FORM D 1. Slide 6 shows galaxies that are billions of parsecs away.
Astronomers have difficulty studying the structure of the Milky Way Galaxy.
A G main sequence star, an M main-sequence star, an RR Lyrae star, and a Cepheid variable star all have the same apparent magnitude. The spiral arms of the galaxy M51 appear blue, but its central bulge is red. Consider a quasar several billion parsecs away. All the interstellar gas is lost from both galaxies. A spiral galaxy is often formed.