Red-giant branch hydrogen fusion continues in stars of roughly solar mass until the helium core increases in mass sufficiently that it becomes degenerate. The core then shrinks, heats up, and develops a strong temperature gradient. The hydrogen shell, fusing via the temperature-sensitive CNO cycle, greatly increases its rate of energy production and the stars is considered to be at the foot of the red-giant branch. For a star the same mass as the sun, this takes approximately 2 billion years from the time that hydrogen was exhausted in the core. Subgiants more than about reach the Schönberg–Chandrasekhar limit relatively quickly before the
Gas torus no clear evidence of such a torus. While neutral nitrogen could not be measured, the ions near the orbit of Titan were primarily hydrogen or water group (O, OH, HO and HO) from the Enceladus torus. Trace amounts of nitrogen ions were detected but at levels consistent with an Enceladus source. A fictional gas torus is the setting for Larry Niven's novels "The Integral Trees" and "The Smoke Ring", in which a gas giant in orbit around a neutron star generates a gas torus of sufficient density and free oxygen to support life (including humans). Gas torus A gas torus