<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> <P> Subsequent generations of stars formed from such nebulae also tend to have higher metallicities . Although these metals are present in stars in relatively tiny amounts, they have marked effects on stellar evolution and fusion reactions . When stars formed earlier in the universe they theoretically contained smaller quantities of heavier elements . Known examples are the metal poor Population II stars . (See Stellar population). Identification of stellar metallicity content is found by spectroscopy . </P> <P> A typical planetary nebula is roughly one light year across, and consists of extremely rarefied gas, with a density generally from 100 to 10,000 particles per cm . (The Earth's atmosphere, by comparison, contains 2.5 × 10 particles per cm .) Young planetary nebulae have the highest densities, sometimes as high as 10 particles per cm . As nebulae age, their expansion causes their density to decrease . The masses of planetary nebulae range from 0.1 to 1 solar masses . </P> <P> Radiation from the central star heats the gases to temperatures of about 10,000 K . The gas temperature in central regions is usually much higher than at the periphery reaching 16,000--25,000 K . The volume in the vicinity of the central star is often filled with a very hot (coronal) gas having the temperature of about 1,000,000 K . This gas originates from the surface of the central star in the form of the fast stellar wind . </P>

Which of the following are characteristics of a planetary nebula