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Representative lifetimes of stars as a function of their masses The change in size with time of a Sun-like star Artist's depiction of the life cycle of a Sun-like star, starting as a main-sequence star at lower left then expanding through the subgiant and giant phases, until its outer envelope is expelled to form a planetary nebula at upper right Chart of stellar evolution
As stars evolve, so do their emissions; younger stars tend to be the most active, meaning they have stronger winds, larger flaring events, and an increased frequency of CMEs. [13] This means that planets orbiting younger stars would endure more volatile stellar events that impact their habitable and abiogenesis zones, perhaps even making them ...
Star formation theory, as well as accounting for the formation of a single star, must also account for the statistics of binary stars and the initial mass function. Most stars do not form in isolation but as part of a group of stars referred as star clusters or stellar associations .
The study of stars and stellar evolution is fundamental to our understanding of the Universe. The astrophysics of stars has been determined through observation and theoretical understanding; and from computer simulations of the interior. [99] Star formation occurs in dense regions of dust and gas, known as giant molecular clouds.
It also does not explain how star formation ceases in galaxies. Theories of galaxy evolution must therefore be able to explain how star formation turns off in galaxies. This phenomenon is called galaxy "quenching". [16] Stars form out of cold gas (see also the Kennicutt–Schmidt law), so a galaxy is quenched when it has no more cold gas ...
Typical boundary conditions set the values of the observable parameters appropriately at the surface (=) and center (=) of the star: () =, meaning the pressure at the surface of the star is zero; () =, there is no mass inside the center of the star, as required if the mass density remains finite; () =, the total mass of the star is the star's ...
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The radiation thus created must traverse the interstellar dust in the surrounding dense core. The dust absorbs all impinging photons and reradiates them at longer wavelengths. Consequently, a protostar is not detectable at optical wavelengths, and cannot be placed in the Hertzsprung–Russell diagram, unlike the more evolved pre-main-sequence ...