THE ELECTRONIC JOURNAL OF THE ASTRONOMICAL SOCIETY OF THE ATLANTIC Volume 5, Number 7 - February 1994 ########################### TABLE OF CONTENTS ########################### * ASA Membership and Article Submission Information * A History of the Study of Planetary Nebulae and Basic Models of Their Formation - Curtis A. Deer ########################### ASA MEMBERSHIP INFORMATION The Electronic Journal of the Astronomical Society of the Atlantic (EJASA) is published monthly by the Astronomical Society of the Atlantic, Incorporated. The ASA is a non-profit organization dedicated to the advancement of amateur and professional astronomy and space exploration, as well as the social and educational needs of its members. ASA membership application is open to all with an interest in astronomy and space exploration. 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When sending your article submissions, please be certain to include either a network or regular mail address where you can be reached, a telephone number, and a brief biographical sketch. Back issues of the EJASA are also available from the ASA anonymous FTP site at chara.gsu.edu (131.96.5.29). Directory: /ejasa DISCLAIMER Submissions are welcome for consideration. Articles submitted, unless otherwise stated, become the property of the Astronomical Society of the Atlantic, Incorporated. Though the articles will not be used for profit, they are subject to editing, abridgment, and other changes. Copying or reprinting of the EJASA, in part or in whole, is encouraged, provided clear attribution is made to the Astronomical Society of the Atlantic, the Electronic Journal, and the author(s). Opinions expressed in the EJASA are those of the authors' and not necessarily those of the ASA. No responsibility is assumed by the ASA or the EJASA for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use of operation of any methods, products, instructions, or ideas contained in the material herein. This Journal is Copyright (c) 1994 by the Astronomical Society of the Atlantic, Incorporated. A HISTORY OF THE STUDY OF PLANETARY NEBULAE AND BASIC MODELS OF THEIR FORMATION by Curtis A. Deer Faintly shining through the eyepiece of your telescope, a ghostly gray-green ball appears as you sweep the night sky, with perhaps a flicker of a point star at its center. What is its origin? What is its nature? This is the typical description of a planetary nebula, the out- blown gasses of a dying star on its way to becoming a white dwarf. They display a spectacular array of shapes, sizes, and structures. Most of them are shaped as rings, bipolars, and disks; a few fall into a category called irregulars. The rings range from a single layer, looking like a smoke ring, to nested configurations concentrically billowing outwards. Disks appear as circular or oval, uniformly illuminated, without any ring structure visible. In ring and disk forms there is a star in the exact center, though some are too faint to see by eye alone. By far the most unusual and interesting shapes are the bipolar nebulae: They look like two cones with their points touching each other and a star at the intersection of the cones. Within the various classes of planetaries, a few show fascinating structures as if someone had torn or punched holes through them. Some have bright spots or glowing streaks; several with the right surface brightness even appear to blink in and out of view as the eye moves over them, allowing them to fall on areas of greater or lesser sensitivity. This variety of shapes and structures has created a wide range of whimsical names for some of the brightest objects, such as the Ring, Butterfly, Clown Face, Little Ghost, Box, Little Gem, Egg, Eskimo, Owl, Cat's Eye, Dumbbell, Little Dumbbell, Bug, and Saturn Nebulae. Most planetary nebulae are green or blue, but some appear red and a few are even visually multi-colored. The colors have inspired names such as Blue Snowball, Blue Flash, Ghost of Jupiter, Phantom Streak, and the Blue Nebula. To understand the history of planetary nebulae, we must start with the history of nebulae in general. This began in the ancient Golden Age of astronomy when the Greek astronomer/philosopher Hipparchus of Nicaea (first quarter of the Second Century B.C. to 127 B.C.) noted two hazy patches in his catalogue of stars. Another Greek astronomer/ philosopher, Ptolemy (100-170 A.D.), observed five more and put all seven in his book called the ALMAGEST. In this famous work, Ptolemy proposed a geocentric Universe with the planet Earth as the center of the system. Today, none of Hipparchus' works have survived the centuries, but much information about his accomplishments is found in the ALMAGEST, from which it is clear that Hipparchus was probably one of the greatest astronomers in antiquity. Of course, no telescopes were then available: The hazy patches were only visible by the unaided eye. Two are easy naked-eye objects from dark skies: The Orion Nebula (also known as Messier 42) and the Andromeda galaxy (Messier 31). The others can be seen only in superb conditions or using optical aid due to the present light