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?

Question

Quizplus · Accepted Answer

The size of the nuclear bulge is directly related to the tightness of the spiral arms, as spiral galaxies with tighter arms tend to have larger nuclear bulges. This parameter can be easily seen even when the spiral arms cannot be observed due to the galaxy's orientation. The overall spectral color, intrinsic brightness, and diameter of the galaxy would not provide direct information about the tightness of the spiral arms.

Spiral Galaxies Are Classified in Terms of Increasing Tightness of Their