Stellar fusion doesn’t stop at helium

Nelemans replies: Thanks to Mason Osborne for the clarification. Indeed, stars that are initially lighter than 8 solar masses also fuse helium into carbon and oxygen and end up with a degenerate carbon–oxygen core. Stars with initial masses of less than about 2 solar masses first develop a degenerate helium core, which in the helium flash is turned into a helium-burning core. Apart from the RR Lyrae stars, several other types, such as subdwarf B stars and horizontal-branch stars in globular clusters, are in the helium-burning-core stage (references in the original article and in Osborne’s letter). The point of my simplification in the box text was to distinguish between stars that develop a degenerate core to withstand gravity and more massive stars that do not form such a core. The less massive stars form white dwarfs after losing their hydrogen mantle, and the heavier ones ultimately become neutron stars or black holes.