pollution problem. In the year 1609, the Italian astronomer Galileo Galilei (1564-1642) used the newly invented telescope to look into the sky and observe the various nebulae and stars. He saw that some nebulae resolved themselves into clusters of stars, while others remained a uniform glow. He came to an important conclusion about the Milky Way galaxy, stating: "The next object which I have observed is the essence or substance of the Milky Way. By the aid of a telescope, any one may behold this in a manner which so distinctly appeals to the senses that all the disputes which have tormented philosophers through so many ages are exploded at once by the irrefrangeable evidence of our eyes, and we are freed from wordy disputes upon this subject, for the Galaxy is nothing else but a mass of innumerable stars planted together in clusters." This was the first time that the great veil of light that stretches across the sky was identified as only a myriad collection of faint stars. Galileo also looked at the several smaller spots of separate nebulosity then known and discovered, to his surprise, that "the stars which have been called by every one of the astronomers up to this day nebulous, are groups of small stars set thick together in a wonderful way." Galileo was describing star clusters. He was close to the truth, but he also believed that the reason the remaining nebulae could not be resolved into stars was because they were of "the denser parts of the heavens, able to reflect the rays of the stars or the Sun." Up to this point, most of the explorations of the heavens had halted at the solar system and did not venture out into the great void that stretched to the stars, which were unknown quantities. Edwin Hubble stated this well in his book, THE REALM OF THE NEBULAE, that an astronomer's study was first "confined to the realm of the planets, then they spread through the realm of the stars, and finally penetrated into the realm of the nebulae." The exploration into this final realm moved more rapidly than the others and was halted only by the limits of the greatest telescopes. Speculation ran the widest possible gamut, pruned back only as new information was to be had from the newest telescopes and instrumen- tation. The outlandish ideas are long forgotten. The surviving hypotheses all satisfied the principle of uniformity, that any large sample of the Universe is much like any other sample. This principle was applied to the stars early on in making the first estimates of their distances. In early 1750, an English instrument maker and private tutor named Thomas Wright (1711-1786) formulated the concept of a stellar system isolated in space. Since the stars were too far away for any known measuring instrument, they must be very bright. Because the brightest object known was the Sun, Wright assumed that the stars were of intrinsically similar brightness. Then their distances could be estimated from their apparent brightness. Wright's philosophical mind was not satisfied by a single isolated example of a stellar system in the Universe. He imagined other similar systems and believed that the mysterious clouds called nebulae were the visible evidence of their existence. Five years later, the philosopher Immanuel Kant (1724-1804) developed Wright's conception into a form that lasted unchanged for almost two centuries. Using the principle of uniformity, Kant described this theory in the following example: The sequence of ideas that led us to it is very simple and natural. They are as follows: Let us imagine a system of stars gathered together in a common plane, like those of the Milky Way, but situated so far away from us that even with the telescope we cannot distinguish the stars composing us from the stars of the Milky Way, is in the same proportion as the distance of the Milky Way is to the distance from the Earth to the Sun; such a stellar world will appear to the observer, who contemplates it at so enormous a distance, only as a little spot feebly illumined and subtending a very small angle; its shape will be circular, if its plane is perpendicular to the line of sight, elliptical, if it is seen obliquely. The faintness of its light, its form, and its appreciable diameter will obviously distinguish such a phenomenon from the isolated stars around it. We do not need to seek far in the observations of astronomers to meet with such phenomena. They have been seen by various observers, who have wondered at their strange appearance, have speculated about them, and have suggested sometimes the most amazing explanations, sometimes theories which were more rational, but which had no more foundation than the former. We refer to the nebulae, or, more precisely, to a particular kind of celestial body which M. de Maupertuis describes as follows: "These are small luminous patches, only slightly more brilliant than the dark background of the sky; they have this in common, that their shapes are more or less open ellipses; and their light is far more feeble than that of any other objects to be perceived in the heavens." Just over one century later, a different interpretation