Novae & Supernovae
Novae and Supernovae are possible stages in the lives of stars. Supernovae occur only in stars significantly heavier than the Sun. Solitary stars that start with a mass below about eight or nine solar masses, and end with a mass less than about 1.4 solar masses, become white dwarfs, and without a companion star, cannot accrete sufficient matter to produce a Nova or Supernova. An important measurement of mass in any discussion of Novae and Supernovae is the Chandrasekhar Limit, which is approximately 1.4 solar masses for stars that are not revolving; the faster a star revolves, the greater its Chandrasekhar Limit. It is the point at which degenerate matter is unable to support itself against further gravitational collapse, and must collapse into either a neutron star or a black hole.
Novae are much less cataclysmic than Supernovae, and contribute only a tiny proportion of matter to the interstellar medium compared to Supernovae. Astronomers categorize Novae depending on how quickly they dim; that is, by their light curve. There are also some other sub-classes, for example Luminous Red Novae may result from the merging of two main sequence stars, and Recurrent Novae that happen periodically with anything from 10 years to 100,000 years between eruptions. The material ejected generally comprises elements heavier than hydrogen; for example, helium, carbon, nitrogen, oxygen, neon, and magnesium but certainly nothing heavier than iron.
Supernovae are truly spectacular, and at peak brightness, a single star can outshine its entire galaxy. There are two primary mechanisms that trigger a Supernova. Accretion of material from a companion star by a white dwarf, or the collapse of the core of a massive star into a neutron star or black hole. The power inherent in Supernovae is sufficient to produce elements heavier than iron, that are then spread into the interstellar medium.
Throughout this section, it is important to understand that, apart from the Chandrasekhar Limit, which is reasonably well defined by verified equations of state, the masses at which other phenomena occur are not well defined. They have changed over time, as our understanding improves, and will continue to adjust as more information, both observed and theoretical, becomes available.
Astronomy & Cosmology -
Stars - Life & Death of Stars