Nuclear burning

Figure 1.1 also gives a schematic illustration of the complex interactions between the ISM and stars. Stars inject energy, recycled gas and nuclear reaction products ( ashes of nuclear burning ) enriching the ISM from which other generations of stars form later. This leads to an increase in the heavy-element content of both the ISM and newly formed stars the subject of galactic chemical evolution (GCE) is really all about these processes. On the other hand, nuclear products may... 

Wolf-Rayet (WR) stars, which are hot stars of high luminosity like O stars, but have peculiar spectra with broad emission bands of He+, N++ (WN), C++ and C+++ (WC) or 0++++ (WO) and with hydrogen weak or absent. These are now believed to be massive stars (initially > 40 M ) undergoing extensive mass loss in the course of which their surface temperature steadily rises and outer layers are successively peeled off revealing the results of more advanced nuclear burning stages. 

Astronomers use a variety of methods to determine the distance to objects in the universe. One of the most effective is the standard candle provided by Type la supemovae. These supemovae originate in a binary star system when a white dwarf star accretes matter from its companion. When the white dwarf reaches the Chandrasekhar limit of 1.4 solar masses, a thermonuclear runaway occurs that completely disrupts the star in a cataclysmic explosion that makes the supernova as bright as an entire galaxy. Because Type la supemovae occur in stars with similar masses and because the nuclear burning affects the entire star, they all have essentially the same intrinsic brightness and their apparent brightness observed from Earth can be used to derive the distance to the supernova. 

The red giant stage ends when helium in the core is exhausted. Again the core contracts and the thermal structure of the star becomes unstable. Convective mixing again reaches down toward the layers that have experienced nuclear burning. This mixing event is... 

Schematic cross-section of a 1.5 M star showing concentrations of various nuclei as a function of depth within the star at the end of its main sequence lifetime. The position within the star is given in units of mass normalized to the solar mass, with zero being the center and 1.5 M being the surface of the star. The outer portion of the star from 1 M to the surface has the initial abundances of the nuclei because this portion of the star was never hot enough for nuclear burning. The region from...

Observations of isotopic abundances provides information on the nucleosynthesis operating in the compact core of stars and supernova explosions and on the chemical evolution of the Galaxy. The CNO nuclides in late-type stars are affected by freshly synthesized core material brought up by dredge-up events. On the other hand, the Si isotopes are involved in later phases of nuclear burning, a narrow span of the red giant lifetime before planetary nebulae or supernovae. Therefore relative abundances of Si isotopes we observe remain unchanged from those of interstellar matter from which a star was formed. 

See 5 Fe (to which5 6 Co decays) for a discussion of the nucleosynthesis of56 Ni (parent of56 Co) by equilibrium and quasiequilibrium processes during supernova nuclear burning. 

See 5 Fe (to which 5 Ni decays) for discussion of the nucleosynthesis of 5 Ni by equilibrium and quasiequilibrium processes during supernova nuclear burning. Roughly 0.5 to 0.8 solar masses of 56Ni is created by the thermonuclear explosions of white dwarfs (Type la supernovae). Only about l/loth as much is created by each core-collapse supernova (Type II), but they are about 4 to 5 times more frequent than Type la explosions. This means that about two-thirds of the galactic total of 5 Ni is synthesized by la explosions and about one-third by core-collapse Type II. 

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