of the nature of nebulae was proposed by Sir Edmond Halley (1656-1743), the second Astronomer Royal, famous now for his prediction of the return of a comet which now bears his name. In the PHILOSOPHICAL TRANSACTIONS for 1715, Halley published a memoir entitled "Of Nebulae or Lucid Spots Among the Fixt Stars". It contained descriptions of six nebulous objects (Messier 11, M13, M22, M31, M42, and Omega Centauri). Halley discovered M13 by chance in 1714 and had seen Omega Centauri while at Saint Helena in 1677. The other four objects were found by other observers. Halley proposed that the nebulae "in reality are nothing else but the Light coming from an extraordinary great Space in the Ether; through which a lucid Medium is diffused, that shines with its own proper lustre" and that this lustre was from a "Region of Light, beyond the Fixt Stars". He stated this because there is "no sign of a star" seen in them. Not everyone agreed with Halley's opinion: In 1733, twenty years later, an English clergyman and - for the most part - an armchair amateur astronomer, William Derham (1657-1735) wrote a paper expressing an opposing point. In his PHILOSOPHICAL TRANSACTIONS, Derham listed sixteen nebulous stars, none of his own discovery (they were all listed in Hevelius' PRODROMUS ASTRONOMIAE), of which he had only observed one himself (the Andromeda Nebula). Relying on Hevelius' MAP OF THE CONSTELLATIONS for their appearance, he stated that he disagreed with Halley's concept of "proper lustre," believing that "the patches are openings in the firmament through which the fiery Empyrean is seen." For this view, Derham was made a canon of Windsor in 1716 and became chaplain to the Prince of Wales, later King George II. Although astronomers remained unsure what the nebulae were, they continued to list them. One of the most famous catalogue compilers was the French astronomer Charles Messier (1730-1817). His real interest was in hunting down new comets. He discovered his first comet in 1764 and virtually all of the comets for the next fifteen years, which is how he earned his title "Ferret of the Comets", bestowed by King Louis XV. Since dim nebulae can easily be mistaken for a comet, Messier created his catalogue of nebulae so he could avoid confusion in his search for comets. His early catalogue had forty-five objects and was published in 1771. The version known today listed 103 objects initially (like the canon of Haydn symphonies, it has continued to grow through the well-meaning addition of entries which Messier may never have seen), and was published in 1784. Messier's list had both galactic and extragalactic bright objects. Of this conspicuous field, some thirty-two are extragalactic nebulae. The true nature of nebulous objects is now known to be quite varied. They are now grouped into four main classes: Star clusters, galaxies, planetary nebulae, and diffuse nebulae. Diffuse nebulae are further divided into emission nebulae, reflection (bright) nebulae, and absorption (dark) nebulae. However, in the Eighteenth Century, when telescopes were small and not well developed, the image quality and scientific understanding were not sufficient to make this classification clear. Although astronomers soon agreed that Derham was mistaken, it was not at all clear whether Halley or Galileo was nearer to the truth. A German-born English astronomer, Sir Frederick William Herschel (1738-1822) was famous for his own catalogue of nebulae and the discovery of the Jovian planet Uranus in March of 1781. He was given a copy of Messier's catalogue, which inspired him to become very much involved with the question of the nebulae. Herschel attempted to see if more powerful telescopes could resolve the hazy patches into stars. Observing all the objects with 31 and 46-centimeter (12.4 and 18.4-inch) telescopes, he concluded that most of the nebulae could be resolved into individual stars. The nebulae that could not be resolved into stars bothered him so much that he constructed reflecting telescopes up to a diameter of 1.3 meters (four feet), with a focal length of nearly thirteen meters (forty feet)! The unresolved nebulae remained impenetrable, even with this behemoth. Herschel finally commented that he was "so far from resolving these nebulae into stars, [it] seemed to prove that their nebulosity was not different from what I had called milky." He attempted to enlarge Messier's list and found it an easy task. Between 1777 and 1802, Herschel compiled a listing of over 2,500 new nebulae, the first installment of which was published in 1785. His son, Sir John Frederick William Herschel (1782-1871), extended this list by adding five hundred more and then observed in the Southern Hemisphere for many years, adding over 1,700 objects. In 1864, the complete list was published, comprising over five thousand entries. In a 1785 edition, the elder Herschel classified all entries by appearance: One class of nebula seemed to be obser- vationally distinct from the rest. He called this class "planetary nebulae", for they vaguely resembled the greenish disk of his recently discovered planet