Main-sequence stars sustain core hydrogen fusion. As core hydrogen is depleted, core and envelope structure change and subsequent evolution depends strongly on mass. Sun-like stars can develop giant phases and helium fusion, while sufficiently massive stars undergo further nuclear-burning stages.
Stellar fuel stages involve fusion; this relation makes no claim that fission powers main-sequence stars.
An alpha channel changes (A,Z) to (A−4,Z−2) plus ⁴He. A positive Q supplies relative kinetic energy but does not bypass the daughter–alpha Coulomb barrier. In an s-wave exterior model V(r)=κ/r, the opaque-barrier factor is T≈exp[−2∫_R^(κ/Q)√(2μ(κ/r−Q))dr/ħ]; an effective channel attempt frequency f gives λ_α≈fT, so the rate depends exponentially on the barrier as well as on nuclear structure.
Consistent bare-nucleus channel mass accounting determines Q and relative exterior kinetic energy; it does not determine the rate.
For projectile rest mass m_a and target m_A initially at rest, squared center-of-momentum energy is E_cm²=(m_a+m_A)²c⁴+2m_Ac²K_a. If the minimum allowed final invariant mass is M_f, an endothermic channel requires K_thr=[M_f²−(m_a+m_A)²]c²/(2m_A). With Q=(m_a+m_A−M_f)c² this becomes −Q(1+m_a/m_A)+Q²/(2m_Ac²), showing why laboratory threshold generally exceeds −Q.
The channel Q uses the same complete rest-mass difference; target recoil distinguishes laboratory threshold from −Q.
Fission can emit neutrons that induce further fissions, while capture without fission and leakage remove neutrons from the chain. In a fixed linear generation model the effective multiplication factor k is the ratio of successive expected neutron generations: N_g=N₀k^g. A chain is subcritical for k<1, critical for k=1 and supercritical for k>1. Criticality balances production and losses; it is not a universal material-mass threshold independent of geometry and environment.
Neutron-emitting fission can feed a next generation; consistent reaction energy does not alone establish net multiplication.
In a dilute nondegenerate gas of independently distributed reacting nuclei at common temperature T, reaction events per volume/time are R_ab=n_an_b⟨σv⟩/(1+δ_ab), where δ_ab=1 for the same species and 0 otherwise. The nonrelativistic Maxwell relative distribution gives ⟨σv⟩=√[8/(πμ)](k_BT)⁻³ᐟ²∫₀∞σ(E)Eexp[−E/(k_BT)]dE, using relative energy E=μv²/2. The identical-species factor counts each incoming pair once.
A channel's Q controls rest-energy accounting while its rate requires actual cross sections and a specified reactant distribution.