Supernova -
Cont'd
Type II -
Cont'd
As the density in the collapsing core skyrockets, electrons and
protons are pushed together until their electrical attraction
overcomes their inherent nuclear repulsion from each other. This
combination, a process called "electron capture", creates a neutron
and releases a neutrino. The neutrinos escape from the core,
carrying away energy and further accelerating the collapse, which
proceeds in milliseconds as the core detaches from the outer layers
of the star and reaches the density of nuclear matter, where the
neutrons press against each other and the entire core is the
density of an atomic nucleus. This is the core collapse. At this
point neutron degeneracy pressure is sufficient to balance gravity;
however the core has actually overshot the equilibrium point and
undergoes a slight bounce, creating a shock wave which slams into
the collapsing outer layers of the star. A "proto-neutron star"
begins to form at the core, though if it is massive enough, it will
continue collapsing to form a black hole.
Currently it is not understood how this shock wave completes the
process of pressuring the outer layer of the star. This is the
"supernova problem", and various models suggest different ways that
the huge amounts of energy present are formed into a single
explosion. One such model is the convective overturnmodel, which
suggests that convection, either from neutrinos from below, or
infalling matter from above, completes the process of destroying
the progenitor star. Heavier elements than iron are formed during
this explosion by neutron capture, and from the pressure of the
neutrinos pressing into the boundary of the "neutrinosphere",
seeding the surrounding space with a cloud of gas and dust which is
richer in heavy elements than the one the star originally formed
from.
Neutrino physics, which is not well understood, is crucial to the
understanding of this process, since half of the energy of the core
collapse is radiated out in the form of neutrinos. The other
crucial area of investigation is the "hydrology" of the plasma that
makes up the dying star, how it behaves during the core collapse
determines when and how the "shock wave" forms and when and how it
"stalls" and is re-energized.
The remaining core of the star may become a neutron star or a black
hole, depending on its mass, although because the processes of
supernova collapse are poorly understood, it is unknown what the
cutoff mass is.
Type II supernovae can be further classified based on the shape of
their light curves into Type II-P and Type II-L. Type II-P reach a
"plateau" in their light curve while II-Ls have a "linear" decrease
in their light curve ("linear" in magnitude versus time, or
exponential in luminosity versus time). This is believed to result
from differences in the envelope of the stars. II-Ps have a large
hydrogen envelope that traps energy released in the form of gamma
rays and releases it slowly, while II-Ls are believed to have much
smaller envelopes converting less of the gamma ray energy into
visible light.
One can also sub-divide supernovae of Type II based on their
spectra. While most Type II supernova show very broad emission
lines which indicate expansion velocities of many thousands of
km/s, some have relatively narrow features which may be produced by
the interaction of the ejecta with circumstellar material; these
are called Type IIn, where the "n" stands for "narrow".
A few supernovae, such as SN 1987K and 1993J, appear to change
types: they show lines of hydrogen at early times, but, over a
period of weeks to months, become dominated by lines of helium. The
term "Type IIb" is used to describe the combination of features
normally associated with Types II and Ib. These are likely massive
stars which have lost most, but not all, of their hydrogen
envelopes. As the ejecta expand, the hydrogen layer quickly becomes
optically thin and reveals the deeper layers.
Some exceptionally large stars may instead produce a "hypernova"
when they die, a theoretical type of explosion. In the hypernova
mechanism, the core of a very massive star collapses directly into
a black hole and two extremely energetic jets of plasma are emitted
from its rotational poles at nearly light speed. These jets emit
intense gamma rays, and are one of many candidate explanations for
gamma ray bursts.
Naming of Supernovae
Supernova discoveries are reported to the International
Astronomical Union's Central
Bureau for Astronomical Telegrams, which sends out a circular
with the name it assigns to it. The name is formed by the year of
discovery, and a one- or two-letter designation. The first 26
supernovae of the year get a letter from A to Z. After Z, they
start with aa, ab, and so on.
Notable supernovae
* 1006 SN 1006 Extremely bright supernova; accounts found in
Egypt, Iraq, Italy, Switzerland, China, Japan, and possibly France
and Syria
* 1054 SN 1054 the formation of the Crab Nebula, recorded by
Chinese astronomers and possibly by Native Americans
* 1181 SN 1181 Recorded by Chinese and Japanese astronomers,
supernova in Cassiopeia most likely left as its remnant the strange
star 3C 58.
* 1572 SN 1572 Supernova in Cassiopeia, observed by Tycho
Brahe, whose book De Nova Stella on the subject gives us the word
"nova"
* 1604 SN 1604 Supernova in Ophiuchus, observed by Johannes
Kepler; last supernova to be observed in the Milky Way
* 1885 S Andromedae in the Andromeda Galaxy, discovered by Ernst
Hartwig
* 1987 Supernova 1987A in the Large Magellanic Cloud, observed
within hours of its start, it was the first opportunity for modern
theories of supernova formation to be tested against
observations.
The Crab Nebula is an expanding
cloud of gas created by the 1054 supernova.
The 1604 supernova was used by Galileo as evidence against the
Aristotelian dogma of his period, that the heavens never
changed.
Supernovae often leave behind supernova remnants; the study of
these objects has helped to increase our knowledge of
supernovae.
Role of supernovae in stellar evolution
Supernovae tend to enrich the surrounding interstellar medium with
metals (for astronomers, metals are all the elements after helium).
Thus, each stellar generation has a slightly different composition,
going from an almost pure mixture of hydrogen and helium to a more
metal-rich composition. The different chemical abundances have
important influences on the star's life, and may decisively
influence the possibility of having planets orbiting it.
Issue #20
