Chapter 21 Stellar Explosions

21.1 Life after Death for White Dwarfs

  • A nova is a star that flares up very suddenly and then returns slowly to its former luminosity

  • A white dwarf that is part if a semidetached binary system can undergo repeated novas

  • Material falls onto white dwarf from its main sequence companion

  • When enough material has accredited, fusion can reignite very suddenly, burning off the new material

  • Material keeps being transferred to the white dwarf, and the process repeats

21.2 The End of a High-Mass Star

  • A high-mass star can continue to fuse elements in its core right up to iron (after which the fusion reaction is energetically unfavored)

  • As heavier elements are fused, the reactions go faster and the stage is over more quickly. A 20-solar-mass star will burn carbon for about 10,000 years, but its iron core lasts less than a day

  • Iron is the crossing point; when the core has fused to iron, no more fusion can take place

  • The inward pressure is enormous, due to the high mass of the star

  • There is nothing stopping the star from collapsing further; it does so very rapidly, in a giant implosion

  • As temperature increases, the photons have tremendous energy. They are capable of breaking heavy elements into protons and electrons. This is called photo-disintegration

  • As it continues to become more and more dense, the protons and electrons react with one another to become neutrons:

    • p+e → n+neutrino

  • The neutrinos escape; the neutrinos are compressed together until the whole star has the density of an atomic nucleus, about 1015 kg/m³

  • The collapse is till going on; it compresses the neutrons further until they recoil in an enormous explosion as a supernova

21.3 Supernovae

  • A supernova is incredibly luminous—and more than a million times as bright as a nova

  • A supernova is a one-time event—once it happens, there is little or nothing left of the progenitor star

  • There are two different types of supernovae, both equally common:

    • Type 1, which is a carbon-detonation supernova

    • Type 2, which is the death of a high-mass star just described