Supernovae and Neutron Stars


There are two Types of supernovae.  Casually, both types appear as brilliant "temporary" stars in the sky, some even visible in daylight.  Both types are violent explosions but the models describing them are very different.  They are referred to as Type I and Type II

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.  Gravitational contraction raises the internal temperature creating convection zones.  A star like the Sun will contract until matter at the center reaches an electron degeneracy state.  An incompressible electron "crystal" forms containing carbon and oxygen nuclei.  Radiation pressure from the degenerate core will drive about 40% of the star's mass off into space as an expanding shell.  This creates a planetary nebula surrounding a white dwarf star.  Initialially, the white dwarf has an apparent (black body) surface temperature of about 100,000 degrees.  Its surface emits intense ultraviolet radiation which causes gasses in the surrounding planetary nebula to flouresce.  Over bllions of years, the star cools becoming a frozen cinder and the halo dissipates into space.

Stars greater than 3 solar masses 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 slightly larger than the size of the earth.  Gravitational pressure around the iron core increases until the core collapses into a neutron degeneracy state.  It takes about 10 seconds for the "Earth sized" core to collapse into a ball of neutrons 10 miles in diameter.  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).  As stellar matter outside the core falls inward and collides with the neutron core.  Near the surface of the neutron core, temperatures reach several hundred million degrees and the infalling matter recoils.  The resulting shockwave produces a brilliant cosmic display and contains elements heavier than iron, for example, nickel, gold, silver, uranium, etc.  These heavy elements are created by fusion reactions near the core which require extremely high temperatures and pressure.  Enough gamma radiation is produced to cause damage to carbon-based forms within 200 light-years (1,200,000,000,000,000 miles!).  The ball of neutrons, a neutron star remains and is, in many cases, ejected from the center of the explosion. 

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. The surface of the neutron star slowly decays into proton-electron pairs which are trapped or ejected by the magnetic field.  The neutron star slowly decays via synchrotron radiation emission and particle ejection into polar magnetic jets. 

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.