5 NUCLEOSYNTHESIS IN MASSIVE STARS

The majority of the heavy elements observed in the world around us are made in massive stars. Less massive stars (up to M 8 M _?) may also go through a few hydrostatic burning stages. However, they lose their envelopes because of strong stellar winds, especially during the red giant stage, eventually leaving behind a star with a mass smaller than the Chandrasekhar mass ( ? 1.4 M _?) that can be stabilized against collapse by the pressure of the degenerate electron gas. This star, a white dwarf, will not undergo further burning but instead slowly cools off, unless more matter is added. Any elements produced in the star's previous burning phases (e.g., ⁴He or ¹²C) remain in the star and thus do not contribute to the observed elemental abundance curve shown in Fig. 1.1 as long as the star remains a white dwarf. Note, however, that the situation can be quite different if the white dwarf is part of a binary system. Then it can accrete mass from its companion star (especially if the companion is a red giant). This steady mass flow onto the surface of the white dwarf will eventually trigger explosive burning in the form of a nova (for low accretion rates), or a supernova of Type Ia (for high accretion rates), the latter case leading to the complete destruction of the white dwarf. For a discussion of the production of heavy elements in...