This document discusses key information about stars including: - Stars are massive luminous balls of plasma that form within molecular clouds in interstellar space and undergo nuclear fusion at their cores to produce energy. - Characteristics like a star's mass, age, composition and evolution can be determined by observing its spectrum, luminosity and motion. - Stars vary greatly in size, from neutron stars only 20-40 km wide to supergiants like Betelgeuse over 600 times the diameter of the Sun. They can exist as single stars, binary systems, and larger groupings in star clusters and galaxies. - The star with the closest proximity to Earth's solar system is Proxima Centauri at 4.2