Uranus. However, Herschel was not the first to do this: In 1779, the French astronomer Antoine Darquier de Pellepoix (1718-1802) described one of his discoveries (Messier 57, also called the Ring Nebula) as "A very dull nebula, but perfectly outlined; as large as Jupiter and looks like a fading planet." One person who is given too little credit for her contributions to William Herschel and astronomy in general is William Herschel's sister, Caroline Lucretia Herschel (1750-1848). She came to England to act as housekeeper for her brother. When he became engaged in astronomy she acted as his secretary and general assistant. It was not uncommon for her to stay up all night recording William's observations. Caroline became an observer in her own right. Using a Newtonian "sweeper" of 67.5-centimeter (27-inch) focal length (20x magnification), she found many star clusters and nebulae which made their way into her brother's catalogue. One of them was NGC 205, the second companion of Messier 31, which she observed in 1783. Although Messier claimed, in 1807, to have seen this object in 1773, the discovery of NGC 205 is usually credited to Caroline. She also found eight comets in a period of eleven years, with the most notable ones discovered in 1786 and 1788. Joseph-Jerome le Francais de Lalande (1732-1807) was so impressed by Caroline Herschel's observational feats that he suggested to the French Academy of Sciences that she should be honored with a "suitable award". The learned members resisted this on the basis that they did not wish to be "accused of an excess of gallantry!" Caroline published a catalogue of 561 stars compiled from John Flamsteed's observations and a zone-by-zone catalogue of all the nebulae and clusters observed by her brother. For this work, she was awarded the Gold Medal of the Royal Astronomical Society in 1828 and was made an honorary member of the Society in 1835. She later retired to her native Hanover and in 1846 received a gold medal from the King of Prussia. Caroline died two years later at the ripe age of 98. In a paper published in 1791, William Herschel reported an observation made on November 13, 1790: "A most singular phenomenon! A star of about the 8th magnitude with a faint luminous atmosphere, of circular form. The star is perfectly in the centre, and the atmosphere is so diluted, faint, and equal throughout that there can be no surmise of its consisting of stars; nor can there be a doubt of the evident connection between the atmosphere and the star." The nebula that Herschel had under consideration was not listed in the Messier catalogue, but is known by its New General Catalogue number as NGC 1514. Herschel's argument that the nebulosity cannot consist of stars was simple. He was certain that the star at the center of the nebula and the nebula itself were associated, for the chance coincidence of a bright star perfectly centered in front of the nebula was highly improbable. Thus the star and the nebula were at the same distance in space. Assuming that the nebula was composed of stars, two possible solutions were that the central star was an ordinary star and the nebulosity was composed of stars that must be very faint, or that the nebula are normal stars and the central star must be of enormous size. Herschel rejected both possibilities. Herschel found many other examples of bright stars in the middle of nebulae, which clearly indicated a physical connection. He finally concluded that the nebulosity in these objects was not composed of stars and that these objects constituted a new class. In 1845, William Parsons, third Earl of Rosse (1800-1867), a wealthy amateur astronomer and telescope maker, completed an even larger telescope than Herschel's largest instrument, with a two-meter (six-foot) diameter mirror. He started to work with his telescope at Birr Castle in Parsonstown, Ireland. Parsons found that even this huge telescope could not resolve some of the nebulae, though he was reasonably sure that even larger instruments might be able to. In the middle of the Nineteenth Century, more evidence became available to support Herschel's conclusion that these nebulae were of a separate class. The spectroscope, an instrument that splits white light into its individual wavelengths or colors, became available for use on telescopes. Joseph Fraunhofer (1787-1826) used the spectroscope to discover that the Sun emitted a continuous spectrum with sharp lines of absorption, dark lines of missing color in the spectrum. The planets and the stars showed features similar to the solar spectrum, but each had its own set of additional absorption lines. In 1859, Gustav Kirchoff (1824-1887), working in the laboratory of Bunsen in Heidelberg, discovered that certain gases emit lines at just the wavelengths we see in the solar absorption lines. In this way, over twenty five elements in the Sun's atmosphere were identified. By the Nineteenth Century, the focus of nebular study had begun to shift from the morphological and qualitative analysis of earlier times to the quantitative methods of modern scientific practice. A revolution in instrumentation