Knowledge Organiser: Life Cycle of Stars

Sun-Like Star Path

1. Nebula
2. Protostar
3. Main sequence (billions of years)
4. Red giant
5. Planetary nebula
6. White dwarf

Massive Star Path

1. Nebula
2. Protostar
3. Main sequence (millions of years)
4. Red supergiant
5. Supernova
6. Neutron star OR black hole

Key Concepts

• Main sequence: radiation pressure balances gravity
• Elements up to Fe: made in stellar fusion
• Elements heavier than Fe: only in supernovae
• More mass = shorter lifetime
• Neutron star: core 1.4–3 solar masses
• Black hole: core over 3 solar masses

Key Terms

• Nebula — gas and dust cloud
• White dwarf — dense remnant, no fusion
• Supernova — massive star explosion
• Neutron star — billion tonnes per teaspoon
• Black hole — no light can escape
• Protostar — collapsing cloud, not yet fusing

Key Equations

• Main sequence fusion: 4H → He + energy (hydrogen → helium)
• Gravitational collapse balanced by radiation pressure during main sequence
• Massive stars: fusion continues to iron (heaviest element formed in stars)
• E = mc² — mass converted to energy during nuclear fusion

Common Mistakes

• Confusing the end states of different mass stars: Sun-like stars end as white dwarfs; massive stars end as neutron stars or black holes — the final stage depends entirely on the star's mass
• Saying a white dwarf still undergoes fusion: A white dwarf is a stellar remnant with no fusion — it simply cools over billions of years; fusion has stopped completely
• Getting the stellar sequence wrong: Sun-like: nebula → protostar → main sequence → red giant → planetary nebula → white dwarf; massive: nebula → protostar → main sequence → red supergiant → supernova → neutron star/black hole
• Confusing nebula (start) and planetary nebula (end): A nebula is a gas/dust cloud from which stars form; a planetary nebula is the ejected outer layers of a dying sun-like star — they are different objects despite the similar name
• Saying supernovae only destroy stars: Supernovae scatter heavy elements (heavier than iron) into space — these elements formed the material for planets and life; supernovae are creative as well as destructive