A primordial star condensing from gas (in 3D-stereo)

NUCLEOSYNTHESIS


Soon after the formation of the Universe in the Big Bang, some of the hydrogen, the only element present shortly after birth, underwent nuclear fusion into helium whilst the temperature and density of the Universe was still extremely high, leaving the present ratio of hydrogen to helium of 3:1, or 24% helium-4 to be more precise. A little lithium-6 and lithium-7 was also produced at this time, as were small amounts of deuterium, helium-3 and tritium (which has decayed). After the Universe had expanded and cooled, further nucleosynthesis of the elements could only proceed when the Universe had cooled sufficiently to allow these gases to condense and to eventually form stars, where hydrogen and helium are fused to produce first more helium, then carbon, silicon, oxygen, sulphur and heavier elements up to iron. The fusion releases energy which heats the star promoting additional fusion as the nuclear reactions proceed.

Fusion can progress no further than iron because this element has the highest binding energy and will thus release no more energy when fused, but rather consume energy to fuse it, thus reducing the temperature of the star. Synthesis of elements heavier than iron had to wait until the supermassive stars turned supernova, the central region having consumed all its nuclear fuel and being no longer able to support the stars' interior against gravitational collapse into a neutron star. This event was akin to the simultaneous beta decay of all the 1057 protons in the supernova when they suddenly transformed into neutrons and released 1057 neutrinos within 10 seconds. This vast flux of neutrinos irradiating the outer layers of the supermassive star with ultra-high energy particles to such an extent that they were blasted away from the neutron star out into interstellar space and at the same time transmuted into heavier elements by r-process neutron capture events resulting in elements ranging from iron to plutonium, and perhaps up to the still undiscovered element 118 and beyond. From such dust, Earth was eventually created.

See 'Stellar Fusion'.