Both mean the same thing - either the sky, or the outer space above it, depending on what the writer was talking about at the time. Sky or space are quite accurate modern substitutes for heaven. The sun is an average-size star.
Some stars are much smaller than the sun, others are millions of times bigger.
From the ejected matter, new stars will eventually be born. Constellations are merely chance alignments of stars that seem to draw an outline of something - a human, animal or an object. There are thousands more that anyone can decide for oneself on any starry night. Galaxies are very large aggregations of stars, gas, dust, and physical matter. Throughout the universe there are billions of galaxies.