What is the Hubble constant essentially a measure of?

Answer to q. 1

b

Answer to q. 2

Much of the interstellar gas is cold, and its density is very low. Because atoms in the ISM move slowly and collide only rarely, interstellar absorption lines are extremely narrow.

Answer to q. 3

c

Answer to q. 4

One way to investigate properties of the ISM is to measure its effects on the starlight that passes through it. Astronomers observe that distant stars appear fainter than they should for their distances and luminosities, an effect called interstellar extinction. Distant stars also appear redder than they should for their spectral types, an effect called interstellar reddening. Astronomers also observe interstellar absorption lines in the spectra of stars. Lines that cannot be produced in stars must be produced by atoms located between the stars and Earth.

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