What observation has prompted astronomers to ponder the possible ...

[chandani]

What observation has prompted astronomers to ponder the possible existence of dark energy?
 
  a. discovery of quasars
  b. discovery of superclusters
  c. supernovae appear fainter than expected at large redshifts
  d. uniform nature of the cosmic microwave background (CMB)

Question 2

Explain the sequence of fusion shell formation in massive stars and thus the generation of nested shells.
 
  What will be an ideal response?

[C.mcnichol98]

Answer to Question 1

c

Answer to Question 2

First, massive stars consume the hydrogen in their cores and ignite hydrogen shells. As a result, they expand into giants or supergiants. Next, their cores contract and fuse heliumfirst in the core and then in a shell, producing a carbon-oxygen core. Massive stars become hot enough to ignite carbon fusion, which produces more oxygen and neon. As soon as the carbon is exhausted in the core, the core contracts, and carbon ignites in a shell. This pattern of core ignition and shell ignition continues with fuel after fuel, and the star develops a layered structure, with a hydrogen-fusion shell above a helium-fusion shell above a carbon-fusion shell, and so on. After carbon fuses, oxygen, neon, and magnesium fuse to make silicon and sulfur, and then the silicon fuses to make iron.

[chandani]

Excellent

[cici]

Great answer, keep it coming