Red-giant branch through several distinct stages which are reflected in the outward appearance. The evolutionary stages vary depending primarily on the mass of the star, but also on its metallicity. After a main-sequence star has exhausted its core hydrogen, it begins to fuse hydrogen in a thick shell around a core consisting largely of helium. The mass of the helium core is below the Schönberg–Chandrasekhar limit and is in thermal equilibrium, and the star is a subgiant. Any additional energy production from the shell fusion is consumed in inflating the envelope and the star cools but does not increase in luminosity. Shell
Fleming 1 a torus of expanding hot gas forming the inner bright ellipse. Fleming 1 is probably 5,000 years old. Like any other planetary nebula Fleming 1 was formed when an old asymptotic giant branch (AGB) star lost its outer hydrogen-rich envelope, leaving behind a hot core (young white dwarf)—the central star of the nebula. The star in the center of Fleming 1 has a temperature of and mass of 0.56 . Observations performed by European Southern Observatory showed that the central star is in fact a double degenerate (made of two white dwarfs) binary with a period of days. The companion