E.5.4—Stellar mass and evolution

Start when core hydrogen is depleted

Reduced core fusion lowers outward pressure, so gravity contracts and heats the core. Hydrogen shell fusion then expands the outer layers: lower-mass stars become red giants, while high-mass stars become red supergiants.

Initial mass	Later pathway	Final remnant
Lower or Sun-like	Main sequence → red giant → planetary nebula	White dwarf
High mass	Main sequence → red supergiant → supernova	Neutron star or, for a sufficiently massive remnant, black hole

Connect mass to lifetime

A more massive main-sequence star has more fuel, but its fusion rate and luminosity rise much more strongly. It therefore uses core hydrogen faster and has a shorter main-sequence lifetime.

Keep the path conditional

Mass controls the pathway; not every star becomes a supernova, and not every supernova leaves a black hole. A planetary nebula is expelled gas from a red giant, not a planet-forming stage.

How it is assessed

Questions compare main-sequence lifetimes or describe stages after a massive star leaves the main sequence.

Command terms

Describe / Compare

What earns marks

Link mass to luminosity and lifetime, then give the ordered evolution and conditional remnant endpoint.

Watch for

Giving the evolution sequence without the mass/luminosity reasoning or naming only one remnant without its condition.

Retrieve stellar balance

Fusion releases energy, outward thermal/radiation pressure balances inward gravity, and high temperature and density allow fusion in the core.

Retrieve stellar inference

Mass controls evolution; HR regions classify stars; parallax gives distance; and $L=4\pi R^2\sigma T^4$ gives stellar radius from luminosity and temperature.

E.5.4—Stellar mass and evolution

Relate Mass to Evolution

E.5.4 Exam Analysis

Assessment in practice

Retrieve the Stellar Model