Red Barns and the Physics of Dying Stars

The pigment in red paint is made from plentiful elements cooked inside dying stars (image credit: Ian Britton)

The pigment in red paint is made from plentiful elements cooked inside massive dying stars (image credit: Ian Britton)

Why are so many barns painted red? The answer to this seemingly simple question goes deep into the physics of massive stars at the end of their lives. In 2013, a Google employee named Yonatan Zunger posted a long explanation on his Google+ page.  Summarized here, Zunger explains that barns are painted red because red paint is cheap. And red paint is cheap because its made of Fe2O3 (red ochre) composed of iron and oxygen. And iron and oxygen are cheap because they are plentiful on Earth because they formed readily in the innards of massive stars when the end their lives as supernovae explosions.

First, iron. Big stars, with a mass more than 8-10 times that of our Sun, burn through their stock of atomic fuel turning hydrogen into helium, helium into carbon, and so on, and releasing energy until the core of the star contains atomic nuclei with 56 nucleons (protons and neutrons). At that point, the game is up because atoms with 56 nucleons can’t fuse into heavier elements and release energy to hold the star up against its own gravity. So the star collapses and explodes, scattering huge amounts of atoms with 56 nucleons into space. And what’s the most stable atom with 56 nucleons? It’s iron, with 26 protons and 30 neutrons.

Heavy elements produced in the cores of massive stars

Heavy elements produced in the cores of massive stars

As for oxygen, it’s also formed in the innards of stars that explode, but more towards the outer layers of the core. It’s also extremely reactive with other elements like silicon and carbon. That’s why it comprises about 30% of the Earth’s crust. Iron makes up about 6% of the Earth’s crust, and the two atoms react readily together to form iron oxide. The pigment red ochre in red paint is a hydrated form of iron oxide.

The production of heavy elements in stars was laid out in 1957 in a landmark paper by Geoffrey and Margaret Burbidge, William Fowler, and Fred Hoyle. It was one of the great scientific discoveries of all time.

Some say astronomy and astrophysics aren’t practical sciences. But what’s more practical than explaining the composition of the material world?