University of Michigan - Department of Astronomy




Version: Worksheet

The Structure of the Milky Way Galaxy

Be sure to print out the Table and the star maps and bring them to the planetarium. In this Activity, you will locate the Galactic Center on the sky, based on the assumption that the globular cluster orbits are centered on the Galactic Center.


Mark the first 2 globular clusters from each letter group in the table on your sky map with an "x" and circle the constellation name. You can also circle nearby constellation names to help you locate this position later, in the planetarium. 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.

Part 1: Finding the Galactic Center

  1. Your GSI will divide you into groups of 4 - 5, and give each group a red flashlight(s) and laser pointer. Each group will be assigned some globular clusters to identify on the sky for the class. Please distribute this responsibility among your group members equitably.
  2. The GSI will dim the lights and project the night sky. S/he tell you the visible range of right ascension. Locate your objects.
  3. The GSI will ask everyone to point out their clusters on the sky. As they are pointed out, add the positions of the other groups' clusters to your own sky chart.
  4. The positions of the globular clusters allow you to estimate the location of the Galactic Center. Carefully mark your estimated location on your star chart. In which constellation is the Galactic Center located?

  5. 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?

    Are they in the same constellation?

    The GSI will give you the equatorial coordinates of Sgr A*. Write them here:

  6. The GSI will point out the position (l, b) = (90°, 0°). If you connect this point with the Galactic Center along the shortest distance, does it trace the Milky Way plane?

  7. We now have poles for each coordinate system. The GSI will point out the North Galactic Pole, the North Ecliptic Pole, and the North Celestial Pole. Mark these on your sky charts. The GSI will bring the North Galactic Pole to the zenith. How does the Milky Way appear in the sky?

Concluding Questions

  1. Carefully mark the true location of the Galactic Center on your star chart, using the position given by your GSI. How does it compare to your estimated position from Question 4 above? Give at least 2 reasons 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. Which is most likely to be found at the position (l, b) = (0, 70): a globular cluster or a massive star, or both equally? Explain.

  4. 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.

  5. Using the position of Baade's Window given in the Introduction, mark its position on your star chart and label it. Measure the angular distance between Baade's Window and the Galactic Center. (Remember that declination is measured in degrees of arc, so you can use the declination scale as a ruler to measure the angular distance, as you do on the planetarium sky.)

    Write your answer here: ___________

Updated: 08/28/14 by SAM & MSO

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