Spiral galaxies are classified in terms of increasing tightness of their spiral arms as types Sa, Sb, and Sc. However, many spiral galaxies are aligned edge-on to Earth and their spiral arms cannot be seen. Which other parameter that is directly related to spiral-arm tightness can be easily seen edge-on and can therefore be used to provide this classification?
A) overall spectral color
B) overall intrinsic brightness of the galaxy, or its absolute magnitude
C) size of the nuclear bulge
D) overall diameter of the galaxy

Question

Quizplus · Accepted Answer

The size of the nuclear bulge is directly related to the tightness of the spiral arms in spiral galaxies. In general, galaxies with larger bulges tend to have more tightly wound spiral arms (type Sa), while those with smaller bulges have more loosely wound arms (type Sc). This characteristic can be observed even when a galaxy is viewed edge-on.

Spiral Galaxies Are Classified in Terms of Increasing Tightness of Their