loading...
running...
How to use
Choose a preset or set the star mass and drag the evolution clock to scrub through a life; Play runs it. The yellow marker on the HR diagram is the star now, the orange line its full track, the green curve the zero-age main sequence. Switch the view to Cluster to see what a population of stars of one age looks like and watch the turn-off point move with age. Drag to orbit, wheel to zoom. Space pauses, R restarts.
The observatory table, star sphere, planet, orbit and habitable-zone rings, HR board and instanced star models were modelled and baked in Blender; Three.js r160 sizes and colours the star from the same evolutionary model as the 2D lab.
The science
A star's life is a path across the Hertzsprung-Russell diagram of luminosity against surface temperature. Radius is not an independent input; it follows from the Stefan-Boltzmann law, which also draws the dashed lines of constant radius:
L = 4π R² σ T4 ⇒ R / R☉ = √(L / L☉) (5772 K / T)², λpeak = b / T (Wien)
Main sequence. Hydrogen burning converts 0.7% of the fuel mass into energy and only the core, about a tenth of the mass, burns. So the lifetime is tMS ≈ 0.1 · 0.007 M c² / L; the model uses tMS = 7 Gyr · (M/M☉) / (LZAMS/L☉) so the Sun lasts 10 Gyr and the lifetime curve falls steeply because L rises roughly as M3.5. The zero-age position comes from a table of published ZAMS values (log-interpolated). During the main sequence the star brightens as helium accumulates (Gough 1981): L = LZAMS / (1 - 0.625 t/tMS), which gives exactly 1 L☉ at 4.57 Gyr for 1 M☉.
After the main sequence. Stars below 8 M☉ climb the subgiant and red giant branch, burn helium (a red clump, or a blue loop above 2 M☉), ascend the asymptotic giant branch, eject a planetary nebula and cool as a white dwarf whose mass follows the empirical initial-final mass relation Mf = 0.109 Mi + 0.394 and whose radius follows the Eggleton mass-radius fit, so it must stay below the Chandrasekhar mass MCh = 1.46 M☉ (derived below from constants). Stars above 8 M☉ cross to a red supergiant (above 25 M☉ a Wolf-Rayet phase) and end in a core-collapse supernova, leaving a neutron star (below about 20 M☉) or a black hole.
Clusters. Cluster mode draws stars from a Salpeter mass function dN/dM ∝ M-2.35 and evolves each to the same age. The main sequence ends at the turn-off mass where tMS(M) equals the cluster age, which is how clusters are dated.
Checks. Stefan-Boltzmann for the Sun, the model Sun at 4.57 Gyr (L, T, R), Wien's law, the Chandrasekhar mass from ħ, c, G and mH, the energy budget against the model lifetime for 1 and 5 M☉, and the cluster turn-off mass against a sampled population.
Simplifications. This is a parametric track model, not a stellar-structure code: post-main-sequence phases are interpolated between representative points (luminosity, temperature and duration), solar metallicity only, no rotation, binaries, convective overshoot or detailed mass loss, a stage-weighted clock so short phases stay visible, and rough remnant masses. Treat positions as accurate to tens of percent in L and T and lifetimes to a factor of about two for massive stars.