made this possible: For planetary nebulae, the spectrograph and the challenge of nebular spectra matured the field and ushered in a new era of research. Stellar and solar spectra were the first recorded of astronomical objects. The stellar spectrum itself opened up the study of the true nature of stars. The history of spectrum analysis began with Joseph Fraunhofer, who in 1817 perceived, and in 1823 described in detail, the fact that in the spectra of Castor and Sirius dark lines were present which were clearly different from those in the solar spectrum. Unfortunately, Fraunhofer was too busy with his optical work and died too soon to follow up this intriguing observation. The nature of the dark lines awaited the qualitative model of Gustav Kirchoff, who described the conditions leading to the three types of observed spectra: Continuum, emission, and absorption. In Kirchoff's model, absorption spectra, as seen in stars, are produced by relatively cool outer layers of the star absorbing light from the hotter interior. Kirchoff and Bunsen were able to identify many of the lines observed with those of known chemical elements seen in a laboratory spectroscope. This led to the natural assumption that stars with different patterns of lines were made of different elements. This was later shown to be incorrect. After the fundamental work of Kirchoff, the field was thoroughly cultivated by an Italian Jesuit priest, Father Angelo Pietro Secchi (1818-1878) at the Specola Vaticana in Rome, and Sir William Huggins (1824-1910) in his private observatory at Tulsa Hill in England. Huggins took his first spectrogram, or photographic plate of a spectrum, in 1863. It was of very poor quality. Visual measurement of the spectrum at the telescope remained the principal method used until the 1890s. Huggins studied the spectra of bright stars, identifying the dark lines of sodium, iron, calcium, and magnesium. He stated in 1863 that the same elements are present in the stars as in the Sun and on the planet Earth. The assumption that the entire Universe is made of somewhat uniformly constituted material was thus first demonstrated by actual measurement. Huggins was also the first to determine the radial velocity of a star, with an uncertainty of tens of kilometers, through the measurement of the displacement of its spectral lines by the Doppler shift. Improvements in instrumentation, such as the temperature stabilization of spectroscopes, resulted in increasingly precise velocity measurements. Hermann Vogel, at Potsdam Observatory, first made measurements in 1887 which displayed the orbital velocity of Earth imposed as a small periodic change in the measured radial velocity of stars. Doppler's principle was thereby validated by experimental test and the doubting astronomers could rest. Vogel was also the first to turn a spectroscope on a nova, namely Nova Corona Borealis of 1866. Father Secchi served as director of the Collegio Romano Observatory. Between 1863 to 1868 he examined the spectra of over four thousand stars and found that they could be classified into four types based on their color and the characteristics of their spectra. This classi- fication scheme remained in use until it was later superseded by the modern Harvard system. Secchi named the classes I, II, IIIa, and IIIb. The first type was distinguished by the presence of four strong absorption lines of hydrogen. It was typical of white or bluish stars. The second type showed a spectrum resembling the Sun and was observed in yellow stars. The third type were marked by dark bands sharply cut at the violet side and gradually fading towards the red, which were typical of reddish stars. In 1868, Secchi noted that the fourth and rarest type was marked by strong bands fading towards the violet end of the spectrum. This was observed in faint and "fiery-red" stars. By the end of the Nineteenth Century, others had taken up and extended the work of Secchi with more detailed analyses. One of the most notable programs was led by astronomer Edward Charles Pickering (1846-1919) at Harvard College Observatory. Pickering's group set up a sequence of spectral categories and set out to classify over 200,000 spectra: All the stars in the sky brighter than ninth magnitude. Annie Jump Cannon (1863-1941), the founder of modern spectral class- ifications, performed the bulk of this immense task personally. Cannon placed the stars in categories according to the prominence of certain spectral lines and simply labeled the categories with letters of the alphabet. The letter A was the beginning of the series. It indicated stars which showed the strongest hydrogen lines. Based on the relative strengths of the hydrogen lines and other elements, the other classes were assigned consecutive letters of the alphabet. The physical principles underlying the variations in spectral line intensity were not known then, but it was found that the spectral types would form a smooth sequence if they were arranged in the order O, B, A, F, G, K, M. Only later was it found that this sequence was sorted by stellar temperature, from the hottest stars to the