Type I and type II supernovae have some characteristics in common, while others are very different.
Type I supernovae consist of explosions of white dwarf stars composed mainly of oxygen and carbon. The white dwarf absorbs the mass of a nearby colliding neutron star to increase to a mass 1.4 times our own. The resulting density and temperature conditions cause the carbon to ignite explosively. In a second, a nuclear fireball is created and the entire star is launched into the kingdom to come. There is no remnant left. The entire mass of the star is ejected into space at speeds of 6,000 to 8,000 miles per second. These projectiles consist mainly of heavier elements resulting from the nuclear fusion process, in addition to a small amount of oxygen and carbon. White dwarfs contain almost no hydrogen, and post-explosion measurements have been consistent with this. Very little hydrogen has been found in the spectrum of Type I supernovae.
This is not true for Type II supernovae. Type II supernovae occur when stars with masses greater than eight solar masses run out of nuclear energy and implode on themselves asymmetrically. The exact causes of the Type II explosion remain undetermined. The ejection of neutrinos from the condensed core is known to be a factor, as neutrinos contain hundreds of times the energy needed to cause the explosion. However, it has been speculated that neutrinos may actually carry too much energy away from the star. The core is left with very little energy for the necessary combustion. Theories have been proposed in which the emission of streams of mass energy known as “jets” or the creation of acoustic shock waves is responsible for the explosion. Computer simulations hope to shed more light on these theories in the future.
Another known difference between Type I supernovae and Type II supernovae lies in the characteristics of the light show emitted during the explosion. Type I supernovae always have a brightness of almost 4 billion times our sun at the time of explosion. It follows a constantly diminishing pattern of light. The subsequent decrease in light at this constant rate is due to the radioactive decay of the heavier elements mentioned above. Radioactive decay follows the universal time law of half-lives, with different elements having different half-lives as one of its properties. This can be used to measure the distance to nearby stars by considering Type I supernovae as the so-called “standard candles”.
In Type II supernovae, the “light curve” increases to a plateau a few months after the explosion. This comes from the expansion and cooling of the outer limits of the resulting gas ball. Computer simulations verify this through the presence of large amounts of helium and hydrogen in the Type II light spectrum, gases that would be expected to be found after the decomposition of stellar materials from this type of explosion.
Type II supernovae are never found in elliptical galaxies. Rather, its stars are generally found in the disks of the spiral arms of galaxies. For this reason, they are believed to be Population I stars. Population I stars make up about two percent of stars and tend to form from the heavier elements of earlier giant stars. They are young, warm and bright.
Type I supernovae, on the other hand, generally occur in the nuclei of elliptical galaxies. They are believed to be from Population Stars II. Population II stars are older, cooler, less luminous, and composed of lighter elements.
Although the differences between Type I and Type II supernovae make them appear as different as apples and oranges, both have their origin in supermassive star explosions due to the collapse of their nuclei and their consequent fusion processes. Therefore, they belong to the same class of natural phenomena. Both play critical roles in stellar evolution, and both contain enough unanswered questions to keep astrophysicists curious about the unpredictable future.