What happens when a star runs out of core hydrogen? The radiative pressure which the star has been relying on will then decrease, fusion slows down, and the star's core contracts inwards! # The Red Giant Phase and Horizontal Branch Once the core begins to contract, the star actually initially shrinks a bit - the radiative pressure is diminished, after all. However, residual heat from the degenerate helium, which in comparison with the outer layers of the star are incredibly hot, allowing for the formation of a hydrogen-burning shell around the core, creating the radiative pressure required to fight against gravity. >[!Tip]- Red Giant Star cross-section >%%excalidraw diagram using the first chunk you drew!%% However, as the helium core is much, much hotter than the initial hydrogen core, the hydrogen shell will also be much hotter, resulting in heightened rates of fusion. This makes the outer layers expand - making its temperature drop, but its luminosity increase, thanks to the larger surface area. The rapid expansion can also be attributed to red giants having opaque atmospheres, trapping photons within them. This shell arises from the residual heat from the degenerate helium, and is also veyr hot, forcing the star to puff up and turn red (as the total flux remains the same) %%this section would be better done with loads of diagrams with one-line texts for clarity! it's hard to read%% as a star moves up the red giant phase, its temperature drops but its luminosity increases. this is because of a larger surface area + the hydrogen-burning shell is much smaller. red giants have opaque atmospheres - which will reduce their masses. While this happens, the core continues to contract, where the degenerate helium gets denser and denser. ***DURING THE RED GIANT PHASE, THE CORE DOES NOT FUSE HELIUM. THE HELIUM IS DEGENERATE AT THIS CURRENT MOMENT IN TIME.*** Eventually, the core helium ignites, triggering a violent expulsion of material called a "thermal pulse" as well as allowing the star to contract. This is the **horizontal branch** - a stable, core helium-burning configuration which lasts for around 10-100 million years, depending on the star. ## The Helium Flash ![[Pasted image 20230717165134.png]] ![[Pasted image 20230710094754.png]] //helium flash ![[Pasted image 20230710094732.png]] ![[Pasted image 20230710094715.png]] # Horizontal Giant Branch Once a red giant hits the pressures and densities required for helium fusion in its core, the helium ignites, becoming a ![[Pasted image 20230710094623.png]] # AGB Stars When this helium runs out, the star's core will contract once again thanks to diminished radiation pressure. Helium will then fuse through the triple-alpha process to form both carbon and oxygen, creating a degenerate core in the middle. This core contracting forms both helium and hydrogen shells around the core, allowing the star to expand further. Eventually, the star's helium shell expands to the point where it overtakes the hydrogen shell. When this happens, it is only a matter of time until the helium shell runs out of fuel to sustain the outer layers of the star, as the helium will be fused into carbon or oxygen. This will lead to a **thermal pulsation**, where shells pick up more energy leading to the outer layers being blown off. Eventually, the mass of the degenerate core is great enough that one more thermal pulsation will result in the star expelling its outer layers. This forms a **White Dwarf**, a degenerate ball of carbon and oxygen [^1]. [^1: Some white dwarfs may actually be helium dwarfs! Those are very rare and only really occur in turbulent binaries where the outer layers of a red giant have been stripped away by a companion, or if very small stars (late K-M type) enter their later evolutionary stages.] ![[Pasted image 20230710094556.png]] # Red Supergiants - Past the Carbon Threshold # What's Next?