The 1 solar mass star is much more luminous than the 2 solar mass star. It tells astronomers the mass of the biggest star that can form in that cluster. It tells astronomers the age of a star cluster. It tells astronomers which stars will explode as supernovae. It tells astronomers what the minimum mass star is in the cluster.
Most stars are too faint to see without a telescope.
Parallax requires a spectrum which only became possible in the 1830s. The Hertzsprung-Russell diagram was not discovered until this time. An HII region is a cloud of ionized hydrogen around a hot star. Star formation is too rare and is not occurring anywhere near us. Protostars do not give off any light until they are formed.