What happens when we stop assuming what can't be assumed?
Astrophotography by GrandpaJunkrat - ----- Nucleosynthese
Cosmology from my armchair..............................
I do it metric .............................................................. Location:
bastion@protonmail.com The Netherlands
Bortle 7 - 9
The r‑Process
The r‑process, or rapid neutron capture process, is responsible for producing roughly half of all atomic nuclei heavier than iron, including gold, platinum, and uranium. It requires an extreme flux of free neutrons, allowing atomic nuclei to capture neutrons much faster than they can undergo radioactive decay. This rapid sequence of captures produces very heavy, neutron‑rich isotopes, which later stabilize through a chain of beta decays.
Such conditions occur only in catastrophic astrophysical environments. For many years, core‑collapse supernovae were considered the primary site of the r‑process, since the collapse of a massive stellar core can briefly create neutron‑rich outflows driven by intense neutrino fluxes. However, modern observations have shown that neutron‑star mergers are far more efficient producers of the heaviest r‑process elements. The kilonova associated with GW170817 in 2017 provided direct evidence that the merger of two neutron stars ejects extremely neutron‑rich material, creating ideal conditions for rapid neutron capture. Current models indicate that neutron‑star mergers are likely the dominant cosmic source of the heaviest r‑process nuclei.
Reference:
(1) Coulter et al. (2017), kilonova GW170817.
Post‑Main‑Sequence Evolution of Low‑ and Intermediate‑Mass Stars
A star of low to intermediate mass (M < 8 M☉) spends most of its lifetime on the main sequence, sustaining hydrogen fusion in its core through the proton‑proton chain or the CNO cycle. When the hydrogen in the core is exhausted, the star undergoes a profound structural transformation.
With core fusion halted, the main source of thermal pressure disappears. The inert helium core contracts under gravity, releasing potential energy that raises its temperature. Hydrogen fusion ignites in a thin shell surrounding the core, and the enormous energy output from this shell causes the stellar envelope to expand and cool. The star ascends the red‑giant branch.
The contracting helium core becomes electron‑degenerate and continues to heat until it reaches the temperature required for helium ignition, around 10⁸ K. In stars of this mass range, helium ignition often occurs as a helium flash, a brief thermonuclear runaway that does not disrupt the star. After the flash, the core stabilizes and burns helium into carbon and oxygen through the triple‑alpha process.
This phase is temporary. When helium is depleted, the star is left with a carbon‑oxygen core. For stars below roughly 8 M☉, the core never reaches the temperatures required for carbon fusion. Fusion ceases permanently, and the star enters its final evolutionary stages.
The degenerate carbon‑oxygen core is supported by electron degeneracy pressure and cannot exceed the Chandrasekhar limit of about 1.44 M☉. During the asymptotic giant branch phase, the outer envelope is expelled through strong stellar winds. The exposed hot core ionizes this ejected material, forming a planetary nebula.
The remaining core becomes a white dwarf: an extremely dense object with a mass around 0.6 M☉ compressed into a volume comparable to Earth. Its initial surface temperature exceeds 100,000 K. With no internal energy source, the white dwarf cools slowly over billions of years, eventually fading into a cold, inert black dwarf.
Reference:
(2) Bressan & Shepherd, The Post‑Main Sequence Evolution of Low‑ and Intermediate‑Mass Stars to the White Dwarf Phase.
