A star the size of the Sun will burn hydrogen into helium until the hydrogen in the core is exhausted. At this point, the core of the star contracts and heats up until the fusion of three ⁴He nuclei into ¹²C can begin. Stars in this stage of evolution are known as red giants. Low mass stars such as our Sun will then evolve into a compact object called a white dwarf. All nuclear reactions in a white dwarf have stopped. Higher mass stars have internal temperatures (10⁸ K) that allow the fusion of carbon with helium to produce oxygen nuclei and excess energy. For very massive stars, the exothermic fusion of low-mass nuclei into successively more massive nuclei can proceed all the way up to nuclei in the iron region (A ~ 60). Table 10-1 shows the temperature (1 keV is equivalent to 1.16x10⁷ K), interior density, and process lifetime that occur in stellar evolution of a star 25 times more massive that the Sun. Note the accelerating time-scale as higher mass nuclei are burned.

Once the core of the star is converted into iron-region nuclei, the star has nearly reached the end of its life. Because the average binding energy per nucleon reaches a maximum at this point, there are no further energy-generating reactions possible and the star collapses because the gravitational force cannot be counter balanced by the high-temperature, high-pressure interior.

As the collapse of the core occurs, the density grows to the point where it becomes energetically favorable for electrons to be captured by protons via the weak interaction, producing neutrons and neutrinos. This process turns the core of the star into neutrons and produces a huge burst of neutrinos. When the collapse reaches nuclear density, the star rebounds explosively, throwing off much of its mass consisting of elements up to iron. This explosive expansion is called a supernova, one of the most spectacular events in astronomy. If the mass of the remnant core is less than two to three times the mass of the Sun, the core will settle down as a compact neutron star with no further nuclear reactions. More massive cores continue to contract due to the intense gravitational force until the size of the core diminishes to a point, a singularity called a black hole. The object is black because the gravitational force is so strong nothing, not even light can escape.

In February of 1987 a supernova in a nearby galaxy was observed in the Southern sky. Underground neutrino detectors saw the neutrinos emitted during the few seconds of the collapse and the birth of either a neutron star or a black hole. The supernova continued to glow for months in the night sky due to the decay of radioactive isotopes that were produced in the explosion. Neutron-capture reactions on iron-region nuclei during the few moments of the explosion produced nuclei more massive than A = 60. A sequence of such reactions can produce elements all the way up to uranium, A = 238.