All stars form from nebulae (clouds of gas and dust that collapse under gravity)

Astrophysics & Cosmology - OCR A-Level Physics

All stars form from nebulaeClouds of gas and dust in space; the birthplace of stars when regions collapse under gravity. (clouds of gas and dust that collapse under gravity).
During collapse, the protostar heats up until hydrogen fusion begins in the core, and the star joins the main sequenceThe diagonal band on the HR diagram where stars spend most of their lives, fusing hydrogen into helium in their cores..
Stars spend most of their lifetime on the main sequence. More massive stars burn fuel faster and have shorter lives.

Low/medium mass starsStars like the Sun that end their lives as white dwarfs after passing through the red giant phase. (like the Sun, up to ~8 $M_{\odot}$):
Main sequence -> Red giant (hydrogen shell burning, helium core) -> Helium flash and further fusion -> Planetary nebula (outer layers ejected) -> White dwarfThe remnant of a low-mass star after it has shed its outer layers as a planetary nebula. Supported against gravitational collapse by electron degeneracy pressureForce per unit area. Measured in pascals (Pa), where 1 Pa = 1 N m⁻².. (hot, dense remnant that slowly cools).
White dwarfs are supported against gravity by electron degeneracy pressureForce per unit area. Measured in pascals (Pa), where 1 Pa = 1 N m⁻²..
Maximum white dwarfThe remnant of a low-mass star after it has shed its outer layers as a planetary nebula. Supported against gravitational collapse by electron degeneracy pressureForce per unit area. Measured in pascals (Pa), where 1 Pa = 1 N m⁻².. mass = Chandrasekhar limitThe maximum mass of a stable white dwarf (~1.4 solar masses). Above this, the remnant collapses further. $\approx 1.4$ $M_{\odot}.$

High mass starsStars much more massive than the Sun that evolve faster, becoming supergiants before exploding as supernovae. (above ~8 $M_{\odot}$):
Main sequence -> Red supergiant (successive shell burning of heavier elements up to iron) -> Core collapse supernova (catastrophic explosion when iron core can no longer fuse) -> Neutron starThe extremely dense remnant of a high-mass star after a supernova. Supported against gravitational collapse by neutron degeneracy pressure. Mass typically 1.4-3 solar masses. or black holeA region of spacetime where gravity is so strong that nothing, not even light, can escape. Formed when a stellar remnant exceeds approximately 3 solar masses..
If remnant mass < ~3 $M_{\odot}$: neutron starThe extremely dense remnant of a high-mass star after a supernova. Supported against gravitational collapse by neutron degeneracy pressure. Mass typically 1.4-3 solar masses. (supported by neutron degeneracy pressure).
If remnant mass > ~3 $M_{\odot}$: black holeA region of spacetime where gravity is so strong that nothing, not even light, can escape. Formed when a stellar remnant exceeds approximately 3 solar masses. (no known force can prevent further collapse).
Supernovae create and distribute elements heavier than iron into the interstellar medium.

Examiner Tips and Tricks

Learn the two pathways precisely.
A 6-mark question on stellar evolution is very common.
Include: what drives each stage (fuel running out, core collapse), what supports the final remnant (electron/neutron degeneracy or nothing for black holes), and the mass thresholds.