Supernovae and Neutron Stars
Three subatomic particles comprise an atom. Positively charged protons, fairly massive, reside in the nucleus. Negatively charged electrons, very low mass, surround the nucleus and electrically neutral neutrons reside in the nucleus with protons. The positively charged protons strongly repel each other. Neutrons act as mediators and bind the nucleus together. Every element, except normal hydrogen with one nuclear proton, has at least one neutron in the nucleus.
Most stars shine by radiating energy produced in their cores from hydrogen fusing into helium. This process operates, for main sequence stars, such as the Sun, for around 10 billion years. Massive stars (GT 3 solar masses) fuse hydrogen more rapidly. Eventually all stars develop a core of helium "ash" and hydrogen begins to fuse in a shell around the core. As the helium core accumulates, the shell temperature drops below that required for hydrogen fusion (about 14 million degrees). When hydrogen fusion ceases, the star contracts by self-gravitation raising the temperature high enough in the core for helium to fuse into carbon and oxygen. A carbon "ash" core accumulates inside the helium fusion zone and a helium fusion shell develops. Helium fusion is unstable, characterised by large core pulsations which push surface material outward where it cools. The star becomes larger and more reddish in color. Eventually, the helium fuel exhausts, fusion ceases, and gravitational contraction again occurs.
At this point, less massive stars, such as the Sun, contract slowly but the core does not get hot enough to fuse carbon or oxygen. The Sun will shed about 40% of its mass into a halo and the rest will contract until it reaches an electron degeneracy state. The core basically becomes an incompressible electron "crystal" containing carbon and oxygen nuclei. This is a white dwarf star. Initialially, it has an apparent surface temperature of about 100,000 degrees. Its surface emits intense ultraviolet radiation which causes gasses in the halo or planetary nebula to flouresce. Over bllions of years, the star cools becoming a frozen cinder and the halo dissipates into space.
Massive stars, however, continue contracting until the core reaches carbon / oxygen fusion temperature. A number of fusion reactions occur producing primarily magnesium, neon and silicon. A complex series of core contractions and toggling of fusion reaction zones between the core and shells eventually results in accumulation of an iron "ash" core. Stars in this phase pulsate and their energy output varies. Iron cannot be fused to release energy so an iron ash core builds up until its volume is approximately the size of the earth. Abruptly, the iron core collapses into a neutron degeneracy state, i.e., within about 10 seconds the core becomes a ball of neutrons 10 miles in diameter. This ball of neutrons is a neutron star. The chaos that follows is a type II supernova. An incredible amount of energy is released, mostly in the form of neutrinos (tiny electrically neutral subatomic particles). The remainder radiates into infalling stellar material heating it to several hundred million degrees producing a brilliant cosmic display and creating elements heavier than iron, ex., nickel, gold, silver, uranium, etc. Enough gamma radiation is produced to cause damage to carbon-based forms within 200 light-years (1,200,000,000,000,000 miles!).
The star's outer layers are blasted into space at thousands of km/sec producing a massive shock wave. This expanding shell is so bright that it can be seen with the unaided eye in daylight if the event occurs within our Milky Way galaxy. These explosions can even be observed with small telescopes in galaxies millions of light-years away. After several months, the blast cloud dims as it expands and cools producing a supernova remnant.
When core collapse occurs, the neutron star "spins up" like a skater pulling their arms in. Rotational speeds of 3000 RPM are not uncommon. The progenitor star's magnetic field is compressed and wrapped up to extremely high flux levels which generates "jets" of protons and high energy radiation aligned with the magnetic field axis. These jets are detectable at great distances if pointed at an observer. Because the magnetic and spin axes are seldom coincident, the jets trace circles in the cosmos. A distant observer located near the beam path will observe a regularly pulsing radiation source, hence, the term pulsar also refers to neutron stars.