ISP 205, Section 3, Spring 1997

Homework 9

Due: Thursday, 10 April 1997

                                        Name: ________________________


                                        Student Number: ______________
1. (5 pts) Structure of the Milky Way Galaxy.
Sketch the Milky Way twice, showing both a top and side view. Label the Sun. Also, indicate on both views where you would find open clusters, globular clusters, two locations for old, red stars and young, blue stars, and dust clouds.
Edge-on View of the Milky Way

Add the open clusters to this diagram. They are in the disk, both in the spiral arms and in the inter-arm region.

For a face-on diagram, see Figure 20-8 in your book.

2. (4 pts) History of Observations of the Milky Way
What component of the Milky Way's disk, unknown in the 1920's, erroneously led astronomers like Kapteyn to conclude that the Sun was the center of the Galaxy (Hint: Read Section 3.5)?

Clouds of gas and dust.
The darkest and coldest clouds block starlight altogether and make it seem like
there are "rifts" where there are no stars.

3. (3 pts) Mass of the Galaxy
Which information do you need to calculate the mass of the Milky Way?
(a) The time it takes the Sun to orbit once around the center of the Galaxy (one Galactic Year).
 (b) The mass of the Sun.
(c) The distance to the nearest star from the Sun.
(d) The distance to the nearest Giant Spiral galaxy, Andromeda.
(e) All of these.

This is another example of how Kepler's 3rd Law is used to find the mass of an astronomical body.

4. (3 pts) Normal Galaxies
Which type of galaxy has a stellar component most like the Nuclear Bulge of the Milky Way?
(a) An elliptical galaxy.
(b) An irregular galaxy.
(c) The disk part of a spiral galaxy.

5. (4 pts) Peculiar Galaxies
What is the primary evidence for there being a super-massive black hole in the center of an Active Galactic Nucleii?

The high velocities of objects in a tiny space in the center of such nucleii. Yet another example of Kepler's 3rd Law, because high velocity of an orbiting object is the signature of a high central mass.

The phenomena observed as a result of these velocities are fast-moving clouds of gas and synchrotron radiation. The synchrotron radiation, given off by fast-moving electrons, has a flat radio spectrum very unlike stellar spectra.

6. (4 pts) Galaxies Far, Far Away
On the Internet, go to the URL http://www.ifa.hawaii.edu/~cowie/tts/tts.html
This is the DEEP image from the Hubble Space Telescope, showing what was previously thought to be a blank part of the sky. When you click on an object in this image, details about the object will be accessed. Record the object name and redshift for one example of each type of object.

           Type of object          DEEP Object No.   Redshift
 
          Star                          1              -2
          Elliptical Galaxy             5              1.006
          Galaxy like the Milky Way     3              0.517
          Interacting Galaxy           61              2.300

7. (4 pts) Galaxies a Long Time Ago, Chapter I
Explain the pattern of redshifts you found in problem 6 above.

Your answer here depends on the particular galaxies you selected. In other words, with such a tiny sample, chances are everyone will get a different result. There are many more possible answers than the sample given above.
However, stars are always seen only nearby, within the Milky Way. They could be coming toward us (blueshifted) or be moving away at low velocities (small redshifts).
In the particular sample above, the elliptical galaxy and the spiral galaxy are closer than the interacting galaxy. This is consistent with the idea that in the distant and long-ago Universe, the higher density of matter resulted in more interacting or even merging galaxies.

8. (4 pts) Galaxies a Long Time Ago, Chapter II
An explanation for why there are more elliptical galaxies in the current Universe than in the early Universe is

If there were more interacting galaxies in the past than now, what we should see in the local Universe is their remants, the product of galactic mergers. These could be the elliptical galaxies.
or
any other reasonable explanation you thought of.

Visions of the Universe
Beth Hufnagel's home page, email: hufnage4@pilot.msu.edu
Bob Stein's home page , email: steinr@pilot.msu.edu