star

A star is one of the light-colored dot-like points seen in the night sky that do not shift around the celestial sphere. Science accepts that these dots are distant astronomical objects much like the Sun, that emit EMR using energy produced by fusion, and science uses the term star for such objects, thus the Sun is considered a star. Such criteria requires some specifics, and exact criteria does vary with context, even within science. The general scientific definition is a body of gas that is large and hot enough that it initiates fusion. Main sequence stars clearly qualify as do post-main-sequence stars, e.g., on the red-giant branch. The eventual stellar remnants (which no longer host fusion) are sometimes still termed stars or a special kind of (qualified) star: neutron stars and white dwarfs do shine, but a stellar-mass black hole arguably does not, yet is often termed a star, e.g., as a binary companion. YSOs and pre-main-sequence stars, presumed to have fusion in their future, are likely to be termed stars if they shine like stars; they begin such shining some time before they trigger fusion, shining due to their hot temperature caused by the Kelvin-Helmholtz mechanism. Brown dwarfs (as a group) are often classified as stars: most are not currently undergoing fusion, but they are defined as bodies that host deuterium fusion sometime during their life. The distinction between brown dwarfs and gas planets is clear in principle, but classifying a particular object from observation data can be difficult or impossible. A rule of convenience that has been standardized is that lacking a reasonable determination, any such body for which the mass is determined to be less than 13 Jupiter masses is nominally classified as a planet, thus presumed not a brown dwarf and not a star. Numerous aspects of stars are of interest in astrophysical research, among them:

• classification: spectral types and luminosity classes.
• stellar designation.
• star formation.
• stellar structure.
• stellar evolution.
• stellar demographics.
• stellar dynamics.
• stellar parameter determination.

Cataloging and tracking stars (astrometry, star catalogs) has constituted a large part of astronomy for millennia, which has been of use for determination of time and place, useful, e.g., for navigation. Much of what is known about the physics of stars was learned studying the Sun, and a surprising amount is from the study of binary stars and variable stars. Spectrography and parallax measurement have been key to their study. See also entries on individual stars, star types, and other star topics.