HKDSE • Elective 1: Astronomy and Space Science • Stars

Stellar Evolution & H-R Diagram

Set a star's initial mass, watch it glow in 3D, and track its path on the Hertzsprung–Russell diagram — from the main sequence through the red-giant phase to a white dwarf or a supernova remnant. Values are teaching approximations, not a precision stellar model.

Teaching simplification: the time axis is schematically compressed; tracks use mass–luminosity and Stefan–Boltzmann relations suitable for HKDSE, not detailed stellar-structure codes.

Focus for observation: Compare the evolutionary tracks and main-sequence lifetimes of stars with different masses on the H-R diagram — massive stars leave the main sequence sooner and pass through the supergiant region.

Parameters

Low mass → long life, red dwarf; high mass → short life, blue giant / supernova.

Time is heavily compressed for classroom use — not real years per second.

Presets

Mini-challenge

Tap a challenge, then click the H-R diagram where you think that star belongs.

Live readings

MAIN SEQUENCE
Mass 1.0 M☉
Surface temperature 5772 K
Luminosity 1.0 L☉
Radius 1.0 R☉
Main-sequence lifetime 10 Gyr

HKDSE relation

$$ L = 4\pi R^{2}\sigma T^{4} $$
$L$: luminosity (power radiated)
$R$: stellar radius
$T$: surface (effective) temperature
$\sigma$: Stefan–Boltzmann constant

A cooler but much larger red giant can outshine a hotter main-sequence star because luminosity grows with $R^{2}$ and $T^{4}$.

3D star (colour ← T, size ← R)
Drag to rotate • pinch / scroll to zoom
Temperature $T$ Radius $R$ Luminosity $L$

Hertzsprung–Russell diagram

$T$ → cooler right · $\log L$ up

Physics note

Most stars lie on the main sequence, where hydrogen fuses to helium in the core. Higher-mass stars are hotter and more luminous, so they exhaust fuel sooner. After the main sequence a star expands into a red giant. Stars like the Sun end as white dwarfs; stars above about 8 solar masses explode as supernovae, leaving a neutron star or black hole.