The colourful planetary nebulae in the various corners of the sky remind us of the penultimate fate of the sun. It is very well known that these colourful rings have a tiny dot at the centre, identified as a white dwarf.
The beautiful yellow ball, our life-maker, will eventually bloat into a red giant. Though it is matter of another four – five billion years easily, we can still envisage the chronology of events right now.
The core of the sun, by then, would be enriched with carbon and oxygen. Right now the sun is losing about a trillionth of its mass every year.
As it swells into the red giant phase it would lose more and more material, almost half of today’s mass. The last phase results in a relatively quiet process of shredding the outer layers, which constitute the colourful planetary nebula.
The final mass is well within the limit defined as “Chandrashekhar limit” and so the sun also will end as a white dwarf. The sun would then be size of earth (of today). That is the “death”, since there are no more nuclear fusion; no more energy production.
The heat gradually is lost and the dead body is no more recognisable. This is not going to be very spectacular event like a supernova, when the star can shine as bright as Venus.
Many brethren of the sun, however, have a second chance to put up a great show. While for all practical purposes white dwarf marks the death of the star, some are destined to have a “second” death. Courtesy – a cooperative companion.
Almost half of the number of stars we see in the sky are binaries. They are seen next to each other, not by virtue of the line of sight coincidence but they are gravitationally bound too. The twins – assuming that they were born together – are not identical. The massive of the two evolves faster and may end up as a white dwarf. The opportunity to display a colourful planetary nebula is not guaranteed.
The fate of the binary takes a very interesting course of action. The “slow and steady” companion now swells into a giant; it sheds material just as his elder brother did some time ago. However, the material now has a channel to flow freely onto the “dead” star - the white dwarf. The white dwarf now gains mass.
There may be periodical “puffs” when the matter thus accreted gets ignited and glows. Such events were very well known almost two hundred years ago. The sudden brightening gave a false impression that a “new” star was seen, justifying the name “nova”. About 70 years ago, it was established that “novae” could be identified with very faint dots on photographs taken earlier. A systematic study revealed their binary nature.
The differences among them requires that they be classified as “classical novae”, “dwarf novae”, “recurrent novae” and so on.
The “obese” white dwarf now has a second chance put up a big show. Fred Hoyle and William Fowler initiated this scheme of calculation four decades ago.
As the mass reaches the Chandrashekhar limit, the interior wakes to activity. At a specified instant, the carbon in the core gets ignited. A huge bubble of ash forms within a second and moves towards the surface.
The movement is not very smooth and the bubble is restricted to the surface of the white dwarf. The bubble quickly engulfs most of the unfused material. Within a second it can spread to the entire sphere. That results in a catastrophe; the entire sphere crashes releasing vast amounts of energy.
This “gravitationally confined detonation” model is now gaining importance and is being used for understanding the supernovae classified as Type I.
It may be recalled here that the word supernova generally is used to specify the penultimate stage of a star about eight times the mass of the sun.
After going through successive stages of nuclear fusion at the core, the star gains a core with iron ash. What follows is the event releasing billion times more energy than the sun, producing a neutron star at the core. To distinguish this from the scenario (of the second death) of white dwarf, this is referred to as supernova Type II.
A similar scenario can be visualised for a binary with a massive companion. While the more massive of the two ends up as a supernova earlier, leading to the formation of a neutron star. It can still be bound in an orbit to the “quiet” companion, which will eventually bloat.
Just as it happened in the case above, there can be mass transfer associated with frequent small scale eruptions. Last few decades saw a new class emitting X rays earning the name “X-ray novae”, which is a manifestation of this scenario.  It is interesting that most of the Type Ia subgroup occur in galaxies with old stellar population.
 The life span of a star is decided by the mass; the more massive it is, lesser is the life span. In the scenario of the gravitationally confined detonation, the stars involved are both sun like and/or less massive than the sun. Quite naturally, they represent an older generation.