<P> For the more massive asymptotic giant branch stars that form planetary nebulae, whose progenitors exceed about 3M, their cores will continue to contract . When temperatures reach about 100 million K, the available helium nuclei fuse into carbon and oxygen, so that the star again resumes radiating energy, temporarily stopping the core's contraction . This new helium burning phase (fusion of helium nuclei) forms a growing inner core of inert carbon and oxygen . Above it is a thin helium - burning shell, surrounded in turn by a hydrogen - burning shell . However, this new phase lasts only 20,000 years or so, a short period compared to the entire lifetime of the star . </P> <P> In either scenario, the venting of atmosphere continues unabated into interstellar space, but when the outer surface of the exposed core reaches temperatures exceeding about 30,000 K, there are enough emitted ultraviolet photons to ionize the ejected atmosphere, causing the gas to shine as a planetary nebula . </P> <P> After a star passes through the asymptotic giant branch (AGB) phase, the short planetary nebula phase of stellar evolution begins as gases blow away from the central star at speeds of a few kilometers per second . The central star is the remnant of its AGB progenitor, an electron - degenerate carbon - oxygen core that has lost most of its hydrogen envelope due to mass loss on the AGB . As the gases expand, the central star undergoes a two - stage evolution, first growing hotter as it continues to contract and hydrogen fusion reactions occur in the shell around the core and then slowly cooling when the hydrogen shell is exhausted through fusion and mass loss . In the second phase, it radiates away its energy and fusion reactions cease, as the central star is not heavy enough to generate the core temperatures required for carbon and oxygen to fuse . During the first phase, the central star maintains constant luminosity, while at the same time it grows ever hotter, eventually reaching temperatures around 100,000 K . In the second phase, it cools so much that it does not give off enough ultraviolet radiation to ionize the increasingly distant gas cloud . The star becomes a white dwarf, and the expanding gas cloud becomes invisible to us, ending the planetary nebula phase of evolution . For a typical planetary nebula, about 10,000 years passes between its formation and recombination of the star . </P> <P> Planetary nebulae may play a very important role in galactic evolution . Newly born stars consist almost entirely of hydrogen and helium, but as stars evolve through the Asymptotic Giant Branch phase, they create heavier elements via nuclear fusion which are eventually expelled by strong stellar winds . Planetary nebulae usually contain larger proportions of elements such as carbon, nitrogen and oxygen, and these are recycled into the interstellar medium via these powerful winds . In turn, planetary nebulae greatly enrich the Milky Way and their nebulae with these heavier elements--collectively known by astronomers as metals and specifically referred to by the metallicity parameter Z . </P>

Star left at the core of a planetary nebula