- Long history: Supernova observations have history and date back several
thousand years. Chinese Astronomers already reported "guest
stars", e.g. in 185 A.D.. Another famous Supernova
was observed in 1006. It was reported to be more luminous
than a quarter of the moon. Further prominent observations
were recorded in 1054, 1572 and 1604. And since the systematic
search of Zwicky and Baade in the 1930's ,
innumerable supernova observations have been registered.
The energy liberated in core collapse is tremendeous,
it streams away at a rate of a few times 10^53
erg/s in the form of neutrinos (10^7 erg/s = 1 Watt).
The electromagnetic luminosity
of ~10^48 erg/s can outshine the whole galaxy, about 10^41 erg/s is
in the visible range. Baade and Zwicky speculated correctly:
"In the supernova process mass in bulk is annihilated".
- Many observations: In the mean time, powerful telescopes amplify the sensitivity
of supernova recordings and the observations extend to very
distant galaxies. The classification of supernovae is based
on the most basic observation: the lightcurve and its time-dependent
spectral information. In some events,
the velocities and composition of the ejecta can be measured
and spectropolarimetry helps to determine the global asymmetry
in the explosion geometry. Mixing between the different
ejected stellar layers needs to be considered to interpret
the measurements. The lightcurve is formed in the outer layers
long after the time when the explosion was launched close to the
center of the star. The recombination of hydrogen releases
trapped radiation, followed by the decay of ejected
56Ni to 56Co on a timescale of 6 days and 56Co to 56Fe
on a timescale of 77 days.
Neutrinos, that interact only weakly with matter and therefore
have a long mean free path, can help to observationally constrain
the evolution of the deep layers. The
most direct observational evidence of a core collapse has
been produced by the (unfortunately unique and scanty) detection
of neutrinos from SN1987A in the large Magellanic cloud
at detectors in Kamioka, IMB and Baksan. We look forward to
future and more detailed neutrino recordings from galactic
Other observations focus on the information imprinted on
the environment of past supernova events. The expanding
shell of the shocked interstellar medium around the explosion
can be observed over thousands of years and a central object may
reveal its presence in the form of a pulsar, an accreting neutronstar
or a black hole. The velocity distribution of the neutron
stars and the geometry of the supernova remnant with respect
to a possible central object give further clues on the
asymmetry in the explosion. Last but not least, the explosion
mechanism is likely to affect the composition of ejected matter.
Metal-poor stars formed in the environment of an early supernova explosion
may have been contaminated by the ejecta of just one supernova event.
The analysis of the spectra of the contaminated star allows a quantitative
reconstruction of the composition of the ejecta for many more events than
the direct observation of closeby supernovae would have allowed. On more
metal-rich stars, the ejecta of multiple supernova events have merged
during galactic evolution. The mixed composition should explain the solar
abundance pattern together with possible other astrophysical events
contributing to the galactic nucleosynthesis.
- Rich theory:
One distinguishes at least two fundamental types of
supernovae: thermonuclear explosions initiated from a white
dwarf stage; or the explosive ejection of outer layers in
the aftermath of the collapse of the inner stellar core.
Core collapse is inevitable when the energy generation by
nuclear fusion at the center of the star dies away because
the nuclei have reached the configuration with maximum binding energy.
Many different fields of physics contribute to the understanding
of stellar core collapse and the subsequent supernova explosion:
Involved are at the beginning stellar evolution theory for the
progenitor model and nuclear and weak interaction physics in
experimentally accessible and inaccessible regimes: e.g. the
equation of state which determines the composition and pressure
as a function of the thermodynamical conditions, the properties of
weak interactions with heavy nuclei during collapse, the phase
transition from isolated nuclei to bulk nuclear matter shortly before
bounce, the compact object at the center of the event where nuclear
densities are exceeded, relativistic fluid dynamics in curved
space-time, neutrino radiation transport and magneto-hydrodynamics
in a convectively unstable hot plasma surrounding the compact object,
the nucleosynthesis of the ejecta and the formation of the light
curve, and finally the galactic evolution resulting from the
superposition of ejecta from many different supernova explosions.
Matter is subject to extreme conditions that cannot be attained in
- Open questions: In spite of the long time
since the basic energy source of supernovae has been identified with the
gravitational energy of collapsing stars and in spite of the many
supernova observations recorded since then, supernova simulations have
still great difficulty to reproduce the event based on known physics and
within the technical limitations set by current computer hardware.
And even if more supernova models would succeed in explaining the
explosion itself, it would still be a long way to coherently and
quantitatively understand the event that connects the stellar evolution
to an essential part of element production in the galactic evolution.
How important is the heating by emitted
neutrinos behind the shock, where do fluid instabilities modify the
transport processes, what is the role of small rotation
and magnetic fields, how does the neutronstar receive a kick
velocity, what geometrical shape does the explosion take in different
phases, how are the observables formed, etc.?
- Human timescale (of the event - of
course - not the simulations!): The supernova "event"
is a real event. Many allegations to human life can be made
and an action-loaded terminology is used in popular articles
(e.g. in "the end of a stars life", "nuclear
fuel" (star = working engine), "collapse",
"shock", "burst", "breakout",
"explosion", "nascent neutronstar").
And indeed, most of these stages are perceived as dramatic
because they occur on human time scales, reaching from milliseconds
to weeks. With an extremely generous portion of luck, one
could even observe a supernova in our galaxy without any
special equipment . Moreover, many materials
in our daily life rely on heavy elements that are believed
to have once been synthesized in supernova explosions.