The last topic ended on a wall. Fusion happens only above tens of millions of kelvin, and nothing ordinary gets that hot. Yet every clear night shows thousands of things that fuse all the time: the stars.
In the fields topic, you saw that every mass attracts every other mass: that attraction is gravity. You also saw why fusion is hard: the nuclei repel, and only enormously fast collisions get them close enough to fuse. This lesson shows how a star meets that condition.
A star begins as a huge ball of gas, mostly hydrogen.
Every part of the ball attracts every other part, so the star's own gravity squeezes the ball inward.
The squeeze is fiercest at the center, so the core becomes enormously hot and dense.
In that core, hydrogen nuclei collide fast enough to get close despite their repulsion, and they fuse into helium.
Each fusion releases energy, and that released energy is what makes the star shine.
The sun converts about four million tonnes of mass into radiated energy every second.
A huge cloud of hydrogen gas collapses inward under its own gravity. Predict what happens at its center.
Gravity squeezes the cloud, and the squeeze is fiercest at the center.
The center becomes hot and dense enough for hydrogen nuclei to collide at enormous speed.
Nuclei that collide fast enough get close, and the strong nuclear force binds them.
Fusion starts at the center, and the released energy makes the new star shine.
The sun has shone for billions of years. What is happening to its total mass?
The sun's shine is energy radiated away.
A system's mass can fall only as the equivalent energy leaves it, and here energy is leaving.
Each second, the sun converts about four million tonnes of mass into radiated energy.
The sun's total mass is slowly falling: about four million tonnes of it leave as radiated energy every second.
Which force squeezes a star's core hot and dense enough for fusion to start?
Deep in a newly formed star, the core has just been squeezed hot and dense. Why does that squeeze start fusion?
Eitan says: 'A star's own gravity squeezes its core hot and dense enough for hydrogen nuclei to fuse into helium.' Is Eitan correct?
Which chain of events makes a star shine?
A huge ball of hydrogen gas collapses inward for millions of years, then begins to shine steadily. What switched the shine on?
The sun converts about four million tonnes of mass into radiated energy every second. What is happening to the sun's total mass?