A star pulls on itself. The gravity at any depth comes from the mass lying below it, so the deeper in, the harder the pull.
With nothing to hold it up, a star falls. For a star the size of the sun, falling freely would bring it down in about minutes.
The real sun has not collapsed in billion years. Something pushing back from inside is holding gravity in balance, exactly.
What does the pushing, and why it lasts for billions of years, are the two things the structure of a star comes down to.
Take one thin layer out of a star. The weight bearing down from above and the pressure pushing up from below must differ by exactly the gravity acting on that layer.
A star where this holds at every depth is in hydrostatic equilibrium. It is the condition for keeping its shape.
The deeper the layer, the more mass lies above it, so the pressure rises toward the center. Temperature does the same, and the center is the hottest place.
The center of the sun is about million K at about billion times the pressure of the air. The surface is K, orders away. What pushes back is the pressure of gas at that temperature.
At that temperature and pressure, hydrogen nuclei come close enough to overcome their electric repulsion and join. Four hydrogen nuclei make one helium nucleus.
The helium that results is percent lighter than the four that went in. That missing mass becomes energy just as it is.
This is . Since is large, a small amount of mass gives a great deal of energy. The sun turns million tonnes of mass into light every second.
It happens only in a narrow region at the center, because that is the only place hot enough. Most of a star is the thick layer wrapped around that region.
Light born at the center does not travel straight out. It is absorbed by a nearby particle, emitted again, and works its way outward while changing direction.
Traveling that way, it takes more than years to get from the center to the surface. The sunlight arriving now left the center that long ago.
Further out it is carried by motion as well, with warm gas rising and cooled gas sinking. The grain seen on the surface of the sun is that rising and sinking showing through.
What leaves the surface is the luminosity. As long as the amount made at the center matches the amount leaving the surface, the star looks unchanged.
Suppose the fusion runs a little strong. The center gets hotter, the pressure rises, and the star swells a little. Swelling cools the center, and the fusion weakens.
When it runs weak the reverse happens. There is not enough push, the star contracts a little, and contracting heats the center so the fusion strengthens.
It returns either way. Because it returns, a star can go on shining at the same brightness for billions of years.
The main sequence of the HR diagram is the collection of stars where this balance is still holding. When the hydrogen at the center runs out and the balance breaks, a star leaves the main sequence.