coolest. Hydrogen presents its strongest lines only in stars of moderate temperature: Hotter stars ionize the hydrogen and remove its lines from the spectrum, cooler stars present spectra dominated by stronger lines of other elements. The results of this massive analysis and classification were compiled in the HENRY DRAPER CATALOGUE, named after amateur astronomer Henry Draper (1837-1882), who took some of the best early photographs of the Moon (1870), the Orion Nebula (1880), and also achieved early success in photographic spectroscopy. Henry was the son of the photographer John William Draper, who made the first photographic representation of the Moon, by Daguerreotypy, in 1840. Draper's widow funded research in astronomical spectroscopy and enabled completion of the catalogue in 1924. The HD designation remains a fundamental one in all astronomical literature. It serves as a common reference for all citations to most bright stars. Sir William Huggins was the first person to examine a planetary nebula with a spectroscope on August 29, 1864. That bright planetary was NGC 6543 in the constellation of Draco, also known as the Cat's Eye Nebula. Huggins had been observing the spectra of stars for years, but what he saw was a complete surprise. He wrote in his famous paper, "Spectra of Nebulae": "No spectrum such as I expected! A single bright line only." This provided a means of distinguishing between star light and the light of truly gaseous nebulae. Though some gaseous nebulae have many lines of spectra, they do not show a continuous spectrum as stars do. In contrast, the Great Nebula in Andromeda, now known as the Andromeda galaxy, was shown to have a continuous spectrum, indicating star light as the source. This was a great leap forward, as it became possible to distinguish between a gaseous nebula and a stellar object by using the spectro- scope. In 1865, Huggins was able to resolve the "single bright line" into three separate lines with the use of a spectroscope of higher resolving power. One of the three lines was identified as the beta Balmer line of hydrogen (H-b). The other two lines remained unidentified and it became clear that no element known in the laboratory would produce the lines seen. A new element, "nebulium", was assigned to these lines. This was not the first time that a new element was named in this way. In 1859, an unidentified line was observed in the solar chromosphere during a total solar eclipse, leading to the fabrication of the element "helium". The matching physical substance was found only in 1895. Ten years later, in 1869, an element called "coronium" was found in the solar corona. Helium was an easy element to identify in the laboratory. Nebulium and coronium resisted identification as separate elements, but were finally identified as being from emissions of existing elements in states difficult to reproduce in the laboratory. Nebulium proved to be doubly ionized oxygen (O-III), an oxygen atom that has lost two of its electrons. Coronium was identified in 1939 as an extremely ionized form of iron (Fe-XIV), an iron atom missing thirteen electrons. John Ludwig Emil Dreyer (1852-1926), a Danish astronomer, continued the search for nebulae. In 1888, he updated and expanded the 1864 Herschel publication to 7,840 numbered objects, calling it the New General Catalogue (NGC). He also published two supplements, Index Catalogues (IC), in 1895 and 1908, with a total of 5,386 additional objects. This brought the total object count up to more than thirteen thousand. The use of spectroscopes to routinely find very small planetary nebulae began just before Dreyer compiled the NGC. Thus virtually all of the NGC planetaries are extended objects, while many of the IC and later nebulae are almost stellar. The NGC and IC designations were the most commonly used ones until very recent times, when the number of objects catalogued made general catalogues slowly obsolescent. Many specialized lists identifying specific types of objects are now used in the research literature. Many list objects by numerical designations which indicate their crude position. One example is the 1932 Harvard College Observatory survey of nebulae brighter than thirteenth magnitude, which contained 1,249 nebulae (1,188 NGC, 48 IC, and thirteen others). Isaac Roberts (1829-1903) photographed some planetary nebulae and other nebulae in the late Nineteenth Century. Roberts was a successful businessman of Liverpool, England. As an amateur astronomer, he became a pioneer in celestial photography, specializing in star clusters and nebulae. His first plates were taken in 1885 at Maghull in Lancashire. By 1886, he was taking the first very good photographs of M42 and M45. In Crowborough in Sussex, in the southern part of England, he built an observatory with a fifty-centimeter (twenty-inch) reflector, which is now in the Science Museum in South Kensington, London. In 1888, Roberts made a three-hour exposure of the Andromeda Nebulae (M31), clearly showing its spiral structure. Roberts published the first volume of his photographs in 1893. The second, published in 1899 and titled