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In forming the stellar structure equations (exploiting the assumed spherical symmetry), one considers the matter density (), temperature (), total pressure (matter plus radiation) (), luminosity (), and energy generation rate per unit mass () in a spherical shell of a thickness at a distance from the center of the star.
This stellar model, technically the spherically symmetric quasi-static model of a star, has stellar structure described by several differential equations derived from basic physical principles. The model is constrained by boundary conditions , namely the luminosity , radius, age and composition of the Sun, which are well determined.
Fluid computer models are often coupled with radiative transfer, (Newtonian) gravity, nuclear physics and (general) relativity to study highly energetic phenomena such as supernovae, relativistic jets, active galaxies and gamma-ray bursts [3] and are also used to model stellar structure, planetary formation, evolution of stars and of galaxies ...
the structure and dynamics of the Hyades cluster [25] kinematics of Wolf–Rayet stars and O-type runaway stars [26] subdwarf parallaxes: metal-rich clusters and the thick disk [27] fine structure of the red giant clump and associated distance determinations [28] unexpected stellar velocity distribution in the warped Galactic disk [29]
Schwarzschild's work in the fields of stellar structure and stellar evolution led to improved understanding of pulsating stars, differential solar rotation, post-main sequence evolutionary tracks on the Hertzsprung-Russell diagram (including how stars become red giants), hydrogen shell sources, the helium flash, and the ages of star clusters.
Normal stars fuse gravitationally compressed hydrogen into helium, generating vast amounts of heat. As the hydrogen is consumed, the stars' core compresses further allowing the helium and heavier nuclei to fuse ultimately resulting in stable iron nuclei, a process called stellar evolution. The next step depends upon the mass of the star.
The horizontal branch (HB) is a stage of stellar evolution that immediately follows the red-giant branch in stars whose masses are similar to the Sun's. Horizontal-branch stars are powered by helium fusion in the core (via the triple-alpha process) and by hydrogen fusion (via the CNO cycle) in a shell surrounding the core.
IMF and PDMF can be linked through the "stellar creation function". [2] Stellar creation function is defined as the number of stars per unit volume of space in a mass range and a time interval. In the case that all the main sequence stars have greater lifetimes than the galaxy, IMF and PDMF are equivalent.