Semper simplicissimam, fundamentalem solutionem quaere. Si simpliciter explicare non potes, non intellexisti. (R. Feynman)
Astrophotography by GrandpaJunkrat - ----- Nucleosynthese
Cosmology from my armchair..............................
I do it metric .............................................................. Location:
bastion@protonmail.com The Netherlands
Bortle 7 - 9
Neutronstarfusion Heaviest Elements (Pt,Au), The merger of two neutron stars (kilonova) is an even more powerful source of the r-process for the rarest heavy elements.
The r-process (rapid neutron capture) is the mechanism responsible for creating roughly half of all atomic nuclei heavier than iron, including elements like gold (Au), platinum (Pt), and uranium (U). This process requires an extreme flux of neutrons, where atomic nuclei capture neutrons much faster than they can undergo radioactive decay. This rapid capture builds up very heavy, neutron-rich nuclei, which then stabilize through a series of beta decays to form stable, heavy elements.
This catastrophic environment is found in two main astrophysical sites:
Core-Collapse Supernovae: For decades, supernovae were thought to be the primary source of the r-process. During the gravitational collapse of a massive star's core, intense neutrino winds and shock waves can create the necessary neutron-rich conditions for a short period.
Recent evidence, most notably the kilonova associated with GW170817 in 2017, has shown that the merger of two neutron stars is an exceptionally efficient and powerful site for the r‑process. Current models indicate that such mergers are likely the dominant cosmic source of the very heaviest r‑process elements in the universe. The material ejected during these events is extremely neutron‑rich, creating the ideal conditions for rapid neutron capture.
Reference:
(1) kilonova GW170817 (Coulter et al., 2017)
Step 1 to 2:
The Post-Main Sequence Evolution of Low- and Intermediate-Mass Stars to the White Dwarf Phase (2) Alessandro Bressan, Kendall Gale Shepherd
A star of low to intermediate mass (M < 8 M☉) spends the majority of its existence on the main sequence, sustaining core hydrogen fusion via the proton-proton chain or CNO cycle. Upon the exhaustion of hydrogen in the core, the star enters a period of profound structural change.
The cessation of core fusion removes the primary source of thermal pressure support. Consequently, the inert helium core undergoes gravitational contraction, releasing potential energy that increases its temperature. This elevated temperature ignites hydrogen fusion in a thin shell surrounding the core. The immense energy output from this shell-burning source causes the stellar envelope to expand and cool dramatically, transitioning the star into the red giant branch.
The contracting helium core evolves degenerately, growing in mass and temperature until it reaches the threshold for helium ignition (T ~ 10⁸ K). For stars in this mass range, this ignition often occurs under degenerate conditions as a helium flash, a thermal runaway event that does not disrupt the star. The core subsequently stabilizes and burns helium into carbon and oxygen via the triple-alpha process.
This helium-burning phase is, however, transient. Upon the depletion of helium in the core, a carbon-oxygen (C/O) remnant is left. For stars below the critical mass of ~8 M☉, the gravitational potential is insufficient to achieve the core temperatures required for carbon fusion (T > 5 x 10⁸ K). Fusion ceases permanently at this stage.
The star now faces its terminal evolutionary state. The degenerate C/O core, supported by electron degeneracy pressure, has a maximum stable mass, the Chandrasekhar limit (~1.44 M☉). The remaining stellar envelope, weakly bound by gravity, is ejected through strong radiative-driven winds during the asymptotic giant branch phase. This ejected material is illuminated by the intense UV radiation from the exposed, hot core, ionizing it and creating a transient planetary nebula.
The remnant core, no longer capable of fusion, is now a white dwarf. It is an extremely dense object, with a mass of roughly 0.6 M☉ contained within a volume comparable to that of Earth. Its initial thermal state is characterized by a surface temperature of over 100,000 K. Lacking an internal energy source, its luminosity is derived solely from the gradual cooling of its degenerate matter. This cooling process occurs over timescales of many billions of years, with the white dwarf's evolutionary endpoint being a cold, inert black dwarf.
