Supernovae represent a catastrophic event for a star that e ectively ends its active lifetime. These events can briefly outshine entire galaxies and radiate more energy than our sun will in its entire lifetime. They are also the primary source of heavy elements in the universe. On average, a supernova will occur about once every 25-50 years in a galaxy the size of the Milky Way. Put another way, a star explodes every second or so somewhere in the universe.Exactly how a star ends its life depends in part on its mass. A star can go supernova in one of two ways in which each type is broken up into subtypes.
Type I supernova: star accumulates matter from a nearby neighbor until a runaway nuclear reaction ignites.
Type II supernova: star runs out of nuclear fuel and collapses under its own gravity.
Since neutrinos are thought to originate only from the cores
of massive stars as they die, HALO is only sensitive to supernovae resulting from gravitational collapse (Types II, Ib/c). The gravitational
collapse mechanism occurs in massive stars heavier than approximately eight
solar masses. During the main sequence phase of its evolution the star passes through successive
stages of hydrogen, helium, carbon, neon, oxygen, and silicon fusion. At the end of
each stage the supply of nuclear fuel is reduced to a point where the pressure generate
by its burning is not enough to counterbalance the inwardly directed gravitational
force. The stellar core begins to contract causing the temperature to increase to a
high enough level to allow the fuel in the next stage to begin burning. The fuel
that is not used during each stage forms outer layers that surround the core creating
an onion-like structure with the innermost layers containing the heaviest nuclei and
the lighter elements forming the outer layers. As a star enters the late stages of its life it burns heavy elements such as oxygen
and silicon which mainly create 56Fe and other neighbouring heavy elements at its
core. The nuclear binding energy per nucleon is the greatest for the iron group and as a result no further energy can be released by nuclear fusion. Therefore the
iron produced does not burn, and accumulates in the core of the star. As the mass
of iron accumulates in the core it approaches the Chandrasekhar limit, which is the
mass at which the degeneracy pressure cannot counterbalance the gravitational force
of the outlying material. Beyond this mass the iron core will begin to collapse due to
gravity. Eventually the implosion bounces back off the core, expelling the stellar material into space — the supernova.
Approximately 99% of its gravitational potential energy is released as a prompt burst of neutrinos, a few hours before the shock wave reaches the outer envelope of the star and produces optical radiation. This is because neutrinos interact weakly with matter, and thus can pass though the star relatively unhindered. Only about 1% of the energy emerges in the shock wave, and 0.01% as optical radiation. The remnants of the supernova collapse into either a neutron star or a black hole, depending on its mass. HALO will take advantage of this to detect a supernova from its neutrino burst a few hours before the supernova can be seen, allowing ample time to give advanced notice to the astronomical community. It will also help to observe the nuclear and sub-nuclear processes that are involved in core collapse supernovae and the formation of neutron stars and black holes.
By comparing the data from HALO, which is sensitive to electron neutrinos, with the data from detectors which are sensitive to electron antineutrinos, one can do a flavour decomposition of the neutrino flux from a supernova, and look for evidence of flavor-swapping due collective neutrino-neutrino effects in the core of the supernova. The density of neutrinos in the core of a supernova is high enough that neutrinos interact significantly with each other and induce flavour changes. Electron (anti)neutrinos can become muon and tauon (anti)neutrinos, and vice versa. The difficulty with this is that the effect isn't linear, and depends on the intersection angle of the neutrinos' tragectories. Thus, there are two conditions for flux swapping to occur: the mixing angle cannot be zero, and the mass hierarchy must be inverted. A supernova core is the only place in the universe in which we can observe the effect of neutrinos scattering from each other.