Betelgeuse is such as massive star that our Sun has seen the birth of it from a dense cloud of gas and dust, and will see the death of it long before the Sun itself fades away.
Betelgeuse long ago burned the reserves of Hydrogen deep within its core. Catalysed by Oxygen, Carbon and Nitrogen, in a cycle termed the CNO cycle, the immense temperature and pressure within the core ensures that fusion into Helium takes place at an accelerated rate. Once Hydrogen burning ceases the core contracts increasing the temperature and pressure until a point is reached where Helium burning takes place in a process termed the 3a (alpha) process. The temperature rise also causes Hydrogen fusion to begin within a shell around the core, and this, plus radiation pressure, causes the outer regions of the star to expand and cool. As Helium becomes depleted within the core and the reactions slow, the core again contracts and temperature and pressure increase until the Carbon produced by Helium burning begins to fuse with some of the remaining Helium to produce Oxygen. These cycles continue in ever more desperate attempts to continue shining with :
Throughout the giant phase, heavy elements are created away from the core by the s-process (slow), the occasional fusion of atoms.
The reactions taking place within Betelgeuse have thus far been exothermic (more energy is generated that is required to maintain the reaction), however this stops once Iron becomes the dominant element within the core. Once the Iron producing reactions slow, the core contracts yet again, only this time there is no chance of further reactions and the temperature and pressure become so intense that the Iron nuclei begin being broken up by protons producing alpha particles and neutrons and as the process gains pace it reaches supersonic speeds. The collapse continues until electron degeneracy sets in within the core of the star and it is at this point that the collapse really gets going. The mass of the star is too great to support electron degeneracy, and so as electrons are removed, the pressure drops and the collapse accelerates. When the core temperature reaches 1,000,000,000,000 Kelvin, neutron degeneracy sets in and the collapse halts and rebounds slightly. This rebound generates a shock wave and the star explodes. The vast majority (99%) of the 1*1046J (one with 46 zeros) of energy from the explosion is released in neutrinos but energy is also released as kinetic energy and cosmic rays. The liberation of this amount of energy causes the surface area of the star to expand over time by up to 20,000,000,000km and to brighten by a factor of 100,000,000 and about 560 years later we see Betelegeuse go Type II Supernova. During the explosion both stable and unstable heavy elements are created by the r-process (rapid), a brief but very much faster version of the s-process.
For up to 100 days after the explosion the star brightening of the star is governed by the energy of the explosion and the opaqueness of the shell. Once the shell has expanded to a distance where it becomes transparent, the brightness diminishes and continued brightness is governed by radioactive decay of the unstable heavy elements, with liberation of gamma rays, created by the r-process during the explosion.
There are three mechanism that are believed to be capable of causing such as explosion.
The first possibility is that when the core rebound occurs the material in the shell is still collapsing in at extremely high speed, and when it is hit by the shock wave these layers are blown apart.
The second possibility is that the shock wave heats the outer layers to such intense temperatures that explosive fusion reactions occur with these layers.
The third possibility is that some of the neutrinos liberated by the collapse interact with the outer layers lifting them away.
Where there was one event taking place, now there are two. The core of the star has been heading on one direction (inwards) and outer shell of the star in the opposite direction (outwards).
The gas and dust of the interstellar medium (including the gas ejected by the stellar wind) surrounding the explosion are resting blissfully unaware of that is about to happen it. The expanding shock wave hits the surrounding gas and dust with a strong shock that heats and compresses it. The vast majority of this, now ionized, material is swept along by the shock wave but a small amount leaks through into a cavity left behind by the shock front. In this low density environment the ionized gas is heated to a few thousand degrees Kelvin.
As expanding shock wave loses kinetic energy ionization of material reduces and radiative cooling takes place. As the temperature falls, so does the pressure and so the material begins being squeezed by its surroundings increasing its density. With this density increase collisions between material occur more frequently and the sweeping up of material increases (but now without the strong shock). Eventually the shock wave subsides leaving as a result regions of interstellar medium containing dense (relatively) material, and behind it a large (300pc) region of hot intercloud medium.
So much for the shell of the star, what of the remaining neutron degenerate core.
During the core collapse, angular momentum has been retained so the resultant neutron star is characterised by rapid rotation and a strong magnetic field. Whether the core of Betelgeuse will manifest itself simply as a neutron star or as a pulsar is likely to depend on its angle of rotation relative to the Sun. Regardless of this the rapidly spinning core will heat the surrounding low density cavity resulting in a glowing gas/dust nebula just as the pulsar at the centre of the Crab Nebula has.
Betelgeuse, in its violent death, has enriched the interstellar medium (ISM), from which new stars will form, in many ways. It has released energy into the ISM in the form of cosmic and gamma rays. It has swept up gas and dust into relatively dense regions and has scattered heavy elements over a wide area. It is also possible that the shock wave may provide the trigger for collapse and fragmentation of local dense clouds leading to new Protostar formation. And so the cycle begins again.
Mark W J Redding, 26th November 2001