Semper simplicissimam, fundamentalem solutionem quaere. Si simpliciter explicare non potes, non intellexisti. (R. Feynman)
Astrophotography by GrandpaJunkrat - ----- Star evolution chemistry
Cosmology from my armchair..............................
I do it metric .............................................................. Location:
Sneek
bastion@protonmail.com The Netherlands
Bortle 7 - 9
Star evolution chemistry
Step 0.1: Gravitational Instability.
A local density fluctuation within a molecular cloud (e.g., the Cepheus Flare) exceeds the Jeans mass. The system begins to isolate itself from the background, initiating a collapse driven by self-gravity.
Step 0.3: Protostellar Accretion (The First Definition of Mass)
A central core (the protostar) forms. Matter falls from the surrounding envelope onto an accretion disk and subsequently onto the stellar surface. At this stage, energy is purely kinetic/gravitational, providing the first major increase in internal pressure.
Step 0.4 - 0.6: Deuterium burning phase, first ignition, still mainly gravity‑powered overall, (overlapping stage).
Step 0.5: T Tauri / Herbig Ae/Be Phase (The Struggle for Stability)
The object becomes optically visible but remains energetically unstable. The internal pressure forces temperatures upward, yet the primary energy source is still gravitational contraction and accretion rather than nuclear fusion. The system is characterized by high variability and a "diffuse" boundary between the star and its environment.
Step 0.7: Deuterium Burning (The First Ignition)
Central pressure reaches the threshold where deuterium fusion becomes possible. This "temporary engine" provides a brief period of stabilization. In objects with insufficient mass (Brown Dwarfs), this is the final stage of active manifestation before gradual cooling.
Step 0.9: ZAMS Approach (The Final Pressure Squeeze)
The contraction continues until the core pressure raises the temperature to approximately 10,000,000 K. At this point, the minimal internal conditions required for hydrostatic equilibrium are established, marking the transition to Step 1.
Deuterium fusion (~1 million K)
Lithium fusion (~2.5 million K)
Hydrogen fusion via the p–p chain (~4 million K) (This is the moment an object officially becomes a star.)
Hydrogen fusion via the CNO cycle (~15 million K)
Helium fusion (triple‑alpha, ~100 million K)
Step 0.8 D+p→3He+γ
Step 1: H --> He The Basic (Main Sequence) Millions and Billions of Years (All Stars)
Step 2: He --> C + O Triple alpha process. Shorter (Red Giant phase)
Step 3: C --> Ne + Mg + O Merger in massive stars. Very short (Only massive stars)
Step 4: O --> Si + S Merger in massive stars. Extremely short (Only massive stars)
Step 5: Si --> Fe Endpoint of energy-producing fusion. Final phase, lasting only days
Step 6: Supernova (r-proces) Heaviest elements (Au, U, Ag,…), The shock wave creates a huge neutron flux that forms all elements heavier than Fe.
Reference:
(2) Evolution and final fates of low- and intermediate-mass stars, Alessandro Bressan, Kendall Gale Shepherd https://arxiv.org/pdf/2412.13039)
Step 0.4–0.6, Deuterium Fusion (D + p → ³He + γ)
Temperature: ~1 million K
Occurs in: Protostars and brown dwarfs
Role:
First ignition event in stellar evolution
Provides temporary stabilization during early contraction
Only fusion process available to brown dwarfs
Step 1, Hydrogen Fusion (H → He)
Process: Proton–proton chain (or CNO cycle in massive stars)
Phase: Main Sequence
Timescale: Millions to billions of years
Occurs in: All true stars
Role:
Defines the Zero‑Age Main Sequence (ZAMS)
Longest and most stable phase of stellar life
Step 2, Helium Fusion (He → C + O)
Process: Triple‑alpha reaction
Phase: Red Giant / Horizontal Branch
Timescale: Much shorter than hydrogen burning
Occurs in: Low‑, intermediate‑, and high‑mass stars
Role:
Begins once core hydrogen is exhausted
Produces carbon and oxygen
Step 3, Carbon Fusion (C → Ne + Mg + O)
Occurs in: Only massive stars (> ~8 M☉)
Timescale: Very short
Role:
Produces neon, magnesium, and oxygen
Marks the beginning of the rapid “onion‑shell” burning stages
Step 4, Oxygen Fusion (O → Si + S)
Occurs in: Massive stars
Timescale: Extremely short
Role:
Produces silicon, sulfur, and other intermediate‑mass elements
Drives the star toward its final catastrophic phase
Step 5, Silicon Fusion (Si → Fe)
Occurs in: Massive stars
Timescale: Only days
Role:
Produces iron‑group elements (Fe, Ni, Co)
Final energy‑producing fusion stage
Iron core grows until collapse becomes inevitable
Step 6, Core‑Collapse Supernova (r‑process)
Outcome:
Shock‑driven neutron flux creates the heaviest
elements (Au, U, Ag, Pt, etc.)
Explosion disperses all newly formed elements into the interstellar medium
Leaves behind a neutron star or black hole
Bressan, A., & Shepherd, K. G. (2024). Evolution and final fates of low- and intermediate-mass stars. arXiv:2412.13039.
Mass Range Classification Fusion Capability
Planet No fusion at all
13–65 Jupiter masses Brown dwarf Deuterium fusion only
65–80 Jupiter masses Massive brown dwarf Deuterium + lithium fusion
> 80 Jupiter masses (~0.08 M☉) Star Sustains hydrogen fusion (p–p chain)
The Chemical Universe: A Flowchart of Stellar Evolution (CNO-cyclus)
Uitleg bij elementen in het diagram,
Star‑Evolution Diagram
Colour Index in the UBVRI System
The colour index (B–V) used in this overview is calibrated within the UBVRI photometric system. Since its refinement in the 1950s, this system has provided a vast and homogeneous dataset for thousands of stars, enabling a consistent and reliable mapping between stellar colour and effective temperature.
The initiation of stable hydrogen fusion marks a protostar’s arrival on the zero‑age main sequence (ZAMS). This transition requires core temperatures exceeding approximately 4×106 K, achieved through sustained gravitational contraction. At these temperatures, protons can overcome the Coulomb barrier via quantum tunneling, allowing nuclear fusion to proceed.
Once hydrogen burning begins, the dominant fusion pathway depends primarily on stellar mass and core temperature. Two distinct mechanisms govern this phase:
The PP chain dominates in low‑mass stars (M≲1.5 M⊙), including the Sun.
In this sequence, helium is produced through the direct fusion of hydrogen nuclei.
Because the PP chain has a relatively weak temperature dependence, it remains efficient at the moderate core temperatures typical of low‑mass stars. This leads to stable, long main‑sequence lifetimes.
In more massive stars (M≳1.5 M⊙), hydrogen fusion proceeds primarily through the CNO cycle.
This catalytic process uses carbon, nitrogen, and oxygen isotopes as intermediaries to convert hydrogen into helium.
Its reaction rate is extremely temperature‑sensitive (ε∝T4–T18), causing energy generation to be strongly concentrated in the stellar core.
The resulting steep temperature gradient requires efficient convective energy transport, leading to the formation of a convective core.
This fundamental difference in energy generation explains the shorter main‑sequence lifetimes, higher luminosities, and more dramatic evolutionary endpoints of high‑mass stars.
The following sections examine two key aspects in more detail:
De fysische parameters die de latere evolutie bepalen: massa, rotatie, metalliciteit en accretiegeschiedenis.
Hoe massa de interne structuur, convectie, energieproductie en levensduur van de ster bepaalt.
