It seems proper to start our Symposium from the very beginning. The oldest known discovery of a planetary nebula dates back to Messier who in 1764 catalogued the Dumbbell nebula, NGC 6853, as Messier 27. Only 100 years later Huggins discovered the emission line spectra of planetary nebulae and he pointed out that his discovery proves that these objects are not clusters of stars but are “enormous masses of luminous gas or vapor”. In 1887, a curious suggestion was made by Lockyer who thought that the nebulae were clusters of burning meteorites when he incorrectly identified three nebular emission lines with magnesium.

The problems of the spatial and velocity distributions of planetary nebulae in our galaxy are still very much with us. In what follows we comment rather hastily on the older work and more especially on the recent work in these fields. We have not been able to carry out an exhaustive literature search, and therefore trust we shall be forgiven by those whose contributions do not appear herein. We first survey the scene in the greater solar neighborhood. Section 2 deals with the local spatial distribution of planetaries in the galactic plane and perpendicular to the plane. Section 3 focuses on the rather meager results that can be obtained on the kinematical properties of the planetaries in our general part of the galactic system. We conclude by moving farther afield and exploring another part of the galaxy and the galaxy as a whole. Section 4 addresses the distribution of planetaries and summarizes recent estimates of the total planetary population of our galaxy. Finally, Section 5 is devoted to what little one can conclude about the overall kinematical properties of the system of planetaries.

Selection effects in the search for planetary nebulae in the Magellanic Clouds have not seriously distorted the observed space distribution, but have affected the apparent excitation classes. A difference between the properties of the SMC and LMC planetaries remains. A group of high-excitation nebulae with strong lines of He and N has been isolated. If these are excluded, the helium abundance appears to be normal in each Cloud. The oxygen abundance in LMC planetaries is about half the Orion value and may be even lower in the SMC objects. The significance of the abundances, excitation classes and space distribution is briefly discussed.

The identification and observation of planetary nebulae in the Andromeda galaxy (M31) and its companions provide a powerful means of studying their old stellar populations. The direct determination of chemical abundances and radial velocities for even the brightest individual old stars is impossible at the distance of M31. The strongest emission lines of planetary nebulae are as bright as the entire visual continuum of the most luminous giants. Consequently, spectrophotometry of planetary nebulae presently provides the only direct measure of chemical abundances, and, with the exception of globular clusters, the only radial velocity determinations for the old populations.

The spectacular success of the so-called Shklovsky method of finding distances to planetary nebulae which are optically thin to the Lyman continuum (Shklovsky 1956; see also Minkowski and Aller 1954) has produced within us a rather breath-taking boldness. Following the careful application of this method to more than 600 planetaries by Cahn and Kaler (1971), we now have the audacity to use planetaries as reliable distance indicators to derive an improved model of our own galaxy (Cahn 1976), trace the evolution of stars in their post-giant stages (O'Dell 1974, Cahn and Wyatt 1976), and even determine distances to other stellar systems (Ford, Jenner, and Epps 1973). At the Tatranská Lomnica meeting 10 years ago, there seemed little hope that one day soon planetary nebulae distances would become reliable. That day is near if not here already.

Since the “Catalogue of Galactic Planetary Nebulae” 226 new objects have been classified as planetary nebulae. They are summarized in Table 1 which gives the designations, names, coordinates and the references to the discovery. Further 9 new objects have been added to Table 1 and called 1proto-planetary nebulae1, but their status is still uncertain. Only 34 objects have been included in the present list of misclassified planetary nebulae (Table 2) although the number of doubtful cases is much larger.

A planetary nebula has been identified near the center of the Fornax dwarf elliptical galaxy. Quantitative spectrophotometry reveals that it is slightly reddened, and belongs to an intermediate-excitation class, with a normal helium abundance. Since the abundances of all 2 to 3 times lower than in the Orion nebula, the material in the nebula which has a mass ≥ 0.1 M probably represents at least a second generation of processed matter. The implications of this conclusion are briefly discussed from the point of view of heavy element enrichment in low mass systems such as Fornax whose mass is 2×107 M. (Paper will appear in The Astrophysical Journal.)

Very deep, blue and red-sensitive objective-prism plates, taken with the Curtis Schmidt telescope at the Cerro Tololo Inter-American Observatory, were used to survey both Magellanic Clouds for unresolved objects which could be classified as highly probable planetary nebulae. The high sensitivity of the baked Kodak IIIa-J emulsion at 5000 Å made it possible to detect the N1 and N2 lines of [OIII] in fainter objects than previously observed. A number of emission-line stars, compact HII regions, and very-low-excitation objects, formerly considered to be probable or possible planetaries, were recognized and excluded. The final lists contain 27 confirmed planetaries in the SMC and 100 in the LMC, a ratio equivalent to the estimated ratio of total mass in the two systems. As would be expected, most of these objects had been detected in previous objective-prism surveys but some apparently new planetaries were found.

A new distance scale is provided for the Planetary Nebulae (PN). It combines thirteen lists, published between 1950 and 1976, for which the distances have been recalibrated with the aid of a large number of individual distances Dind: 19 values were selected from among individual distances already published, and 62 new distances have been calculated from kinematic and absorption laws.

