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. Characteristics Physical characteristics 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 . (The Earth's atmosphere, by comparison, contains 2.5 particles .) Young planetary nebulae have the highest densities, sometimes as high as 106 particles . As nebulae age, their expansion causes their density to decrease. The masses of planetary nebulae range from 0.1 to 1 solar masses. 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. Nebulae may be described as matter bounded or radiation bounded. In the former case, there is not enough matter in the nebula to absorb all the UV photons emitted by the star, and the visible nebula is fully ionized. In the latter case, there are not enough UV photons being emitted by the central star to ionize all the surrounding gas, and an ionization front propagates outward into the circumstellar envelope of neutral atoms. Numbers and distribution About 3000 planetary nebulae are now known to exist in our galaxy, out of 200 billion stars. Their very short lifetime compared to total stellar lifetime accounts for their rarity. They are found mostly near the plane of the Milky Way, with the greatest concentration near the Galactic Center. Morphology Only about 20% of planetary nebulae are spherically symmetric (for example, see Abell 39). A wide variety of shapes exist with some very complex forms seen. Planetary nebulae are classified by different authors into: stellar, disk, ring, irregular, helical, bipolar, quadrupolar, and other types, although the majority of them belong to just three types: spherical, elliptical and bipolar. Bipolar nebulae are concentrated in the galactic plane, probably produced by relatively young massive progenitor stars; and bipolars in the galactic bulge appear to prefer