University of Michigan - Department of Astronomy




Version: Worksheet long

Stellar Populations and the Milky Way Halo

Part 1: Before Class

In this Activity, you will locate the Galactic Center, based on the assumption that the globular cluster orbits are centered on the Galactic Center.

Before class, map the globular clusters from the Table onto the star maps. Use the equatorial maps for objects with declinations between -60º and +60º, and the polar maps for the few clusters that are outside that range. Using a colored pen may help you find the clusters later.

Part 2: Globular Clusters in the Halo

  1. Your GSI will break you up into 5 groups, and give each group a laser pointer. Each group will be assigned 10 globular clusters. Circle your clusters on your star chart. Figure out how you will identify it on the sky.
  2. When prompted by your GSI, point out your clusters on the sky.
  3. In which constellation is the Galactic Center located? Explain your answer in terms of the location of the globular clusters. Carefully mark your estimated location on your star chart.

  4. Your GSI will point out the exact location of Sgr A* and Baade's window on the sky. Are they in the same general direction of the sky?

  5. Are these in the constellation you identified as the Galactic Center? If not, in which constellations are they located?

Part 3: The Milky Way on the Sky

Your GSI will give each group a celestial sphere. The Milky Way is marked on the outer globe as a slightly opaque band.

  1. Your GSI will give you the coordinates of the Galactic Center. Enter them into Table 1.

  2. For each of the remaining positions in the Table, estimate the RA and Dec, identify the constellation in which it is located, and the nearest star named on the star charts. Figure 4 of the Introduction may be helpful.

  3. The GSI will prompt each group to point to one of the positions in Table 1.

  4. Use the coordinates from Table 1 to draw a line representing the Galactic equator on your star charts. Mark the Galactic north and south poles on your star charts.
Table 1: Reference points in the Galactic Coordinate system
Location RA Dec Constellation Nearest Named Star
Galactic Center        
Apex of
Antapex of
North Galactic Pole        
South Galactic Pole        

Concluding Questions

  1. Using your star chart, what is the angular distance between the point that you estimated to be the Galactic Center, and its true position? Give at least 2 reasons for why your position did not yield the exact position.

  2. Can you ever see the Galactic Center and Galactic Anticenter simultaneously in the sky? Explain.

  3. Using the position of Baade's Window given in the Introduction, mark its position on your star chart. Mark the true position of the Galactic Center on your star chart. Measure the angular distance between Baade's Window and the Galactic Center. Assuming that the Sun is located 25 LY from the Galactic Center, what is the distance of the stars in Baade's Window from the Galactic Center, in the direction perpendicular to our line of sight to the Center? These are some of the few regions of accessible bulge stars. Use the following figure when solving this problem.figure ahowing the relative locatons of the Sun and Baade's Window (not to scale)
  4. Does the ecliptic run through the Galactic Center and Galactic Anticenter? Explain.

  5. Review the positions of the North and South Galactic Poles on your star charts. The stars in these constellations most likely belong to the disk, not the halo. Explain why.

Updated: 3/17/10 by SAM & MSO

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