The planetary nebula PK 261+8°1 is of special interest due to its apparent physical association with the galactic cluster NGC 2818 (Tiff, Connally, and Webb 1972, M.N.R.A.S., 158, 47). During 1975–76 we obtained spectrophotometric observations (both photographic and photoelectric) and interference filter photographs (Hβ, [OIII], [NII], and [SII]) of this object at Cerro Tololo. The nebula has a rather smooth bilaterally symmetric “hourglass” structure in the Hβ and [OlII] photographs, while several bright filaments appear in [NII] and [SII]. The spectrum of PK 261+8°1 is very rich in lines characteristic of both low ionization ([OI], [OII], [NI], [NII], [SII]) and high ionization (He II, [OIII], [NeV]) regions. Using the photo electric observations, we derive Te = 14,500 ± 1500°K from the [Olli] lines and Te = 11,000 ± 1300°K from the [OIII] lines and Te = 11,000 ± 1300°K from the [NII] lines. The λ6717/λ6730 [SII] ratio suggests that Ne ≤ 800 cm−3 in the regions where S+ exists. Additional information μ the density and temperature structure of the nebula derived from the deep photographic spectra will also be presented. Due to the complicated ionization structure, an accurate abundance analysis of PK 261+8°1 is difficult. Consequently, we will present the quantitative results from a variety of abundance calculation techniques. We conclude that He/H = 0.14 ± 0.01, N is substantially enriched relative to H and 0 (N/H ≥ 0/H = 4 × 10−4), while the abundances of 0, Ne, S, Ar, and Cl relative to H are similar to values typical of galactic planetary nebulae. Other characteristics of the nebula derived from its association with the cluster NGC 2818 and its spectral similarity to Henize 97 in the Large Magellanic Cloud will be discussed. This research was supported in part by a grant from the Research Corporation.

This review will contain a brief survey of a few of the issues that have been the concern of optical studies of planetary nebulae since the last IAU Symposium on this subject in 1967. Also included will be a short discussion of advances in optical instrumentation and remarks on some selected important problems which should be the concern of future optical investigations of planetaries.

The ultraviolet spectra of gaseous nebulae have for long been arresting the attention of astrophysicists. The 60's saw the first attempts at plotting the expected spectra of planetary nebulae between 3000 and 912 Å (Code 1960; Aller 1961). This was followed by an examination of the expected forbidden lines (about thirty of them) in the ultraviolet, and of their relation to physical parameters (Gurzadyan 1965; Osterbrock 1963). Even more accurate calculations were made for the intensities of stronger lines in highly excited nebulae (Flower 1968). At the same time preliminary estimates were being made of atomic parameters for various transitions - the collision strengths, the probabilities of forbidden transitions, etc. (Czyzak et al. 1968; Garstang 1968). The computations of various models for planetary nebulae also bear on the above studies (Harrington 1968; Kirkpatrick 1970).

Several groups have been involved in recent years in making ultraviolet observations of planetary nebulae from rockets and satellites. About 30 or 40 nebulae have been measured up to the present, some with reasonably high spectral resolution, more with band pass filters of 100 Å or more in width. All measurements have large diaphragms (or slitless systems) so that usually the nebula and the central star are observed together (there are large nebulae for which this is not true). The photometric accuracy of the later observations is quite good, usually better than 20%.

The discovery of infrared continuum emission from NGC 7027 by Gillett, Low, and Stein in 1967 marked the beginning of far infrared observations of planetary nebulae. These early observations verified the predictions (Delmer, Gould, and Ramsey 1967) of infrared fine structure line emission from the SIV ion and also provided a surprise; namely, that the continuum radiation from planetary nebulae was not free-free emission from the gas, but rather that it was thermal emission from heated dust grains. In the ten years which have elapsed since 1967, numerous infrared emission lines have been observed and interpreted in many of the brighter planetary nebulae. In the middle infrared these lines were principally Ne II at 12.8μ, Gillett et al. (1969); SIV at 10.5μ, Holtz, et al. (1971), Gillett, et al. (1972), Aitken and Jones (1973); and AIII at 9.0μ Geballe and Rank (1973) and Gillett and Forrest (1973).

The most recent review on planetary nebulae at radio wavelengths appeared in 1974 by Thompson. Previous summaries were given by Terzian (1968) and Higgs (1973). Major advances in this subject include extensive continuum observations at 5 GHz of southern planetary nebulae; the highly successful aperture synthesis observations providing brightness temperature distributions with a resolution of a few arc seconds; the high sensitivity observations of radio recombination lines which have allowed the unambiguous detection of collisional broadening in NGC 7027; the very recent detection of molecular CO emission from NGC 7027, IC 418 and NGC 6543, and CO and OH emission from possible proto-planetary nebulae.

An ultraviolet spectrum of NGC 7662 was obtained with a rocket-borne telescope in a 130 s exposure using a microchannel plate detector and film. In addition to the three strong ultraviolet lines of C III], C IV, and He II seen previously in NGC 7027, a new strong line of [Ne IV] at 2440 Å and a prominent continuum were measured in NGC 7662. The observed fluxes are given for both the lines and the continuum on an absolute basis, with a typical accuracy of about 25 percent. The absolute calibration is based primarily on in-flight observations of the stars α Lyr and α And, which have been well measured in the ultraviolet. This calibration is confirmed by laboratory data and by photoelectric observations from the ANS satellite.