The process is fusion, two atoms forced together combine giving out energy due to a mass defect . The core has a massive abundance of Hydrogen ( 1 proton ) which is fused together to form Heliun (4 protons ) but the process is not quite as simple as it first appears. Although four Hydrogens colliding under such extreme conditions would have the required threshold energy to combine producing Helium,the statistical chances of this show that even in the dense conditions of a star not enough interactions would occur to keep the core burning. Instead we must look for a chain reaction , the proton-proton chain.
Two colliding Hydrogen atoms fuse to form Deutrium (heavy Hydrogen (Du))
H + H ---------> Du ( 1.44MeV )
The next collision required involves the Deutrium and another Hydrogen to give Helium three
Du + H ----------> He (5.49MeV)
Then the final stage is for two of these Helium atoms to collide giving the finished product Helium four [AC1]
He + He ------------> He + 2H (12.9MeV)
The energy released at each stage is displayed in brackets following the equation . To give an example of the ammount energy involved it would take 315 tera (315,000,000,000,000) reactions to produce enough heat to boil a kettle of water ( at S.T.P.).
The next layer is the RADIATIVE ENVELOPE a Helium cloud which surrounds the core ,its a gaseous area made up of the Hydrogen released by the proton proton chain. This also produces energy as Helium at the edge of the core burns helping to raise the temperature of the center. The outer layers are the CONVECTIVE ENVELOPE which acts as an insulator ; the PHOTOSPHERE effectively the sun skin and the CORONA but they have far less significance in the final fate of the star.
When stars die they have a number of ways in which they degenerate , some sudden ( in galctic terms) and explosive or slowly and (relatively ) peacefully . This first type is known as a white dwarf and it describes the pattern that the less massive stars and so also the sun follow . The first stage is the using up of the Hydrogen atoms in the center and so the energy supply is halted. The slight cooling down , combined with the ecxess of Helium atoms causes the core to contract and in doing so raises the temperature again. This second temperature increase causes the surrounding radiative envelope of Helium to ignite and burn furiously. This massive increase sets off a chain of reactions due to the increased heat starting with the expansion of the convective envelope . The two sections of the sun separate as the core contines to shrink and the outer layers expand . The core is now a white dwarf , A small very hot ( appearing white) star while the outer layers have formed a red giant ,In the early stages of this process the young red dwarf will totally obscure any sight of the white dwarf which is being formed. In the case of the sun the red Giant will entirely consume the earth along with all the other planets in our solar system.
The larger stars have greater flexability as to their death ,in fact there are three typical results, supernova , formation of either a neutron star or a blackhole. All the results follow a similar path as the formation of supernova and neutron often go hand in hand .
Supernova are quite rare , in the last thousand years six have been seen , but none in recent times . The are the result of a massive collapse of the star which ends in a huge collision blowing the outer layers away and leaving a central core of fatastically dense material a neutron star.
Supernova can be seen for about 2 years and records from the more recently observed nova agree that it takes about 70 to 80 days to reach its brightest point and the rest of the time dispursing the energy released . What happend to supernova 1987A seen in the was that a very massive star ( 18 solar masses) developed an iron core that suddenly became a neutron star. The ensewing collapse blasted the outer layers into space.
Nutron stars as disscused above are are very dense stars ,formed as a result of shrinking cores ( usually because the core has become largely a heavier element)..
If the mass of stars core left after a supernova explosion is greater than 3 solar masses, the remaining stellar material will become a black hole. At this point, the force of gravity is strong enough to overcome the degenerate neutron pressure, and nothing stops the object from collapsing. Because nothing opposes gravity, the object collapses to a point, known as a singularity, where all of the mass is concentrated. It is then deemed to be a Black Hole.
Because black holes are so massive, they exert huge gravitational forces and near the centre even light cannot escape its pull ( this can be calculated using Newtons gravitational law) there is a quantity, known as the escape velocity, that is the speed a body needs to escape the gravitational pull of an object. Because the gravitational force the body experiances decreases as it moves farther from the central object, the escape velocity decreases ,in other words, the closer you are to a massive object, the faster you must go in order to escape the gravitational pull of that object.
The distance at which the escape velocity equals the speed of light is called the Event Horizon. Any object within this radius is unable to escape the pull of the black hole. As a result beyond this point only speculation as no probes can send information back as nothing can move faster than light.
Many thanks to