Stars are among the best-understood celestial objects. If the light of a star is dispersed into its spectrum, the relative intensities at various wavelengths yield considerable information about the star. The surface temperature can be estimated, using the laws of thermal radiation.
If the distance of the star is known, its luminosity can be found by summing the observed intensities over all wavelengths. Its radius can then be found using the fact that the luminosity is the product of the energy emitted per unit area (which depends only on the surface temperature) and the total surface area.
If the spectrum of a star is studied under high resolution, many dark lines are seen at specific wavelengths. These lines are due to the absorption of light from deeper layers by atoms in the cooler layers above. The kinds of atoms present in the star can then be identified by comparing stellar absorption lines with those produced in the laboratory by known gases, and the temperature and pressure of the atmosphere as well as the relative abundances of the chemical elements can be calculated. 
Most stars belong to a "main sequence" (see diagram below) in which both temperature and luminosity increase with mass. Some stars are much brighter and hence much larger than main-sequence stars of the same temperature, and are called red giants. Many stars are much fainter and hence much smaller than main-sequence stars of the same temperature, including white dwarfs (1 per cent of the size of the Sun) and neutron stars (0.001 per cent of the size of the Sun).
Theoretical models of stellar interiors have been calculated based on the theory that an equilibrium exists between the force of gravity, which tends to cause the star to collapse, and the pressure of superheated gases, which tend to expand. High stellar temperatures also drive a flow of heat from inside the star to the outside. If the star is to be in equilibrium, this heat loss must be compensated by the energy released by nuclear reactions in the core. As various nuclear fuels are exhausted, the star slowly evolves, and the core contracts to higher and higher densities.
For stars of low mass, this process ends when the outer layers are gently ejected to form a planetary nebula; the core then cools down to form a white dwarf. More massive stars become unstable; as they evolve, this core suddenly collapses to form a neutron star or black hole, and the energy thereby released ejects the outer layers at very high speed, in a colossal explosion called a supernova.

Hertzsprung-Russell Diagram

The position in the H-R diagram of the point representing a star corresponds to its brightness and temperature. Stars on the left of the diagram are blue because they are hot, while those on the right are red because they are cool. The diagonal band of stars running from the upper left to the lower right is called the main sequence. Stars in the upper right are called red giants: although they are cool and red, they are very bright, because they are big. Stars near the bottom (known as white dwarfs) are very hot, but not very bright, because they are small. This diagram was developed independently by Ejnar Hertzsprung, a Dane, and Henry Norris Russell, an American.

All stars are hot, gaseous bodies like the Sun, but differ from it and from one another in minor ways. The most important physical data about a star are its intrinsic brightness, size, mass, and chemical composition. Although all stars seem much fainter than the Sun because of their great distances from the Earth, some of them are intrinsically brighter than the Sun. Star masses can be determined directly for the Sun and for pairs of stars, in binary systems, that are seen to orbit around each other. Astronomers apply the law of gravitation and Kepler’s laws to determine the stellar masses mathematically. Of the 50 nearest stars for which information is fairly complete, 10 per cent are brighter, larger, and more massive than the Sun. Spectroscopic studies show that the stars are composed largely of hydrogen.

The number of stars visible to the naked eye from earth has been estimated to total 8,000, of which 4,000 are in the northern hemisphere of the sky and 4,000 in the southern hemisphere. At any one time during the night in either hemisphere, only about 2,000 stars are visible. The others are obscured by atmospheric haze, especially near the horizon, and by faint sky light. Astronomers have calculated that the stars in the Milky Way, the galaxy to which the sun belongs, number in the hundreds of billions. The Milky Way, in turn, is only one of several hundred million such galaxies visible through large modern telescopes. The individual stars visible in the sky are simply those that lie closest to the solar system in the Milky Way.

The star nearest to our solar system is Proxima Centauri, one component of the triple star Alpha Centauri, which is about 40 trillion km (25 trillion mi) from the earth. In terms of the speed of light, the common standard used by astronomers for expressing distance, this triple-star system is about 4.29 light-years distant; light travelling at about 300,000 km/s (186,000 mi/s) takes more than four years and three months to travel from this star to the earth.