A star is a massive, luminous sphere of plasma that generates energy by nuclear fusion, and most spend the majority of their lives on the main sequence, fusing hydrogen into helium in their cores. Stars are organized on the Hertzsprung-Russell (H-R) diagram, which plots luminosity against temperature, and they are classified into spectral classes O, B, A, F, G, K, and M from hottest to coolest. Our Sun, for example, is a G2V yellow main-sequence dwarf. Two important properties of any star are its apparent magnitude, or how bright it looks from Earth, and its absolute magnitude, which is how bright it would appear at a standard distance of 10 parsecs. Stars also vary in luminosity, the total energy they emit per unit time, and many are members of groups such as binary systems (two stars orbiting a common center of mass) or eclipsing binaries, where one periodically passes in front of the other from our viewpoint. Loose, young groups called open clusters and dense, ancient spherical collections called globular clusters give astronomers snapshots of stellar populations at different ages.
Stars continually build heavier elements through stellar nucleosynthesis. Once a star exhausts the hydrogen in its core, it expands into a red giant. Low- to medium-mass stars eventually shed their outer layers to form glowing planetary nebulae, leaving behind compact white dwarfs. The maximum mass a white dwarf can stably support is the Chandrasekhar limit, about 1.44 solar masses. Massive stars end their lives in spectacular supernova explosions, which forge elements heavier than iron together with neutron star mergers. The expanding gas and dust left behind is called a supernova remnant. A Type Ia supernova is particularly important in cosmology because it serves as a standardizable "standard candle," an object of known intrinsic brightness used to measure cosmic distances.
After a supernova, the collapsed core can become a neutron star, an ultra-dense object composed mostly of neutrons with a maximum stable mass given by the Tolman-Oppenheimer-Volkoff limit of roughly 2 to 3 solar masses. Rapidly rotating neutron stars that emit beams of radiation are observed as pulsars, whose clock-like pulses make them useful for precise timing experiments, while magnetars are neutron stars with extraordinarily powerful magnetic fields. Fast radio bursts are brief, millisecond-long radio pulses observed from extragalactic sources whose exact origin is still being investigated.
For the most massive stars, gravity can compress the core into a black hole, a region of spacetime where gravity is so intense that nothing, not even light, can escape. The boundary surrounding it is the event horizon, whose radius for a non-rotating black hole is given by the Schwarzschild radius, while at the very center classical general relativity describes a singularity, a point of infinite density. Some pulsating stars, Cepheid variables, are particularly valuable to astronomers because the period of their brightness variation correlates with their true luminosity, making them another important rung on the cosmic distance ladder.