Interstellar gas is composed mainly of A) some hydrogen, but ...

[Jipu 123]

Interstellar gas is composed mainly of
 
  A) some hydrogen, but mainly carbon dioxide.
  B) 10 hydrogen, 90 helium by numbers of atoms.
  C) 90 hydrogen, 9 helium by weight.
  D) ammonia, methane, and water vapor.
  E) only hydrogen.

Question 2

Why did Hubble have to rely on Cepheids to find the distance to M31, unlike Shapley's use of RR Lyrae stars with the Milky Way?
 
  What will be an ideal response?

[blfontai]

Answer to Question 1

C

Answer to Question 2

The RR Lyrae stars were not bright enough to be seen from the distance of the Andromeda Galaxy, so Hubble used longer-period Cepheids in the spiral arms which were also much more luminous and showed up on his photos.

[Jipu 123]

TYSM

[billybob123]

Excellent