Can astronomers predict when a star will become a nova or a supernova?
Nova are produced in binary star systems in which one of the two stars is a white dwarf or a neutron star, and the other is a normal star usually a red giant which has a distended outer atmosphere. Some of this material can be captured by the dense companion. Depending on a variety of factors, the dense companion will flare up as a nova periodically, or simply blow itself to pieces as a 'Type I Supernova'. Astronomers know of many reoccurring novae; some work with near clockwork precision and their outbursts can be roughly predicted. The dynamics of the nova process are sufficiently complex that we as yet have no good way to predict when outbursts will occur from a 'cold start'. Recurrent Novae and Dwarf Novae are classes of stars that produce very large nova-like outbursts, usually involving instabilities in the accretion disks orbiting white dwarfs in binary systems. They can have outbursts every 500 days or less, but are not periodic like clockwork so that each pulse can't be predicted with much accuracy beforehand. Supernova are explosions that detonate the entire star. Some of these are nova-like situations such as the Type I event above. Many others are the so-called Type II events that involve the detonation of a single massive star as it ends its life. We have good theoretical reasons to believe that supernova precursor stars will be red supergiants if their heavy element content is like that of the Sun, or perhaps blue supergiants if they have somewhat more heavy elements as was the case for Supernova 1987A in the 'metal-rich' Large Magellanic Cloud. Just before it becomes a supernova, a star may have a composition that suggests a great deal of chemical enrichment of its surface by convection from its nuclear core. There may be a sudden increase in the velocity of its stellar wind, or the thickness of its circumstellar dust envelope. As we learn more about the theory behind the explosion, we can better anticipate just what kinds of things to look for observationally. Right now, we can identify which star type will explode, but not when this will happen to within a million-year accuracy or worse.