None of the above. None of the above. Today, we believe that only a small number of elements were actually formed during the Big Bang. Open clusters of stars are found primarily within the galactic disk. Cepheid variables are important to us in calculating distances to galaxies the compositions of stars.
Interstellar dust blocks visible light.
The redshift of distant stars makes them difficult to detect. The opacity of the atmosphere blocks most types of radiation. Relative motion of the spiral arms makes them difficult to detect. According to the density wave theory of the spiral arms, there is about the same number density of stars between the spiral arms as in them. The look-back time of an object is how long light from the object takes to reach Earth numerically equal to the distance to the object in light years.