• Nuclear Fusion: The process by which two atomic nuclei combine to form a heavier nucleus, releasing energy in the process.
• Fusion in stars, especially the Sun, provides the energy that lights and warms our planet!

To get the party started (i.e., for hydrogen fusion to begin), stars need

• High Temperature: Ensures energetic collisions, so hydrogen nuclei get super close.
• High Density & Pressure: More collisions, more chances for fusion.

🎉 REAL-WORLD EXAMPLE: Imagine trying to put two repelling magnets together. If you gently push them, they easily repel. But if you push them with enough force (or heat them up enough), they might just stick!

• Also called “hydrogen burning” (because 4 hydrogen nuclei fuse to give 1 helium atom).
• Occurs in Sun-like stars (0.07-4 solar masses). When there’s no hydrogen left, the party ends.
• Here's a breakdown
• Stage I
  • 2 protons fuse → hydrogen-2 + positron + neutrino.
  • Fun Fact: The positron immediately annihilates with a nearby electron, producing 2 gamma photons.
  • But, this stage is rare. The average proton in the Sun waits a few billion years for this to happen.
• Stage II
  • A proton and hydrogen-2 fuse → helium-3 + gamma-ray.
  • This one’s fast, taking just a second after hydrogen-2 is produced!
• Stage III
  • Two helium-3 nuclei fuse → helium-4 + two protons.
  • This takes a while; the average helium-3 waits 400 years to fuse.

• Apart from the main pathway, there are other routes based on temperature
• 10MK - 23MK: Helium-3 and helium-4 fuse via a beryllium nucleus, which eventually turns to helium.
• Above 23MK: Beryllium fuses with a proton to form boron, which then splits into helium.
• Theoretical Branch: Involves helium-3 directly turning to helium-4. But this is like spotting a unicorn—rare and hasn’t been observed in the Sun!

🎉 REAL-WORLD EXAMPLE: Think of fusion pathways as different recipes to make the same dish. Depending on the ingredients (temperature) you have, you might choose one over the other!