Category:Star

A star is a massive, luminous ball of plasma that is held together by gravity. For most of its life, a star shines due to thermonuclear fusion in its core releasing energy that traverses the star's interior and then radiates into outer space. Almost all elements heavier than hydrogen and helium were created by fusion processes in stars. A star begins as a collapsing cloud of material composed primarily of hydrogen, along with helium and trace amounts of heavier elements. Once the stellar core is sufficiently dense, some of the hydrogen is steadily converted into helium through the process of nuclear fusion. The remainder of the star's interior carries energy away from the core through a combination of radiative and convective processes. The star's internal pressure prevents it from collapsing further under its own gravity. Once the hydrogen fuel at the core is exhausted, those stars having at least 0.4 times the standard mass expand to become a red giant, in some cases fusing heavier elements at the core or in shells around the core. The star then evolves into a degenerate form, recycling a portion of the matter into the interstellar environment, where it will form a new generation of stars with a higher proportion of heavy elements. Binary and multi-star systems consist of two or more stars that are gravitationally bound, and generally move around each other in stable orbits. When two such stars have a relatively close orbit, their gravitational interaction can have a significant impact on their evolution. Stars can form part of a much larger gravitationally bound structure, such as a cluster or a galaxy.

Spectral classification is the classification of stars based upon the temperature of the photosphere. Each spectral class is further divided into 10 subclasses, ranging from 0 (hottest) to 9 (coolest). Stars are also placed into one of six categories based upon luminosity: 1a (luminous supergiant), 1b (less luminous supergiant), II (luminous giants), III (normal giant), IV (subgiant), and V (main sequence and dwarf stars). Stellar Remnants are also placed within there own categories. For example, Sol is classified as type G2V. This indicates that is a Spectral Class G star of considerable temperature, and that it is a part of the main sequence.