A Helium Runaway Shell is the fast Fusion of Helium in a shell in an Asymptotic Giant Branch. Such stars can periodically burn Hydrogen in a shall, then helium when enough is present to ignite. The Helium Flash can last as long as 400 years, and are spaced in the 10,000 to 100,000 year range. These flashes can cause the star to emit gas and Dust.
Planetary Nebula (PN)