Multiple star systems are more common than single-star systems. These physical properties change with time. Small stars may exist relatively unchanged for 12-15 billion years eventually fading into a cinder. Mass from one star is gravitationally transferred to a companion star and this star will blow this material into space one or many times. The remaining core will become a fast spinning neutron star.
Stars vary in brightness throughout their lives based primarily on their mass.
Many vary in brightness by high amounts later in life. Star Clusters Stars tend to form in loose clusters in the arms of galaxies. Clusters vary greatly in density, number and types of stars as well as age. The youngest exhibit hot new stars with some original gas gust surrounding them. Some stars form in giant spherical masses of stars.