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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.
Pre-Activity
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
- 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.
- The GSI will dim the lights and project the night sky. S/he
tell you the visible range of right ascension. Locate your objects.
- 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.
- 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?
- 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:
- 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?
- 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
- 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.
- Can you ever see the Galactic Center and Galactic
anticenter simultaneously in the sky? Explain.
- 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.
- 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.
- 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: ___________
- Assuming that the Sun is located 25,000 LY from the Galactic
Center, find the true distance x in LY (see figure below) between
the stars in Baade's Window and the Galactic Center. Use the
figure when solving this problem, noting that
α is the angular distance you just measured. This is one of the few regions of accessible bulge stars.

Updated:
08/28/14
by SAM & MSO
Copyright Regents of the University of Michigan.