Some decades ago, the astronomer Rudolph Minkowski ( 1895 - 1976) identified two kinds of supernovae based on the kinds of galaxies they were found in, their spectroscopic properties, and the way in which they rose to maximum brilliance and then diminished.
Type I supernovae had no spectral lines of hydrogen. They were found in all kinds of galaxies; both elliptical and spiral.
Type II supernova did have strong spectral lines of hydrogen, typically indicating very high velocities of thousands of kilometers per second. They were never found in elliptical galaxies.
Eventually, these observations were developed into a theory in which the Type II supernovae were identified with the detonation of massive stars which for decades had been described as Population I, and Type I supernovae are caused by the detonation of a white dwarf companion star in an evolved Population II binary system. This explains why Type II supernovae are never seen in elliptical galaxies because these galaxies contain primarily low mass stars of Population II. When a helium-rich white dwarf explodes, you will also not see much hydrogen so the spectral lines from this element will be weak or absent. In a Type II supernova, however, the hydrogen lines are very strong because these supernova eject most of the hydrogen-rich gas of the massive star in a blast wave traveling at up to 10,000 kilometers per second. The above image is of Supernova 1987a and its ring system. This was a Type II supernova in the Large Magellanic Cloud.
We believe that Type II supernovae are caused by a massive star reaching a crisis in its last stages of evolution. As the gravitational force of a massive star continues to crush its core regions and drive them to higher and higher temperatures, eventually a core rich in iron develops surrounded by an onion-skin of regions where different elements are being fused to produce heat and pressure to stabilize the star from gravitational collapse. Unfortunately, by the time the star reaches this state, the radiation field in the billion-degree core has become so energetic that every time an iron nucleus forms, it is shattered by a collision with a high energy photon of light. This means that some of the pressure in the core provided by radiation is absorbed by breaking down iron nuclei into alpha particles. This is like throwing cold water on a fire. For a massive star where gravitational forces are carefully balanced by heat and radiation pressure, this cooling effect is catastrophic. Gravitational collapse is no longer counterbalanced by these internal pressures and the star's core begins its last collapse. Within minutes, the core temperature climbs to 10s of billions of degrees and the density of the iron-rich stellar core climbs to nearly-nuclear densities. The matter becomes so tightly packed that even the slippery neutrinos which accompany thermonuclear fusion can no longer get out. This produces a tremendous spike in internal pressure which, according to computer simulations, creates a blast wave which travels from the core of the star, through its interior, and deposits lots of kinetic energy in the outer envelope of the star. This envelope can contain over half of the total mass of the original massive star, and the hydrogen-rich envelope is ejected into interstellar space at speeds of 10,000 kilometers per second. What remains behind is the imploded core of the star which can be a spinning neutron star, or a black hole if the star was more massive than about 8 times the Sun. If the star had a mass somewhere in the range from about 5 - 8 solar masses, the explosion could conceivably destroy the core itself leaving behind nothing at all.
Type I supernovae, on the other hand, seem to favor binary star systems in which one star is an evolved red giant, and the other is a white dwarf. We think what happens here is that as the red giant expands, some of the mass in its outer layers is captured by the orbiting white dwarf. This material is parked in an orbiting accretion disk surrounding the white dwarf, and the matter captured from the red giant companion eventually makes its way from the outer edge of the accretion disk into the inner edge where it then falls onto the surface of the white dwarf. Over time, enough mass collects on the surface of the white dwarf that a fusion reaction can become self- sustaining and lead to explosive detonation. In the so-called recurrent novae, this explosion is limited to the mass collected near the surface of the white dwarf. The accretion disk is destroyed, but eventually re-forms and the process of accumulation and detonation starts again. For Type I supernovae, however, the entire white dwarf is involved and the detonation essentially destroys the white dwarf completely.
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