# Intro - Main Sequence Fusion A star's existence is by no means serendipitous. More specifically, it's a combination of crack fluid dynamics and gravity that keeps one intact, depleting its own core to stay afloat a little longer. Hydrogen fusion releases photons as energy, which by [[Intro to QM (Nuclear & Quantum)#The Heisenberg Uncertainty Principle|Quantum Mechanics]] exerts an outward momentum onto the outer envelope above it. Eventually, a star settles into a state of equilibrium, where the outward 'fusion pressure' equals the inwards force of gravity. Typically, the more massive the star, the larger the core; but more massive stars 'burn out' - reach the end of their [[Stars (Astro)|main-sequence lifetimes]] much more quickly than stars with lower masses. Try guessing why this is the case - spoiler, it's the rate of fusion. With such large disparities between the lifespans of main-sequence stars, there's got to be an explanation. That explanation lies in there being **two** mechanisms of hydrogen fusion available - the **proton-proton chain** and the **CNO-cycle**. # The P-P Chain The proton-proton chain (mostly known as the P-P chain) is the simplest of the two fusion chains, being what we'd generally consider when thinking about fusion. Four hydrogen atoms fuse first into deuterium, releasing energy, then to a larger helium state. ![[proton-proton cycle.png]] Each individual 'stage' of the p-p chain releases energy, but the diagram is (most) of what you need to know about the P-P chain. Pink is for neutrons and blue is for protons; when a proton (hydrogen atom) fuses to become a neutron, a positron and a gamma ray is emitted to dispel the charge and energy of the fusion reaction. And that's it! # What's Next? Naturally, you're going to want to continue down stellar fusion chains, so take a look at the other articles in this 'mini-series': [[The CNO Cycle (Astro)]], [[The Triple-Alpha Process and Beyond (Astro)]] Navigate back to the main menu: [[Everything Astronomy]]