Galaxies are massive collections of stars, gas, and dust.
Matter and energy are exchanged between these various components in a grand cycle that changes the chemical and physical properties of galaxies. Stars form from the interstellar material, synthesize chemical elements in their interiors, and return their products to the interstellar gas during their lives and in their death throes. All naturally occurring elements heavier than lithium are produced in stars. The carbon atoms that form the basis of life, the oxygen we breathe, and the silicon in the sand on our beaches were all formed deep inside of stars. The calcium in our teeth, the copper in our coins, and the iron in the steel frames of our cars were formed in massive stellar explosions called supernovae that occur as stars exhaust their nuclear fuel, explode, and reseed the interstellar gas for a new generation of stars.