IB Physics HLTopic 5 — Fusion & StarsPaper 1 & 2gravity vs pressure~15 min read
How Stars Form
Every star — including the Sun — began life as nothing more than a cold, thin cloud of gas and dust drifting in space. So how do you get from a cloud to a blazing ball of fusion? The answer is a tug-of-war between two forces: gravity pulling inward and pressure pushing outward. When gravity wins, the cloud collapses and heats up until fusion ignites. When the two forces balance, you have a stable star. Let’s follow that story.
📚 What you need to know
Stars form from a giant cloud of gas and dust called a nebula
Gravitational collapse pulls the matter together, heating it up
The collapsing, glowing ball of gas is called a protostar
Fusion begins when the core reaches millions of Kelvin
For fusion, nuclei need enough kinetic energy to overcome Coulomb repulsion
This needs very high temperature (~10–100 million K) and high pressure/density
A stable star sits in equilibrium: outward pressure balances inward gravity
Outward forces = radiation pressure + gas pressure; inward force = gravity (weight)
From nebula to protostar
It all starts with a nebula — a vast cloud of hydrogen gas and dust. Gravity acts between all the particles, pulling them together into denser and denser clumps. This inward movement is called gravitational collapse.
As the cloud collapses, the particles crash into each other more and more. This does work on them, increasing their kinetic energy and so raising the temperature. The clump heats up and begins to glow, forming a protostar. Protostars aren’t hot enough to shine in visible light yet, so we detect them by their infrared radiation.
Gravitational collapse turns a diffuse nebula into a hot, glowing protostar; once the core is hot enough, fusion ignites and a stable star is born.
Conditions for fusion
Fusion doesn’t switch on until conditions in the core are extreme. Remember, the nuclei repel each other electrostatically (the Coulomb repulsion), so they need very high kinetic energy to get close enough for the strong force to fuse them. That requires:
Very high temperature — on the scale of tens to a hundred million Kelvin, so nuclei move fast enough.
Very high pressure and density — so collisions between nuclei are frequent enough.
Once the protostar’s core hits these conditions, four hydrogen nuclei begin fusing into helium, releasing enormous energy and producing an outward radiation pressure.
The chain of cause and effect here is a favourite exam question, so learn it as a story: gravity collapses the cloud → collisions do work on the particles → kinetic energy rises → temperature rises → nuclei move fast enough to beat the Coulomb repulsion → fusion begins. Every arrow in that chain is a mark. Learn the sequence, not just the endpoints.
Equilibrium in stars
Here’s the key idea for a stable star. Once fusion starts, its energy creates an outward push — the radiation pressure from photons, plus the ordinary gas pressure. These outward forces balance the inward pull of the star’s own gravity (its weight). When the two are equal, the star is in equilibrium and stays the same size for millions or billions of years.
Outward radiation and gas pressure balance the inward pull of gravity. As long as they stay equal, the star holds its size — this is stellar equilibrium.
This balance is self-correcting, which is what keeps stars so steady:
If the star gets hotter, the outward pressure rises above gravity, so the star expands — which cools it back down.
If the star cools, the outward pressure drops below gravity, so the star contracts — which heats it back up.
As long as the two forces stay balanced, the star remains stable. This stable phase is called the main sequence, and stars spend most of their lives there.
WE 1
Explain, step by step, how the collapse of a nebula leads to the start of nuclear fusion.
Step 1 — gravity
Gravitational attraction pulls the gas and dust together — gravitational collapse.
Step 2 — work done
Collisions between particles do work, raising their kinetic energy.
Step 3 — temperature
More kinetic energy means a higher core temperature.
Step 4 — fusion
Eventually nuclei move fast enough to overcome Coulomb repulsion, and fusion begins.
collapse → heating → high KE → fusionFollow the causal chain in order. Each link (collapse, work, KE, temperature, overcoming repulsion) is a separate marking point.
WE 2
A main sequence star is described as being “in equilibrium”. State the forces involved and explain what happens if the star’s core temperature rises slightly.
Step 1 — the forces
Outward: radiation pressure + gas pressure. Inward: gravity.
Step 2 — temperature rises
Higher temperature raises the outward pressure above gravity.
Step 3 — result
The star expands, which cools it and restores the balance.
Self-correcting: expands then cools back to equilibriumThe balance is stable because expansion cools the star and contraction heats it — the star automatically pushes itself back to equilibrium.
Outward = radiation + gas pressure; inward = weight.
The balance is self-correcting, which keeps stars stable for aeons.
⚠ Common mistakes
Saying the nebula “explodes” — it collapses under gravity
Forgetting that collisions do work, which is what heats the core
Listing only radiation pressure — gas pressure acts outward too
Thinking equilibrium means “no forces” — it means balanced forces
Confusing a protostar (pre-fusion) with a main sequence star
Quick recap: Stars form when a nebula undergoes gravitational collapse, heating up as collisions do work, forming a glowing protostar. When the core reaches millions of Kelvin, nuclei overcome Coulomb repulsion and fusion begins. A stable star sits in equilibrium, with outward radiation + gas pressure balancing inward gravity — a self-correcting balance that lasts most of the star’s life.
A star reaching equilibrium is only the beginning of its story. What happens next — whether it swells into a red giant, blows up as a supernova, or fades quietly — depends entirely on one thing: its mass. Next page: The Life Cycle of a Star.
Star formation and equilibrium unclear?
Book a free meeting and we’ll drill the collapse-to-fusion causal chain and the force-balance reasoning examiners reward.
