NEBULAE, PLANETARY YERVANT TERZIAN The theory of late stellar evolution indicates that stars with masses between -1 and -6-8 M* (or somewhat higher, see entry on Nebulae, Planetary, Origin and Evolution-Ed.) will undergo core collapses reaching densities of about 10**g cm** and radii on the order of 7000km. The formation of these well known white dwarf stars is accompanied by a mild ejection of the outer parts of the evolved stars. The ejected material forms an envelope that is known as a planetary nebula. These nebulae have low expansion velocities of -20-30 km s** and have a lifetime of about 30,000 yr before they disperse into the diffuse interstellar medium. Initially, the ejected material is cold and neutral and contains molecular species such as H*, CO, and OH and also dust particles. Eventually the surface of the remnant stellar core becomes hot enough and dissociates and ionizes part of the expanding envelope, which in turn emits strongly in the ultraviolet, visible, and radio spectral regions. Although planetary nebulae are not very luminous, they are easily recognizable and can be observed within 10 or more kiloparsecs from the Sun. At the present time, some 1500 planetary nebulae have been identified in our galaxy, and several hundred other such nebulae have been detected in other systems, especially in the Andromeda galaxy and the Magellanic Clouds. Planetary nebulae can be found by examining photographic plates of the sky and distinguishing them by their shell-type or disk shapes. Such a method is limited to nebulae that have angular sizes of at least 10 arcsec, most of which are within a few kiloparsecs from the Sun. Almost all other discoveries of planetary nebulae are made by detecting the nebular emission line spectra, which show strong Balmer emission lines and the [O ***] lines at about wavelength (*) 5000*. The space distribution of planetary nebulae shows a concentration along the galactic plane, and a strong concentration of the smaller nebulae in the direction of the galactic center. This distribution, in galactic coordinates, is shown in Fig. 1. A histogram as a function of galactic latitude, shown in Fig. 2, also indicates this distribution and in addition, reveals a deficiency of nebulae at galactic latitudes near zero. This probably reflects the strong effects of interstellar extinction, particularly in the direction of the galactic center. Even though the population of planetary nebulae in the Galaxy is large and their apparent distribution is well defined, details about their real space distribution remain uncertain because of difficulties in determining their individual distances. The trigonometric parallax method can only be applied to a very few planetary nebulae, and the spectroscopic parallax method is not applicable to the central stars of these nebulae. Hence other methods must be used to estimate the distances to individual objects. One method uses the spectroscopically observed radial velocities of the nebular expansion together with the observed angular expansion rate to derive the distance. Another method uses estimates of the amount of interstellar extinction, due to interstellar dust, in the direction of a nebula to estimate the distance. Still another method uses the *21-cm neutral hydrogen absorption line measurements in the line of sight to a planetary nebula to estimate the distance. Very few planetary nebulae are known members of stellar clusters whose distances are known by other independent methods. One such object is K648 (Ps-1) in the globular cluster M15 at a distance of 10 kpc. These methods apply only to a few dozen nebulae, and the majority have distances derived from approximate statistical methods. In general, the available distance estimates of planetary nebulae are very uncertain and individual distances may be in error by factors of 2 or even 3. The uncertainty in the distance scale of planetary nebulae unfortunately propagates into many vital characteristics of these objects, such as the nebular mass, the absolute luminosity, and the total number of planetary nebulae in the Galaxy. However, from the available information, it appears that the ionized nebular mass in planetary nebulae has a range from -0.01-1 M*. There is also a correlation indicating that the larger the mass is, the smaller is the electron density of the hot gas. The ionized mass can be a small fraction of the total mass. The space within 1 kpc from the Sun contains at least 50 planetary nebulae and perhaps as many as 100, given the distance uncertainties of these objects. The scale height of their distribution above and below the galactic plane is about 200 pc, and the total number of planetary nebulae in the Galaxy can be as low as 10,000 and as high as 100,000 depending on the adopted space density. Statistics of planetary nebulae in other nearby galaxies suggest that there are 1-4x10** nebulae per solar mass. This implies that in our galaxy there must be 20,000-80,000 nebulae if the Galaxy has a total mass of 2x10** M*. These numbers, though uncertain, indicate that the birthrate of planetary nebulae in the Galaxy is -1 yr** which agrees well with the rate of formation of white dwarf stars. The kinematics of planetary nebulae can be investigated by measuring their radial velocities. More than 500 nebulae have such measured velocities and Fig. 3 shows their galactic distribution. It is seen that a large velocity dispersion exists near the galactic center -140 km s** and a total range of about *250 km s**. The kinematics of the planetary nebulae outside the galactic center regions are in agreement with the expected radial velocities assuming circular orbits at various distances from the galactic center. The galactic center objects do not appear to follow the regular differential circular galactic motion, and in this regard they are similar to the motions of Population II objects, such as RR Lyrae stars, globular clusters, and OH/IR stars. The kinematical data indicate that planetary nebulae near the galactic center essentially are members of an older population similar to globular clusters. It is however, also true that planetary nebulae that are confined to the galactic plane, outside the galactic central regions, are of a younger Population I type. It is also important to consider the influence that planetary nebulae have on the chemical evolution of the interstellar matter. Most of the matter returned to the interstellar medium comes from mass ejection from cool giant stars such as Miras and OH/IR stars, which are considered possible progenitors of planetary nebulae. If the mean mass in a planetary nebula is 0.2 M*, and if one such object is formed per year, then 20 M* of processed material are returned to the interstellar medium per century from the formation of these nebulae. This mass is at least comparable to the mass ejected by novae and supernovae, so that the formation of planetary nebulae seems to play a central role in the chemical evolution of the Galaxy. Additiooal Reading Maciel, W.J.(1989). Galactic distribution, radial velocities and masses of PN. In Planetary Nebulae, S. Torres-Peimbert, ed. Kluwer Academic, Dordrecht, p. 73. Pottasch, S.R.(1984). Planetary Nebulae. D. Reidel, Dordrecht. Schneider, S.E., Terzian, Y., Purgathofer, A., and Perinotto, M. (1983). Ap. J. Suppl. 52 399. Terzian, Y.(1980). Planetary nebulae. Quart. Roy. J. Astronom. Soc. 21 82.