M74 is a remarkably symmetrical grand design spiral galaxy of type Sc. It is
seen nearly face-on at a distance of about 10.6 million parsecs (10.6
megaparsecs or 35 million light years).
Center on the dark-subtracted image and adjust
the gray scale so you can
see a full range of its features. The only exposure available is a brief 100
second
test exposure in which the spiral arms appear only very faintly. You will have
to carefully adjust the Scale -> Parameters to display
the faintest parts of the image, and the Color -> Colormap Parameters
to get high contrast with minimum bias. When you have this just right, the
spiral arms will appear distinctly wrapping around the bright central nucleus.
Clearly the nuclear region of the
galaxy is far more luminous than the faint outer spiral arms.
What is the diameter out to the outer spiral arms in seconds of arc? For this you will need the calibration of the pixel scale to angular scale that you did earlier.
Now that you know the angle and the distance to the galaxy, what is its diameter in parsecs? Compare this to the Milky Way, where the Sun is about 8000 parsecs from the center, and the diameter including the brighter spiral arms is roughly 30000 parsecs.