Stars are theoretically born in clouds of hydrogen gas and dust in the space between other stars. Gravity pulls huge masses of hydrogen gas together into a protostar, a mass of gases that will become a star. The protostar contracts, becoming increasingly hotter at the center, eventually reaching a temperature high enough to start nuclear fusion reactions between hydrogen atoms. Pressure from hot gases balances the gravitational contraction, and the average newborn star will shine quietly for billions of years. The average star has a dense, hot core where nuclear fusion releases radiation, a less dense radiation zone where radiation moves outward, and a thin convection zone that is heated by the radiation at the bottom, then moves to the surface to emit light to space.

The brightness of a star is related to the amount of energy and light it is producing, the size of the star, and the distance to the star. The apparent magnitude is the brightness of a star as it appears to you. To compensate for differences in brightness due to distance, astronomers calculate the brightness that stars would have at a standard distance. This standard-distance brightness is called the absolute magnitude.

Stars appear to have different colors because they have different surface temperatures. A graph of temperature by spectral types and brightness by absolute magnitude is called the Hertzsprung-Russell diagram, or H-R diagram for short. Such a graph shows that normal, mature stars fall on a narrow band called the main sequence of stars. Where a star falls on the main sequence is determined by its brightness and temperature, which in turn are determined by the mass of the star. Other groups of stars on the H-R diagram have different sets of properties that are determined by where they are in their evolution.

The life of a star consists of several stages, the longest of which is the main sequence stage after a relatively short time as a protostar. After using up the hydrogen in the core, a star with an average mass expands to a red giant, then blows off the outer shell to become a white dwarfstar, which slowly cools to a black dwarf. The blown-off outer shell forms a planetary nebula, which disperses over time to become the gas and dust of interstellar space. More massive stars collapse into neutronstars or black holes after a violent supernova explosion.

Galaxies are the basic units of the universe. The Milky Way galaxy has three distinct parts: (1) the galactic nucleus, (2) a rotating galacticdisk, and (3) a galactic halo. The galactic disk contains subgroups of stars that move together as galactic clusters. The halo contains symmetrical and tightly packed clusters of millions of stars called globular clusters.

All the billions of galaxies can be classified into groups of four structures: elliptical, spiral, barred, and irregular. Evidence from four different astronomical and physical "clocks" indicates that the galaxies formed some 13 billion years ago, expanding ever since from a common origin in a big bang. The big bang theory describes how the universe began by expanding.