The Milky Way Galaxy

Learning Objectives: (Total points = 80)

• To understand the overall structure of our own galaxy, and how we determine it.

Part I: Viewing Our Sky (20 min) (40 points)

The first panel above is an image of the face-on spiral galaxy M74. SUPPOSE you lived on a planet going around a star, as shown in the second panel (NOT TO SCALE).

Now SUPPOSE that your solar system is located at one of the positions (A, B, or C) on the image. In the following questions, describe what the night sky (say, at midnight, when your side of the planet is facing away from your star) would look like during the course of a year. In all cases, assume that the orbit of your planet lies in the same plane as the galaxy. Remember, as your planet orbits around its sun (see right-hand figure), the objects you see in the night sky would change. Also remember that the galaxy is flat (like a plate), with a spherical (ball-shaped) bulge of stars in the center.

(Start by assuming that your planet is on the side of your star that is away from the center of the galaxy. At midnight, you can see the part of the sky away from your sun. Would the stars you see be evenly distributed in the sky, or would there be a concentration of some kind? If so, would the area with lots of stars be circular, a band, or some other shape? Will there be many or few bright, nearby stars? How will the night sky change when your planet is on the opposite side of its sun?)

EXAMPLE: Sky seen from Position A during the course of a year.

At midnight, you will be able to see the part of the sky away from the Sun. So when the Star is between your planet and position C (the galaxy's center), you will see almost no stars in your night sky. You might see some randomly distributed halo stars and globular clusters. When your planet is between your star and the galactic center, you will see a large number of stars, mostly in a faint band across the sky (although you may see halo stars and clusters that don't lie in this band). This band may contain dark dust lanes. You may or may not (due to the dust) be able to identify the bulge of stars at the center of the galaxy. There will be very few bright, nearby stars in your sky at any time.

1. Sky seen from Position B during the course of a year.

   When your Star is between your planet and position C, you will see a faint band of stars in your sky, due to the spiral arm which is outside of your star's position in the galaxy. When your star and planet have reversed positions, you will see many more stars in a band along the sky, but the band may be somewhat brighter, if there is not too much dust. If there is a lot of dust in the disk of this galaxy, the difference in brightness may not be very great. Since you are inside the disk (and not at the outer edge), you will probably see some bright, nearby stars at all times of the year (more or less randomly distributed).

2. Sky seen from Position C during the course of a year.

   During the year, you will always see lots of stars in ALL directions (not just in a band). You may still be able to pick out a band which contains slightly more stars (depending on how much dust there is), but it will be much less obvious. There will be more bright, nearby stars than in the previous two cases.

   Compare your results with the actual appearance of the sky
   (This is a composite map of the total sky in ALL directions)
3. Are stars evenly distributed around the night sky?

   No. Many of the stars lie in a band of light. We see groups and clusters of stars.

4. Do you think that we are at the center of the galaxy (analogous to C), way out to one side (A), or partway off to one side (B)? Why?

   We are not at the center, because we don't see stars randomly distributed in all directions. We are most likely partway off to one side (B), because 1) we see nearby bright stars year-round and 2) there is not a time of year when we do not see the band of stars from the disk of our galaxy.

Part II: Our Distance from the Center of the Galaxy (15 min) (40 points)

5. SUPPOSE that the observed distribution of globular star clusters looked like that in the panel below. In this case, the globular clusters are distributed in three dimensions, not two (ie in a ball). Where WOULD THESE clusters appear in the night sky? Diagram is NOT TO SCALE. (HINT: Ignore what you know about our night sky, and focus on what the image is telling you. Would the clusters appear randomly in all directions, or be concentrated in one section of the sky?)

   

   These clusters would appear randomly in all directions in the night sky.

6. Where WOULD you conclude the Sun was located in our Galaxy? Why? (HINT: Again, IGNORE what you know about our Sun's location in our Galaxy. This is a hypothetical data set. Focus on what this picture tells you, not what you think the right answer is. This is where you should be using the information given at the beginning of Part II).

   I would conclude that the Sun was located at the center of our Galaxy, because the globular clusters are expected to be gravitationally bound to the center of the galaxy, and therefore distributed uniformly about the center.

7. SUPPOSE that the globular cluster distribution looked like the picture below. Again, they are distributed more or less in a ball (in this case a slightly flattened one). Would the Sun be located at the center of the Galaxy? Why or why not? Diagram is NOT TO SCALE.
   

   No, the Sun would not be located at the center of the Galaxy, because the globular clusters would be distributed uniformly about the Galaxy's center, and in our night sky the globular clusters would appear to be concentrated in a particular direction.

8. Mark where you think the galactic center would be in the above picture.

   It would be more or less in the middle of the distribution of globular clusters.

9. If I gave you a table of the locations of globular clusters, you would notice something odd. Even though the globular clusters seem to be centered around the galactic center, you would find that clusters located near the galactic plane and in the direction of the galactic center are poorly represented. Why are few clusters seen in this direction? (Hint: The answer is NOT that there are no globular clusters in this direction, but rather that few are seen. WHY are they hard to see?).

   Few clusters are seen in this direction because the dust in the plane of the disk blocks, dims, and reddens the light from the globular clusters.