The Difference between Types I and II Supernovae

Type I and Type II supernovae have some characteristics in common while others are vastly different.

Type I supernovae consist of explosions of white dwarf stars composed primarily of oxygen and carbon. The white dwarf absorbs the mass of a colliding nearby neutron star to increase to a mass of 1.4 times our sun. Ensuing density and temperature conditions result in the carbon beginning to burn explosively. Within one second, a nuclear fireball is created and the entire star is blown to kingdom come. No remnant is left. All of the star’s mass is ejected into space at speeds from 6,000 to 8,000 miles per second. These projectiles primarily consist of heavier elements resulting from the nuclear fusion process, in addition to some small amount of oxygen and carbon. White dwarves contain almost no hydrogen and post-explosion measurements have been consistent with this. Very little presence of hydrogen has been found in the spectrum of Type I supernovae.

This is not true of Type II supernovae. Type II supernovae occur when stars with masses greater than eight solar masses run out of nuclear energy and implode upon themselves in an asymmetrical fashion. 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 the neutrinos contain hundreds of times the energy necessary to cause the explosion. However it has been speculated that the neutrinos may actually carry too much energy away from the star. The core is left with too little energy for the necessary combustion. Theories have been proposed in which either emission of mass-energy streams known as “jets” or the creation of acoustic shock waves is responsible for the blast. Computer simulations hope to shed more light on these theories in the future.

Another known difference between Type I supernovae and Type II supernovae lie in the characteristics of the light specta emitted during the explosion. Type I supernovae always have a brightness of nearly 4 billion times our sun at the time of the explosion. A steadily decreasing light pattern follows. The subsequent light decrease at this constant rate is due to the radioactive decay of the heavier elements mentioned previously. Radioactive decay follows the universal time law of half-lives, with different elements having different half-lives as one of their properties. This can be used to measure the distance to nearby stars by considering Type I supernovae as so-called “standard candles”.

In Type II supernovae the “lightcurve” increases to a plateau a few months after the explosion. This comes from the expansion and cooling of the outer limits of the resulting ball of gas. Computer simulations verify this through the presence of large amounts of helium and hydrogen in the Type II light spectrum, gases which would be expected to be found after the breakdown of star materials from this type of explosion.

Type II supernovae are never found in elliptical galaxies. Rather their stars are usually found in the disks of spiral arms of galaxies. For this reason, thse are thought to be Population I stars. Population I stars form about two percent of stars and tend to be formed from heavier elements from previous giant stars. They are young, hot and luminous.

Type I supernovae on the other hand, usually occur in the core of elliptical galaxies. They are believed to be from Population II Stars. 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, they both have their origins in explosions of super massive stars due to the collapse of their core and their ensuing fusion processes. Thus they lie in the same class of natural phenomena. Both play critical roles in stellar evolution and both contain enough unanswered questions to keep astrophysicists curious for the unforeseeable future.

David Craig

M.S. Physics - University of Minnesota

B.S. Computer Science - University of Oregon