R Coronae Borealis variable

[6] The constant features are prominent carbon lines, strong atmospheric hydrogen deficiencies, and obviously the intermittent fadings.

The 20 radii model requires a large and thereby long-time buildup of the obstructing dust cloud, making the fast light decline hard to comprehend.

The alternate theory of photospheric buildup of carbon dust in a 4,500–6,500 K temperature environment could be explained by condensations in the low pressure parts of shock fronts – being detected in the atmosphere of RY Sagittarii – a condensation that causes local runaway cooling, allowing carbon dust to form.

Standard stellar evolution models do not produce large luminous stars with essentially zero hydrogen.

The second model postulates a massive convective event at the onset of burning of an outer helium shell, causing the little remaining atmospheric Hydrogen to be turned over into the interior of the star.