When I was a senior in high school, I read Black Holes and Time Warps: Einstein’s Outrageous Legacy by Kip Thorne and gave a presentation on it for my Honors Physics class (incidentally, in the same classroom that Jake Gyllenhaal attempts to have a cosmological discussion with Noah Wyle in “Donnie Darko”)

I was enamored by the myriad fates of dying stars.

Stars with core temperatures around 15 million Kelvin or less (like the Sun) convert hydrogen to helium via the proton-proton chain reaction.

In time, the hydrogen in the core will run out, and the core will collapse due to gravity. This will cause the outer layers to heat up and expand, transforming the star into a red giant.

The core will eventually stop collapsing due to electron degeneracy pressure

This will cause the helium in the core to heat up, creating a helium flash that converts helium to carbon via the triple alpha process.

Stars the size of the sun won’t get hot enough to ignite the fusion of carbon, and they will become white dwarfs.

Bigger/hotter stars (more than 1.3x the mass of the Sun) have more cataclysmic fates: supernovae, neutron stars, black holes.

Once helium is depleted, carbon fusion predominates, which then gives way to neon fusion, then oxygen fusion, then finally silicon fusion

• ²⁸Si + ⁴He → ³²S + γ
• ³²S + ⁴He → ³⁶Ar + γ
• ³⁶Ar + ⁴He → ⁴⁰Ca + γ
• ⁴⁰Ca + ⁴He → ⁴⁴Ti + γ
• ⁴⁴Ti + ⁴He → ⁴⁸Cr + γ
• ⁴⁸Cr + ⁴He → ⁵²Fe + γ
• ⁵²Fe + ⁴He → ⁵⁶Ni

Nickel-56 then decays to cobalt-56 via electron capture

⁵⁶Ni + e^- → ⁵⁶Co + ν

And cobalt-56 decays to iron-56 via electron capture or positron emission

• ⁵⁶Co + e^- → ⁵⁶Fe + ν
• ⁵⁶Co → ⁵⁶Fe + e⁺ + ν¹

While nickel-56 can fuse with helium to form zinc-60, because the helium comes from photodisintegration of other heavy nuclei (the original helium of the star already having been mostly consumed by carbon burning), it is a net endothermic process.

So fusion basically ends with iron.

So we have the reason why barns are painted red.

Barns Are Painted Red Because of the Physics of Dying Stars • 2013 May 10 • Rose Eveleth • Smithsonian

(TL;DR, barns are painted red ochre—Fe₂O₃—because it’s the cheapest paint and that’s because of the abundance of iron-56.)