Can someone help me on a few astronomy questions?

Question

Humason and Hubble discovered, what?

Main sequence stars show the relationship called, what?

You might see an absorption spectra if you looked at a spectra of, what?

Answer 1

hubble discovered that the universe was expanding, humason carried on his work.

mass-luminosity relation

an absorption nebula or anything whose light has to pass through some cooler substance on the way to the telescope.