Stellar evolution dredge up2/20/2024 ![]() ![]() ![]() The end of the main-sequence phase occurs when hydrogen burning ceases in the stellar core. ![]() For stellar masses greater than \(1.2\,M_\odot\) the contraction defines the end of the main sequence. The release of some gravitational potential energy increases the luminosity slightly, where th effective temperature must increase. When the hydrogen mass fraction reaches about \(X=0.05\) (in the core of a \(5\, M_\odot\) star), the entire star begins to contract. ![]() As a star evolves the convection zone, the core retreats more rapidly with increasing stellar mass and can disappear entirely before the hydrogen is exhausted for \(M \gtrsim 10\,M_\odot\). For a \(5\,M_\odot\) star, the central convection zone decreases in mass during (core) hydrogen burning and leaves behind a slight compositional gradient. The convection timescale (as determined by the mixing length and convective velocity) is much shorter than the nuclear timescale. The convection zone continually mixes the material keeping the core composition nearly homogenous. The evolution of more massive stars on the main sequence is similar to lower mass stars, but with a convective core (an important difference). Main-Sequence Evolution of Massive Stars # Main sequence and post-main-sequence stellar evolution are governed by the timescale of nuclear reactions, which is \(\sim 10^ \simeq 1.34\)?ĩ.1.5. Pre-main-sequence evolution is guided by two timescales: 1) the free-fall timescale and 2) the thermal Kelvin-Helmholtz timescale. To maintain their luminosities, stars must use either gravitational or nuclear energy, where chemical energy cannot play a significant role. Globular Clusters and Galactic (Open) Clusters The Thermal-Pulse Asymptotic Giant Branchĩ.3.2. A Derivation of the Schönberg-Chandrasekhar Limitĩ.2.8. Main-Sequence Evolution of Massive Starsĩ.1.6. ![]()
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