Understandings: Stellar evolution on HR diagrams; Red giants, white dwarfs, neutron stars and black holes
Applications and skills: Sketching and interpreting evolutionary paths of stars on an HR diagram; Describing the evolution of stars off the main sequence; Describing the evolution of stars off the main sequence; Describing the role of mass in stellar evolution
Utilization: An understanding of how similar stars to our Sun have aged and evolved assists in our predictions of our fate on Earth
The radius of a star of given mass (and therefore, indirectly, its surface temperature and colour) are determined by the hydrostatic equilibrium between gravity pulling all parts of the star towards its centre of mass, and the radiation pressure pushing out. The radiation pressure is determined by the nuclear reactions taking place in the core of the star.
The basic initial reaction cycle for a star is the proton-proton cycle, 'burning' hydrogen into helium. As the protostellar mass initiates this nuclear fusion cycle then a star is truly born for the first time. As viewed on a HR diagram it takes its rightful place in the main sequence - the position along the main sequence is determined by the mass of the star. The mass of a star is the single most important variable to describe a star. More massive stars burn much, much hotter than less massive stars and consequently spend much less time on the main-sequence. The mass luminosity relationship is L ∝ M3.5 - so a star twice as massive as the sun will have twice the fuel, but will consume it 23.5 x as quickly!
As a star consumes the available hydrogen in the core then the core will shrink - converting more GPE into thermal energy, possibly enough to start the next reaction cycle, transforming helium into heavier elements. Hydrogen burning will continue in a shell around the core. Fusion of helium only happens if the core can reach a high enough temperature - and that is only true for large enough stars. The increased core temperature means greater radiation pressure - and therefore the star gets bigger! A star similar to our sun (a yellow dwarf star) will swell to become a red giant (the luminosity increases, but due to the increased surface area the power output per square meter (and therefore the temperature of that square meter) will decrease.
Oxford Physics: pp 656 - 659, including the death of stars, which is our next page
Hamper HL (2014): p 548 - 553, including the death of stars, which is our next page
Page 204 - 209