Red Giants
During the long period a star remains on the main sequence of H-R diagram and it continues to burn Hydrogen into Helium in its core. When about 12% of its mass converts into helium by nuclear burning, the star becomes unstable. The central regions contract while the outer layers heat up and expands until it creates a star which is many thousands of times larger than the main sequence star from which it formed. The expansion continues until the star becomes a “red giant”. When this happens, the star moves off the main sequence and crosses the H-R diagram to the region occupied by the red giants. Then, the central temperature of the star can become high enough for the nuclear burning of the carbon into Helium to take place.
All stars spend a much longer time on the main sequence. All other stages take place over a very much shorter time-scale than the long period it remains on the main sequence. The physical reason for this is that the luminosity of the star becomes very much greater when it evolves into red giant and so the available nuclear fuel is burned up much more rapidly.
The star runs out of a nuclear fuel and the central regions collapse. In the most massive stars, the collapse is very violent and results in an enormous explosion in which the star is most likely to end up as a black hole. In less massive star, the death is very more dramatic. Once they reach the end of their life, they become unstable and blow off their outer layers to forms the objects known as” planetary nebulae” while the core of the star shrinks to form a “white dwarf”.