PHOTOGRAPHS OF STARS, STAR CLUSTERS, AND NEBULAE, earned wide acclaim. A third volume was published posthumously in 1928, which was prepared and issued by his widow. The Royal Astro- nomical Society awarded him the Gold Medal for his photographic work. It was not until the latter part of World War One (1914-1918) that detailed studies of planetaries as a class were made. The prime investigator was American astronomer Heber Doust Curtiss (1872-1942), who used the ninety-centimeter (36-inch) Crossley reflector at Lick Observatory. Curtiss made numerous drawings from his photographic plates, showing very clearly the distribution of material in some of the nebulae. He also summarized the spectroscopic classification method, indicating that by 1870 it had been possible to distinguish between spiral nebulae and emission nebulae on the basis of their spectra. The emission nebulae were further subdivided into diffuse nebulae and planetary nebulae on the basis of their structure. Curtiss discussed this issue in the 1918 PUBLICATION OF THE LICK OBSERVATORY, Volume XIII, Part III, in an article titled "The Planetary Nebulae". He stated: "Greater differences of form and structure could scarcely exist than obtained between the small, clear-cut planetaries, and the enormous, tenuous, highly irregular, and cloud-like diffuse nebulosities." In the years between 1864 and 1918, the spectroscope was improved so that it became possible to measure the radial velocity of the nebulae from the Doppler shift. The distribution of different classes of nebulae was determined during this time period. It was found that the diffuse nebulae and planetary nebulae were close to the galactic plane, while the spiral nebulae were uniformly distributed except toward the "zone of avoidance", the galactic plane where the stars in the Milky Way block the view. Curtiss combined the radial velocities and the distribution of different types of nebulae to conclude that both diffuse nebulae and planetary nebulae are "an integral part of our own galactic system." He also concluded that the spiral nebulae were a class of their own and not part of our galaxy, but "perhaps individual galaxies." Curtiss used this information to consider the possible ranking of planetary nebulae in stellar evolution. At that time only about 150 planetary nebulae were known in the entire sky. They were therefore considered rare objects compared with the number of stars. Because of their rarity and evident association with stars, he surmised that they were either a common outcome of stellar processes but with a very short life, or that they were an unusual byway of evolution with a longer life. He found that the shorter lifetime explanation, requiring lifetimes of less than a few thousand years, did not "seem inherently probable." Curtiss' dissatisfaction arose from his consideration of their velocities through space, as revealed by detailed studies with the spectrograph. As planetaries seemed to have an exceptionally broad distribution of space velocities as compared to stars, they appeared to be exceptional objects not related to ordinary stars. He argued that because there were so few, "one one-thousandth of 1 per cent, or less", that "this minute percentage would seem to stamp the planetary at once as an exceptional case, a sporadic manifestation of a path which has been but rarely followed in stellar evolution....Perhaps of even greater weight as a support to the planetary nebula as an exceptional and sporadic case in stellar evolution is the fact that these bodies likewise stand apart from the stars in the very important criterion of average space-velocity." In his paper, Curtiss used the following table to present his main data as the basis for his statement: SPACE VELOCITIES Class Velocities (km/s) (miles/s) Stars Class B 12 8 Class A 21 13 Class F 29 18 Class G 32 20 Class K 34 21 Class M 34 21 Nebulae Diffuse low low Planetary 77 48 Spiral 770 480 As can be seen in the table, it is difficult to position planetary nebulae among the stellar classes due to their average space velocity. Furthermore, the early and erroneous concept of hot stars evolving "down the temperature sequence" into cooler and less luminous stars led Curtiss to classify the very hot central stars of planetary nebulae (known since 1918 to have temperatures as high as 300,000 Kelvin) as primordial, placing planetary nebulae at the top of the evolutionary chart. The problem arose in attempting to reconcile the large differ- ence that exists in the space velocities between Class B stars and planetary nebulae, which have an average space velocity six times greater. Curtiss concluded: "In view both of high average space- velocity and the rare occurrence of the planetary nebulae, it would seem much more reasonable to regard them as merely sporadic cases of stellar evolution, presumably of cataclysmic origin." William Wallace Campbell (1862-1938) and Joseph Haines Moore presented observations in a paper titled "The Spectrographic Velocities of the Bright-Line Nebulae", which was to ultimately show that Curtiss' arguments were wrong. In 1915, they