Stars are celestial bodies which are sufficiently massive to naturally achieve a significant rate of nuclear fusion reaction of hydrogen to helium in their interior at some point in their lives. Most stars are composed mostly of hydrogen and helium, the two most abundant elements in the universe, although this is not the case for some stars, such as Wolf-Rayet stars or white dwarfs.
At the center of a star lies the core, where the temperature and pressure are the greatest. In main sequence stars, this is the region where hydrogen fusion happens by either the proton-proton chain, which dominates in less massive stars, or the CNO cycle, which dominates in more massive stars. Helium ash builds up within this region as the star ages, eventually causing it to become virtually hydrogen-free by the end of the main sequence phase. In more massive stars, it may continue fusing helium and heavier elements up to iron. Energy produced by fusion at the center of the star is transported to the surface via two methods: radiation and convection. This manifests as the radiation zone, where energy is transported via radiation and conduction, and the convection zone, where energy is transported via convection. The placement and thickness of both zones differ depending on stellar mass and the current stage in stellar evolution. Once energy produced at the core reaches the photosphere, the layer where the star becomes transparent and generally regarded as the lowest layer of stellar atmosphere, it is finally emitted as radiation. Additionally, cooler stars containing sufficiently deep surface convection zones have two additional layers of atmosphere: the chromosphere and the corona.
There are two main types of stellar magnetic fields. Hotter stars with thin or nonexistent surface convection zones usually possess simple, stable, and dipolar fields. Cooler stars possessing deep surface convection zones generally have complex and dynamic fields, resulting in various phenomena within the stellar atmosphere. In the photosphere, granules (convection cells of plasma), faculae (bright valleys between granules), and starspots (dark or bright spots) can be observed, while spicules (short-lived vertical jets of plasma) and plages (bright regions analogous to faculae) may be found in the chromosphere. Additional phenomena include flares (bursts of light emission), prominences (large plasma and magnetic structure extending from the surface), and coronal mass ejections (ejection of stellar mass).
Stars emit a constant stream of particles known as stellar wind. The strength of stellar wind varies from star to star, and in extreme cases can lead to mass loss on the order of 1E30 kg over millions of years or less, as happens in extremely massive stars or post-AGB stars. Stellar wind pushes back the interstellar medium wind, forming a bubble surrounding the star that is known as the astrosphere. The boundary separating the astrosphere from the interstellar space, where stellar wind collides with the interstellar medium, is the astropause.
Stars are of immense utility to the Terragens. They are some of the most massive singular objects that can be found in the Terragen Sphere and the universe as a whole, and together they contain a large fraction of all mass-energy available in the Terragen Sphere. The following table contains representative examples of the various uses of stars.
Stars passively generate energy in the form of light that can be utilized even by simple beings, and is the primary source of energy for many biospheres, natural or artificial, directly or indirectly. As such, early Terragens on Earth were dependent on their home star, Sol, to survive, and this is also the case for many low technology cultures across the Terragen Sphere. While the development of nuclear fission and fusion reactors rendered illumination from stellar objects redundant long ago, many Terragens today still prefer to rely on sunlight and starlight to power them, their technologies, and their environments.