A star doesn’t just switch on. It begins as a cold, thin cloud of gas drifting in space, and gravity spends millions of years pulling it into a ball dense and hot enough to ignite fusion. Once it does, a delicate tug-of-war between gravity pulling in and pressure pushing out keeps the star steady — sometimes for billions of years.
📘 What you need to know
Stars form from a giant cloud of hydrogen gas and dust called a nebula
Gravity pulls the cloud together (gravitational collapse), and the collapsing gas heats up to form a protostar
Fusion needs very high temperature (~107–108 K) and very high pressure and density so nuclei have enough kinetic energy to overcome their repulsion
When the core is hot enough, hydrogen fusion begins and the object becomes a main sequence star
The fusion energy creates an outward radiation pressure (plus gas pressure) that balances the inward pull of gravity
This balance is called equilibrium — while the two forces are balanced, the star is stable
Sequence to remember: nebula → protostar → main sequence star
From a Cloud to a Protostar
Every star starts life as a nebula: an enormous, cold cloud of mostly hydrogen gas mixed with dust. It’s incredibly spread out, but it isn’t perfectly smooth. Gravitational attraction between the atoms pulls the denser patches together, and as matter falls inward the cloud begins to shrink. This inward movement of matter is called gravitational collapse.
As the gas falls inward, the particles collide more and more. Those collisions do work on the particles, raising their kinetic energy, which means the cloud heats up and starts to glow. This hot, glowing ball of collapsing gas is a protostar. It isn’t a true star yet — no fusion is happening — but astronomers can spot protostars because they emit infrared radiation.
nebula (cold gas & dust)
→ gravity pulls matter in →
gravitational collapse
→ collisions heat the gas →
protostar (hot, glowing)
Where does the heat come from if nothing’s burning yet? It’s the same reason a bike pump gets warm when you squeeze air into it — squashing gas does work on it and raises its temperature. Gravity is doing the squashing here, on a colossal scale.
Conditions for Fusion
A protostar keeps collapsing and heating until its core reaches the extreme conditions fusion demands. Remember from earlier that two nuclei only fuse if they have enough kinetic energy to beat the electrostatic repulsion between their positive charges. That translates into two requirements at the core:
🧭 What the core needs before fusion switches on
Very high temperature — on the scale of 100 million kelvin, so nuclei move fast enough to overcome their repulsion
Very high pressure and density — so the nuclei are packed close together and collide often enough for fusion to actually occur
Once the core crosses that threshold, hydrogen nuclei begin fusing into helium (the proton–proton chain), releasing enormous amounts of energy. As the protostar keeps drawing in more gas and dust, the core gets hotter and denser still, and collisions become more frequent — which makes fusion even more likely. At this point the object has become a main sequence star.
Equilibrium in Stars
Now the key idea for the exam. Once fusion is running, a star is caught in a permanent tug-of-war between two opposing effects:
Gravity pulls every part of the star inward, trying to make it collapse (this is the star’s weight, F = mg).
Radiation pressure and gas pressure push outward. The fusion energy streaming out of the core, along with the pressure of the hot gas, resists the collapse.
A stable star sits in equilibrium: outward radiation and gas pressure exactly balance the inward pull of gravity.
While these two are balanced, the star holds its size and stays stable — this is equilibrium. And the balance is self-correcting, which is what makes stars so long-lived:
The balancing act
outward pressure ↔ inward gravity
If the temperature rises, the outward pressure grows, so the star expands and cools back down. If the temperature drops, the outward pressure falls, gravity wins slightly, and the star contracts and heats back up. Either way it drifts back toward balance. A star like the Sun can hold this equilibrium on the main sequence for billions of years.
Quick recap: gravity collapses a nebula into a hot protostar; once the core is hot and dense enough, hydrogen fusion starts and it becomes a main sequence star, held stable by outward pressure balancing inward gravity.
WE 1
Describe how a protostar forms from a nebula, and explain why its temperature rises even though no fusion is taking place.
How it forms
a nebula is a large cloud of hydrogen gas and dust
gravitational attraction pulls the matter together (gravitational collapse)
the cloud shrinks and becomes denser
→ the dense, hot, glowing ball is a protostarWhy it heats up (no fusion yet)
as gas falls inward the particles collide more often
these collisions do work on the particles, raising their kinetic energy
→ higher kinetic energy means a higher temperature
WE 2
A main sequence star is described as being “in equilibrium.” (a) State the two forces that are in balance. (b) Explain what happens to the star if its core temperature increases slightly.
Part (a)
inward: the gravitational force (the star’s weight)
outward: radiation pressure and gas pressure
→ these two are balanced → the star is stablePart (b)
a higher temperature increases the outward pressure
outward pressure now exceeds gravity
the star expands
expanding cools it, lowering the pressure again
→ it settles back into equilibrium
💡 Top tips
Learn the order: nebula → protostar → main sequence star. A protostar is before fusion; a main sequence star is after it starts
Protostar heating is not fusion. The heat comes from gravitational collapse doing work on the gas — fusion only starts once the core is hot enough
Two forces, two pressures: gravity acts inward; radiation pressure and gas pressure act outward. Name all three when asked about equilibrium
“Stable” means balanced, not “nothing changing” — the star constantly self-corrects to stay in equilibrium
⚠ Common mistakes
Saying a protostar is powered by fusion — it isn’t; it’s heated by gravitational collapse
Only naming gravity and “radiation pressure” — don’t forget gas pressure also pushes outward
Thinking the outward push is a chemical reaction — it’s the pressure from fusion energy and hot gas, not burning
Confusing nebula with planetary nebula — a nebula forms stars; a planetary nebula is thrown off by a dying low-mass star (a later stage)
Up next: The Life Cycle of a Star — now that our star is stable on the main sequence, we’ll follow what happens when its fuel finally runs low, and how its fate splits depending on how heavy it is.
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