3. Spiral arm
5. Globular cluster
6. Spiral Galaxy
a. The circular but relatively flat portion of the galaxy is the ______.
b. The first portion of the galaxy to form was the ________.
c. A tightly packed goup of a few hundred thousand very old stars is a _______.
d. A ________ stands out in a photo of a galaxy because it shines brightly with light from massive young stars and glowing clouds of gas and dust.
e. Our Milky Way galaxy is a _______.
f. Stars orbiting in the ________ near the galaxy's center can have orbits highly inclinded to the galactic plane.