Collapse triggers only once the iron core exceeds the Chandrasekhar limit (≈1.44 M☉) — the maximum mass electron degeneracy pressure can support. Past that, the core collapses in milliseconds, bounces off nuclear density, and launches the outgoing shock.
Free expansion: R(t) = v_ej·t, v_ej = sqrt(2E/M_ej)
Sedov–Taylor: R(t) = R_ST·(t/t_ST)^(2/5) once swept-up ISM mass = M_ej
The shock coasts at constant velocity while it carries more mass than the medium it sweeps up. Once swept-up circumstellar mass equals the ejecta mass, deceleration begins and the remnant enters the self-similar Sedov–Taylor phase, radius growing as the classic t2/5 power law.
L(t) ≈ L_peak·[ w₁·e^(−t/τ_early) + w₂·e^(−t/τ_Co) ], τ_Co ≈ 111 d (⁵⁶Co decay)
The light curve rises as the expanding photosphere's area grows, peaks near maximum optical depth, then declines — first quickly as the ejecta thins, then settling onto a slower exponential tail set by the ⁵⁶Ni→⁵⁶Co→⁵⁶Fe decay chain (⁵⁶Ni half-life 6.1 d, ⁵⁶Co half-life 77.1 d). This double-exponential is a standard simplified stand-in for the true radiative-transfer curve. Photosphere colour cools from blue-white toward red as the ejecta expands and its temperature drops.
- Iron core mass — below 1.44 M☉ the core is stable; above it, collapse can be triggered.
- Explosion energy / ejecta mass — set the free-expansion velocity and how long it lasts before Sedov–Taylor deceleration begins.
- Circumstellar density — denser surroundings are swept up faster, shortening the free-expansion phase.
- Ni-56 yield — sets how bright the late radioactive tail is.