The Difference between Types I and II Supernovae
By David Craig
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
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