IB Physics HLTopic 5 — Fusion & StarsPaper 1 & 2it all depends on mass~17 min read
The Life Cycle of a Star
Stars are born, live, and die — and the whole path a star takes is decided by one thing at its birth: its mass. Every star starts the same way, from a nebula to a main sequence star. But after that, the story splits. A star like our Sun ends quietly as a fading white dwarf. A star many times heavier goes out in a spectacular supernova, leaving behind a neutron star or a black hole. Here’s the full journey for both.
📚 What you need to know
The early stages are the same for all stars: nebula → protostar → main sequence star
What happens after the main sequence depends on the star’s mass
Low-mass stars (like the Sun): red giant → planetary nebula → white dwarf
High-mass stars: red supergiant → supernova → neutron star or black hole
A star leaves the main sequence when the hydrogen in its core runs out
Massive stars fuse elements all the way up to iron
A remnant core over ~3 solar masses collapses into a black hole
The same start for every star
No matter how big it ends up, every star begins the same way. A nebula of gas and dust collapses under gravity into a hot, glowing protostar. When the core gets hot enough, fusion ignites and the star settles into equilibrium as a main sequence star — the long, stable phase where it fuses hydrogen into helium.
The common beginningnebula → protostar → main sequence star
A star spends most of its life — around 90% of it — on the main sequence. Our Sun has been there for 4.6 billion years and will stay for roughly 5 billion more. But once the hydrogen fuel in the core runs low, the balance breaks and the star’s fate depends on its mass.
All stars share the same start. After the main sequence, low-mass stars fade to white dwarfs, while high-mass stars explode as supernovae and leave neutron stars or black holes.
Low-mass stars (like the Sun)
For a star roughly the size of the Sun, the path after the main sequence is:
Low-mass pathred giant → planetary nebula → white dwarf
Red giant: when the core hydrogen runs out, fusion there slows and the core contracts and heats up. Hydrogen fusion continues in a shell around the core, and eventually the core gets hot enough for helium fusion. This releases so much energy that the outer layers swell and cool, forming a huge, cool, red star — a red giant.
Planetary nebula: once the helium runs out, the core can’t get hot enough for further fusion. The outer layers drift away, forming a glowing shell of gas called a planetary nebula.
White dwarf: the leftover core collapses under gravity into a very hot, dense remnant called a white dwarf. With no fusion left, it simply cools down over billions of years.
High-mass stars
A star much heavier than the Sun lives fast and dies violently:
High-mass pathred supergiant → supernova → neutron star or black hole
Red supergiant: the star swells like a red giant, but its greater mass lets it fuse heavier and heavier elements in successive shells — all the way up to iron. Iron is the end of the line, because fusing it doesn’t release energy (it sits at the peak of the binding-energy curve).
Supernova: with no more fusion energy to hold it up, the iron core collapses catastrophically, and the outer layers are blown off in a colossal explosion — a supernova.
Neutron star or black hole: what’s left of the core depends on its mass. A moderate remnant becomes an incredibly dense neutron star. If the remnant core is more than about 3 solar masses, gravity is so strong that it collapses into a black hole, from which not even light can escape.
The single most important word on this whole page is mass. Everything after the main sequence branches on it: low mass gives you the gentle white-dwarf ending, high mass gives you the violent supernova. If an exam asks you to describe a star’s evolution, your very first move is to check — are we talking low mass or high mass? That decides the entire answer.
WE 1
A star with a similar mass to the Sun leaves the main sequence. Describe and explain the remaining stages of its evolution.
Step 1 — red giant
Core hydrogen runs out; fusion continues in a shell, helium fusion begins, and the outer layers swell and cool into a red giant.
Step 2 — planetary nebula
Helium runs out; the core can’t fuse further, so the outer layers are ejected as a planetary nebula.
Step 3 — white dwarf
The remnant core collapses into a hot, dense white dwarf that slowly cools.
red giant → planetary nebula → white dwarfName each stage AND explain the cause (fuel running out, core contracting, layers ejected). Both the sequence and the reasons score marks.
WE 2
Explain why a massive star can fuse elements up to iron but no further, and what happens to the star as a result.
Step 1 — why iron is the limit
Iron sits at the peak of the binding-energy-per-nucleon curve, so fusing it releases no energy.
Step 2 — loss of support
With no fusion energy, there’s no outward pressure to balance gravity, so the iron core collapses.
Step 3 — result
The collapse triggers a supernova, leaving a neutron star or black hole.
Iron = no fusion energy → core collapse → supernovaThe key physics is the binding-energy peak: once you reach iron, fusion stops giving energy, the star loses its support, and gravity wins dramatically.
⚛ Tracing a star’s life
Common start: nebula → protostar → main sequence.
Check the mass — this decides everything next.
Low mass: red giant → planetary nebula → white dwarf.
High mass: red supergiant → supernova → neutron star / black hole.
Explain each step by what happens to the fuel and the force balance.
💡 Top tips
The path after the main sequence is decided by mass.
Low mass ends as a white dwarf; high mass ends as a neutron star or black hole.
Stars leave the main sequence when core hydrogen runs out.
Massive stars fuse up to iron, then can’t go further.
A remnant core over ~3 solar masses becomes a black hole.
⚠ Common mistakes
Giving the same ending for all stars — it depends on mass
Confusing a planetary nebula (low mass) with a supernova (high mass)
Saying massive stars fuse beyond iron — iron is the limit
Mixing up red giant (low mass) and red supergiant (high mass)
Forgetting that a white dwarf has no fusion — it just cools
Quick recap: All stars start nebula → protostar → main sequence. After that, mass decides the fate. Low-mass stars go red giant → planetary nebula → white dwarf. High-mass stars go red supergiant → supernova → neutron star or black hole, fusing up to iron before the core collapses. A remnant over ~3 solar masses becomes a black hole.
We keep describing stars as “red giants”, “white dwarfs” and “main sequence” — but how do astronomers actually classify them? There’s one famous graph that sorts every star by its brightness and temperature, and reveals these families at a glance. Next page: The HR Diagram.
Stellar life cycles getting jumbled?
Book a free meeting and we’ll sort the low-mass and high-mass paths, the role of iron, and the “describe the evolution” questions examiners love.
