Red giant Red giant A red giant is a luminous giant star of low or intermediate mass (roughly 0.3–8 solar masses ()) in a late phase of stellar evolution. The outer atmosphere is inflated and tenuous, making the radius large and the surface temperature around or lower. The appearance of the red giant is from yellow-orange to red, including the spectral types K and M, but also class S stars and most carbon stars. The most common red giants are stars on the red-giant branch (RGB) that are still fusing hydrogen into helium in a shell surrounding an inert helium core. Other
Power-to-gas produce hydrogen. This microbial power-to-gas reaction occurs at ambient conditions, i.e. room temperature and pH 7, at efficiencies that routinely reach 80-100%. However, methane is formed more slowly than in the Sabatier reaction due to the lower temperatures. A direct conversion of CO to methane has also been postulated, circumventing the need for hydrogen production. Microorganisms involved in the microbial power-to-gas reaction are typically members of the order "Methanobacteriales". Genera that were shown to catalyze this reaction are "Methanobacterium", "Methanobrevibacter", and "Methanothermobacter" (thermophile). The synthetic methane generated from electricity can also be used for generating protein rich feed for cattle,