<P> Stellar nucleosynthesis is the nuclear process by which new nuclei are produced . It occurs in stars during stellar evolution . It is responsible for the galactic abundances of elements from carbon to iron . Stars are thermonuclear furnaces in which H and He are fused into heavier nuclei by increasingly high temperatures as the composition of the core evolves . Of particular importance is carbon, because its formation from He is a bottleneck in the entire process . Carbon is produced by the triple - alpha process in all stars . Carbon is also the main element that causes the release of free neutrons within stars, giving rise to the s - process, in which the slow absorption of neutrons converts iron into elements heavier than iron and nickel . </P> <P> The products of stellar nucleosynthesis are generally dispersed into the interstellar gas through mass loss episodes and the stellar winds of low mass stars . The mass loss events can be witnessed today in the planetary nebulae phase of low - mass star evolution, and the explosive ending of stars, called supernovae, of those with more than eight times the mass of the Sun . </P> <P> The first direct proof that nucleosynthesis occurs in stars was the astronomical observation that interstellar gas has become enriched with heavy elements as time passed . As a result, stars that were born from it late in the galaxy, formed with much higher initial heavy element abundances than those that had formed earlier . The detection of technetium in the atmosphere of a red giant star in 1952, by spectroscopy, provided the first evidence of nuclear activity within stars . Because technetium is radioactive, with a half - life much less than the age of the star, its abundance must reflect its recent creation within that star . Equally convincing evidence of the stellar origin of heavy elements, is the large overabundances of specific stable elements found in stellar atmospheres of asymptotic giant branch stars . Observation of barium abundances some 20 - 50 times greater than found in unevolved stars is evidence of the operation of the s - process within such stars . Many modern proofs of stellar nucleosynthesis are provided by the isotopic compositions of stardust, solid grains that have condensed from the gases of individual stars and which have been extracted from meteorites . Stardust is one component of cosmic dust, and is frequently called presolar grains . The measured isotopic compositions in stardust grains demonstrate many aspects of nucleosynthesis within the stars from which the grains condensed during the star's late - life mass - loss episodes . </P> <P> Supernova nucleosynthesis occurs in the energetic environment in supernovae, in which the elements between silicon and nickel are synthesized in quasiequilibrium established during fast fusion that attaches by reciprocating balanced nuclear reactions to Si . Quasiequilibrium can be thought of as almost equilibrium except for a high abundance of the Si nuclei in the feverishly burning mix . This concept was the most important discovery in nucleosynthesis theory of the intermediate - mass elements since Hoyle's 1954 paper because it provided an overarching understanding of the abundant and chemically important elements between silicon (A = 28) and nickel (A = 60). It replaced the incorrect although much cited alpha process of the B FH paper, which inadvertently obscured Hoyle's better 1954 theory . Further nucleosynthesis processes can occur, in particular the r - process (rapid process) described by the B FH paper and first calculated by Seeger, Fowler and Clayton, in which the most neutron - rich isotopes of elements heavier than nickel are produced by rapid absorption of free neutrons . The creation of free neutrons by electron capture during the rapid compression of the supernova core along with assembly of some neutron - rich seed nuclei makes the r - process a primary process, and one that can occur even in a star of pure H and He . This is in contrast to the B FH designation of the process as a secondary process . This promising scenario, though generally supported by supernova experts, has yet to achieve a totally satisfactory calculation of r - process abundances . The primary r - process has been confirmed by astronomers who have observed old stars born when galactic metallicity was still small, that nonetheless contain their complement of r - process nuclei; thereby demonstrating that the metallicity is a product of an internal process . The r - process is responsible for our natural cohort of radioactive elements, such as uranium and thorium, as well as the most neutron - rich isotopes of each heavy element . </P>

Where do the heaviest elements in our solar system come from