Only type O stars are hot enough to show ionized ________ in their spectra.
Fill in the blank(s) with correct word
Question 2
For finding the distances to globular clusters, Harlow Shapley used
A) RR Lyrae variables.
B) planetary nebulae.
C) eclipsing binaries.
D) Population I classical Cepheids.
E) Type I supernovae.