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Doing Science

Unit One: Investigating Motions of the Sky
Doing Science

1. The Distribution of Stars in the Night Sky

The sun, moon, and planets are close neighbors, parts of the same system as our Earth. All of the planets and even most of the smaller bodes in the solar system are distributed in a flat plane that passes though the sun. But the stars are much, much farther away, and not related to our solar system at all. How are they distributed in space? The bright stars of the constellations appear more or less evenly scattered across the sky. But how accurate is that perception? Can we test it by closer observation?

One way to investigate the distribution of stars in the sky is to count stars in 30 or 40 random places in the sky (it's a big sky!). The amount of work can be reduced by carefully choosing the areas to do the counts. Observers at relatively dark locations usually notice the Milky Way, a band of light that cuts across the night sky at certain times of the year. (An image of the Milky Way across the winter sky—plus a few meteors—can be found at:

The Milky Way is a natural feature that divides the sky into roughly symmetric halves. If the symmetry is reflected in the distribution of the stars, then logic suggests that we pick one counting area in the Milky Way, and one as far away from it as we can get. If star counts in those two areas show large differences, then additional areas can be chosen to more fully explore the distribution of stars.

You will select and download two 1-degree-square images suitable for counting stars. A good web site for obtaining the images is Palomar Sky Survey at: In the space that indicates form, you will need to specify the coordinates of the center of you patch of sky in right ascension (RA) and Declination (Dec). Start with the coordinates of the Milky Way's "north pole" at RA = 12 hours 51 minutes 24 seconds, and Dec = +27 degrees 07 minutes 00 seconds. Type in 60 for the height and width (60 minutes is one degree). Select GIF for the file format and "Save File to Disk," and then click "Retrieve Image." These images are fairly large—5 to 10 MB—and will take a minute or so to download. Alternatively, you can use the SkyView Advanced Server and enter the same information. The files are smaller, but the download time is usually longer.)

Use a star atlas to find the RA and Dec of point along the Milky Way and enter those coordinates to get your second star field. Locations in the Milky Way near Canis Major and Sagittarius are good to use.

Once you have your star fields, open them in ImageJ. To save processing time, reduce the size of your images by selecting Image, then Scale, and type "0.20" in the X Scale and Y Scale boxes. Select Interpolate, Create New Window, type in a Title, and click OK. This will generate a new image about 700 pixels square that is 1/25 the size of the original. The new image will process 25 times faster.

To do a star count, select an image by clicking on it, select Image, Adjust, and then Threshold. In the box that appears, use the slider to adjust the minimum until all the stars in the image turn red. These are now selected for counting. Now select Analyze, then Analyze Particles. In the dialog box that appears, type 2 or 3 in Minimum Size (this eliminates noise in the image from being counted � be sure to keep this number the same for all your counts!). For Show, select Outlines (not necessary but helpful), and click OK. A new image will appear with all the counted objects outlined and numbered. To see the total number of objects counted, select Analyze, and then Measure. A box will appear with the count.

Repeat for your second area.

Your Report:

  1. Prepare a hardcopy or power point report showing your two areas and star counts. Use your data in answering the following:
    Does the night sky look the same in all directions?
    What do your star counts imply about the distribution of stars in the night sky relative to the Milky Way?
    What can you then infer about the nature of the Milky Way?
  2. Repeat your star counts for several other 1-degree areas. Select two or three additional areas along the center of the Milky Way, and three or four areas distributed uniformly between the Milky Way and its "north pole." Graph your counts as a function of angle from the center of the Milky Way and reconsider the questions in #1. Study the whole sky photograph at How do your results compare with this image?