ia supernova is well known, as they are all nearly identical, and because they
are extremely luminous, they are superb standard candles for determining the distances of remote galaxies. They can
be used for virtually any galaxy. However, there is a downside. The appearance of a Type Ia supernova is completely
unpredictable. An observer might have to wait decades, or even centuries for a Type Ia supernova to occur in a
preselected galaxy. A wise observer takes advantage of such a supernova wherever and whenever it occurs, rather than
choosing to wait for one in any particular galaxy.
14-9. What is the difference between a cluster and a supercluster? Are both clusters and
superclusters held together by their gravity?
A cluster is a group of associated galaxies, while a supercluster is a group of associated clusters. Both are held together
by the gravity of either their member galaxies or their member clusters of galaxies.