Saturday, March 9, 2013

The Universe’s Brightest Explosions

Supernovae (plural of supernova) are the most luminous stellar explosions in the universe. Over a period of several days to several months, a typical supernova can produce as much energy as the Sun emits during its entire multi-billion year life span. A supernova can be created either by runaway nuclear fusion in a white dwarf star or by the sudden gravitational collapse of the core of a massive star. There is a rare type of supernova known as a pair-instability supernova and such a supernova can blaze up to around 100 times more luminous than a typical supernova. This makes pair-instability supernovae the brightest stellar explosions known in the universe.

Pair-instability supernovae only happen for very massive stars (140 to 260 solar mass) that have a very low abundance of elements heavier than hydrogen and helium. The low abundance of heavier elements reduces mass loss and keeps these stars sufficiently massive to eventually explode as pair-instability supernovae. Pair instability occurs when the thermal energy in the core of a massive star becomes large enough to produce electron-positron pairs. This process reduces radiation pressure that supports the star and causes the star to contract. The situation rapidly runs out of control as the contraction triggers an explosive thermonuclear burning of oxygen and silicon in the star’s core which occurs over a span of just a few seconds. More thermal energy is released than the star’s own gravitational binding energy and this eventually destroys the star, leaving no black hole or any other remnant object behind.

During the explosive thermonuclear burning event, a significant fraction of the star’s core is transformed into radioactive nickel-56. For the most massive stars, up to 40 solar mass of nickel-56 can be synthesized. Nickel-56 is a radioactive isotope which decays with a half-life of 6.1 days into cobalt-56 which then further decays with a half-life of 77.2 days into stable iron-56. The exceptionally large amount of nickel-56 being produced in a pair-instability supernova means that the radioactive decay of nickel-56 into iron-56 powers a very luminous light curve which lasts for a few hundred days. In comparison, the luminosity of a typical supernova only lasts for up to 100 days or so.

For a star between 100 to 140 solar mass, a true pair-instability supernova does not occur. Instead, the star undergoes a “pulsational pair-instability supernova”. In this case, the explosive thermonuclear burning is insufficient to completely unbind the star is it produces less energy than the star’s gravitational binding energy. Nevertheless, many solar masses of material are still ejected from the star. The core of the star then contracts into a stable burning state before the next explosion occurs. A “pulsational pair-instability supernova” can be extremely luminous as the highly energetic ejecta from the second supernova plows into the ejecta from the first supernova. Finally, a star with over 260 solar mass does not explode as a pair-instability supernova since the temperature in the core of such a star becomes high enough for alpha particles to photo-disintegrate into free nucleons. This process consumes as much energy as that produced by all the preceding thermonuclear burning. As a result, gravity prevails and the star collapses into a black hole.

Population III stars are the first stars to form in the universe. These stars consist only of hydrogen and helium since heavier elements have yet to be synthesized by nuclear fusion in stars. The absence of heavier elements makes it easier to form stars of higher masses than those existing in the current universe. Population III stars between 140 to 260 solar mass serve as suitable progenitors of pair-instability supernovae. Since Population III stars are expected to exist only during the beginning of the universe, they can only be found at the edge of the observable universe. As a result, observations of pair-instability supernovae from Population III stars will be affected by a large amount of cosmological time dilation. Together with the intrinsically slow light curve evolution of pair-instability supernovae, observations of these enormous stellar explosions will require multi-year baselines.