Distance to the Center of the Galaxy: Globular Cluster NGC 6528
First left-click with the mouse on the right-hand side of the cluster (and noting the value of the first number given) and then on the left-hand side (noting the value of that first number) so that you will get a "horizontal" measurement first. The script will return the X (the first number given, just like when specifying coordinates on a graph) and Y pixel numbers of each click for each frame. Record the DIFFERENCE in the X values in Table 1 (see example). The next measurement will be one of the "vertical" diameter. Left-click at the bottom of the cluster (noting the second number given--the Y pixel number) and then at the top of the cluster (again, noting the second number for the Ypixel number). Record the DIFFERENCE in the Y values in Table 1 (see example).
(The X and Y values will change slightly when we are measuring the other, but the effect is so small that we will ignore it.)
The pixel numbers are displayed to the right of the image.
[Image width is 15 arc minutes; 300 pixels. So 300 pixels = 15 arc min
Scale factor: 1 pixel = 0.05 arc minutes (0.05 arc min per pixel.)]
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Other ways to do the same thing:
- Using a ruler to measure from the screen. Determine the width of the IMAGE in millimeters, and calculate the scale factor (just like reading a map). For example, if the width of the image from your screeen is 90 mm, we know that 90 mm = 15 arc minutes. So, 1 mm = 15/90, or 1 millimeter = 6 arc minutes. (Another way of saying this is that there are 6 arc minutes per millimeter.) Then, measure the width of the cluster in millimeters and multiply that number by 6 arc seconds to get the angular size of the globular cluster in arc minutes.
- Print the image and measure from the hardcopy of the picture, following similar steps to those given just above.