made higher resolution spectra than had ever been possible up to that time. The emission lines were broadened and tilted in slit spectra, with the more prominent lines split in 23 of the 43 nebulae studied. The broadening and tilting of the emission lines were not understood by Campbell and Moore, but they believed it to be due to the rotation of the nebula. The splitting was thought to be caused by matter on the outside of the nebula which was supposed to be rotating more slowly than the emitting matter and therefore absorbing the central part of the spectral lines. They published their observational data in 1918. Ironically, it was in the very same journal in which Curtiss had published his article about space velocities! After Curtiss read Campbell and Moore's article and the observational data, he changed his view. Curtiss argued that the kinematic properties of the planetary nebulae placed them closer to Class M stars than to early type stars. Although it has been confirmed since the 1950s that cool giant stars (themselves arising from stars not much more massive than the Sun) are the progenitors of planetary nebulae, Curtiss' arguments were based on bad measurements: His inclusion of nebulae close to the galactic center biased his space velocity sample, which already suffered from the difficulty in measuring true "average" space velocities from an expanding bubble. In 1928, astrophysicist Ira Sprague Bowen (1898-1973), working at the Mount Wilson and Palomar Observatories, finally identified the spectral lines of "nebulium" as "forbidden-line" radiation from doubly ionized oxygen (O-III). Due to the very low density of gas in nebulae, atoms in an excited state which normally decay rapidly by atomic collisions may find themselves stranded in the higher state. In this unusual circumstance, decay to the ground state may take place by much rarer mechanisms, giving rise to the so-called "forbidden lines". The American astronomer Charles Dillon Perrine (1878-1951) first argued against the "slow gas" explanation for the splitting of the forbidden lines. He was soon joined by Dutch astrophysicist Herman Zanstra (1894-1972). They argued convincingly that the observed line profiles could be better explained by expansion of the nebula, resulting in a line shape indicative of the two velocities of the front and rear sides of the dynamic object. Using very rough estimates of the distance of the nebulae, it became possible to calculate the rate of expansion of the nebulae. The expansion velocity and nebular size made it possible to calculate the age of the nebula. The average age turned out to be about 104 years. This was the first direct evidence leading to our present view that most stars go through the planetary nebula stage. As the study and identification of other nebular emission lines continued, it brought new insight into the physical characteristics and new conclusions about the evolution of planetary nebulae. The chemical composition of the nebulae was one of the most important results to came from identification of the emission lines. This work was first done by Bowen and later by another American astronomer, Howard Donald Menzel (1901-1976) and his co-workers. In 1931, Zanstra showed that a nebula surrounding a hot star would completely absorb the ionizing radiation. His method for determining the number of ionizing photons and thus the temperature of the star from the size and density of the ionized region of a nebula is still used today and bears Zanstra's name. By 1940, it was shown that most of the elements in planetary nebulae were also present in the Sun. Improved studies continue to the present day. The big questions have been resolved for the most part. The fine details of creation remain, but they are slowly yielding to improved observational techniques and the availability of computers to model the late life stages of red giant stars. The next major chapters of our understanding are continually being written by current astronomical research. Related EJASA Articles - "Our Closest Neighbors in the Milky Way Subdivision", by Ingemar Furenlid and Tom Meylan - September 1989 "Long-Term Trends in Ground-Based Astronomy", an interview with Dr. Hal McAlister by Edmund G. Dombrowski - January 1990 "Stellar Spectroscopy: At the Heart of Astrophysics", an interview with Dr. Ingemar Furenlid by Edmund G. Dombrowski - March 1990 "Sir William Herschel and the Natural History of the Heavens", by Keith M. Parsons - June 1991 About the Author - Curtis A. Deer worked as an airplane mechanic for Eastern Airlines until the relentless forces of capitalistic imperialism conspired to exploit this hapless proletarian. Happily, Curtis found this to be an opportunity to pursue his true love, astronomy, in which he began study as an undergraduate at Georgia State University (GSU) in 1990. Curtis hopes to complete his degree and continue his studies for an advanced degree, working in the area of planetary nebulae and terminal stellar evolution. THE ELECTRONIC JOURNAL OF THE ASTRONOMICAL SOCIETY OF THE ATLANTIC February 1994 - Vol. 5, No. 7 Copyright (c) 1994 - ASA