Tuesday, December 08, 2009

PISN into the stellar wind

Whilst the lighter chemical elements found on the Earth, such as carbon and oxygen, are produced by nuclear fusion inside stars, the heavier elements from the far end of the periodic table are produced in supernovae, the explosive events by which high-mass stars end their lives.

The type of supernova described in most textbooks is a core-collapse supernova, in which the core of the star is transformed into iron by nuclear fusion, and fusion then ceases. The weight of the core is supported by a pressure gradient ultimately created by the energy released from fusion, hence when fusion ceases, the core collapses, triggering a supernova.

However, this type of supernova only occurs to stars in the range 10-140 solar masses. For stars more massive than this, their lives will actually end with a Pair Instability Supernova (PISN). Such an event occurs before the stage at which the fusion of oxygen would otherwise begin in the core. The temperatures in the core of such massive stars are so high at this stage, that the photons produced are effectively high-energy gamma rays. By virtue of their high-energy, these photons will tend to interact with the nuclei in the core by means of a mechanism called electron-positron pair production. The consequence of this is that the radiation pressure in the core plummets, leading to the collapse of the core, and a PISN supernova.

Nature for 3rd December reports that a supernova first detected in 2007, has now been verified as the first ever observational PISN. Such supernovae are difficult to observe because stars lose mass during their lifetime due to radiation-driven stellar winds, and the threshold mass for a PISN supernova increases for stars with a higher proportion of heavy elements. Each generation of stars forms from interstellar matter created by the supernovae of preceding generations, hence each successive generation of stars in a galaxy possesses ever higher proportions of heavy elements. PISN were therefore most frequent in the early universe, when stars had lower fractions of heavy elements, and when stellar winds were weaker. In fact, it is claimed that a single PISN can release more heavy elements than an entire generation of core-collapse supernovae. It is calculated, for example, that the 2007 supernova released 22 solar masses of silicon alone!

Hence, the heavy elements utilised by much of our technology on Earth, may owe their existence to the production of electron-positron pairs from high-energy gamma rays.

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