Table Of ContentPL~er~,=,
NeaULl'e
•
APractical Guide and Handbook
for
Amateur Astronomers
Includes253finderchartsand acatalog of1340 objects
sreveN J. H':'NeS
Published by:
Willmann-Bell Inc.
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Published by Willmann-Bell, Inc.
P.O. Box 35025, Richmond, Virginia 23235 Preface
Copyright @1991 by Willmann-Bell, Inc.
Scattered among the millions of stars that spangle the heavens, the as
tronomer will find small, seemingly insignificant balls of hazy light, likened
All rights reserved. Except for brief passages quoted in a review, no part of
bytheearlyobservers to thefaint, distant outer planets, and called planetary
this book may be reproduced by any mechanical, photographic, or electronic
nebulae. For many years their nature remained a mystery and they were con
process, normayitbestoredinanyinformationretrievalsystem, transmitted,
sidered to be little more than a curiosity, a comparatively rare phenomenon
or otherwise copied for public or private use, without the written permission
which for the most part seemed to be associated with tiny, centrally situated
of the publisher. Requests for permission or further information should be
stars.
addressed to Permissions Department, Willmann-Bell, Inc. P.O. Box 35025,
A little over a century ago, however, the English amateur astronomer
Richmond, VA 23235.
William Huggins, usingearlyspectroscopic equipment, was able to showthat
the nebulous shells were genuinely ofa gaseous nature, the greatly extended
atmospheresofthecentralstars. Intheearlydecadesofthetwentiethcentury,
First published 1991
American astronomers Campbell and Moore made observations which were
toshowthat planetarynebulaewereexpandingat a rate indicatingan age no
greater than a few tens ofthousands ofyears, very small on the timescale of
Printed in the United States ofAmerica
cosmicevents. Theapparent rarityoftheseobjectswasshowntobemerelyan
artefactoftheirtransient natureandtheirtruesignificancewassoonrevealed;
they represent the penultimatestage in the life ofa medium-mass star. After
a comparatively tranquil existence ofmany thousands ofmillions ofyears on
Library ofCongress Cataloging-in-Publication Data.
the main sequence, such a star will become unstable as its fuel runs low and
Hynes, Steven J. expands to the vast size of a red giant, engulfing any inner planets in its
Planetary nebulae : a practical guide and handbook for amateur system, before shedding its distended outer layers as planetary nebula. The
astronomers / Steven J. Hynes. core ofthestar now settles down as a tiny white dwarf, perhaps onlythesize
p. em. ofthe Earth, yet so dense and massive and with such a high surface gravity
Includes bibliographical references and index. ~at a small coin such as a U.S. Quarter Dollar would weigh over 2 tonnes!
ISBN 0-943396-30-1 hus will be the fate ofour own star, the Sun.
1. Planetary nebulae. 2. Planetary nebulae- Amateurs' manuals. T~e
. purpose ofthis book is to describe for the amateur astronomer the
I. Title.
SCIentificproc b h' h .
QB855.5.H96 1991 90-25545 d . esses yw IC our understandmgofplanetarynebulae has been
523.1'135-dc20 elP .ealrlved and explore the way in which the threads ofobservation and theoret-
IC research h b
Th . ave een drawn together to produce our present day picture.
e st.o~ IS not straightforward, however; a variety ofother celestial objects
can mImIC the characteristics of planetary nebulae and many of these inter-
91 92 93 94 9596979898765432
iii
iv
lopers are not only difficult to root out but are also interesting in their own
right; these deserve a chapter oftheir own.
A major part of this book is devoted to a catalogue of planetary nebu
lae, with notes about objects ofparticular in interest and finder charts based
on photographic sources. The widespread use of large-aperture Dobsonian
mounted reflectors has broughtwithin reach oftheamateurastronomermany
Table of Contents
objectswhichonlya decadeago would havebeenconsideredthesoleprovince
ofthe professionalastronomer. Theselectionofobjectsinthecataloguetakes
account oftheever-increasingsize oftelescope available to amateurs but also
provides for the user of small and medium aperture instruments. In many
cases the planetary nebulae are still not well studied and even basic param
eters are poorly determined but they all represent a challenge to observers. Preface iii
In addition, many references have been provided to enable the advanced am
ateur, with access to professional journals, to pursue his research further. 1 The Discovery ofPlanetary Nebulae 1
1.1 Historical Perspective . 1
1.2 The Observations ofWilliam Herschel 2
Acknowledgements
1.3 The Discovery ofSpectroscopy 4
The preparation of this work has been greatly aided by a number of in 1.4 Huggins and "Nebulium" 5
dividuals and institutions. I would like to thank David A. Allen, Edmund S. 1.5 Early 20th Century Studies .. 7
Barker, andSteveGottleibfor someusefulandinterestingcorrespondenceand 1.6 Modern Discoveries ofPlanetary Nebulae 8
conversations, and Keith Lowe, proprietor of Dark Star Telescopes (U.K.), 1.7 The "P&K" . 9
for use of his facilities. Of particular value, though, has been the numer 1.8 Recent Planetary Nebula Discoveries 10
ous contributions ofBrian A. Skiff. Much ofthe research was carried out at 1.9 Extragalactic Planetary Nebulae 11
the libraries ofthe Royal Astronomical Society, the University ofKeele, and 1.10 References . . . . . . . . . . . . . . . 13
the Nuffield Radio Astronomy Laboratories, Jodrell Bank. I am indebted to
2 The Astrophysics ofPlanetary Nebulae 15
the authorities ofthese institutions, but would like especially to thank Peter
Hingley at the R.A.S. and Joan Adams of Jodrell Bank for their help and 2.1 The Distances ofPlanetary Nebulae .. 15
hospitality. Barry N. Rappaport provided computer programming expertise 2.2 The Spectrum ofa Planetary Nebula . . 17
at several stages of this book's preparation which was particularly valuable. 2.3 The Origin ofPlanetary Nebulae . . . . . . . . . . 20
Finally, a special mention must be given to Perry W. Remaklus, President 2.4 Morphology and Classification ofPlanetary Nebulae 23
and Publisher ofWillman-Bell Inc., for his help, ideas and support. 2.5 Planetary Nebulae Central Stars . . . . . . . . . . . 26
2.6 References . ................ 32
3 Planetary Nebula Mimics 35
Steven J. Hynes
3.1 Introduction. . . . . . 35
3.2 Symbiotic Stars . . . . 36
3.3 Bipolar Nebulae 37
3.4 Compact H II Regions 38
3.5
Be Stars . 39
3.6
Ring Nebulae Associated with Wolf-Rayet Stars. 39
3.7
Galaxies . 40
3.8
References . . . . . . . . . 42
v
vi
4 Amateur Observation ofPlanetary Nebulae 43
4.1 Learning to See 43
4.2 Telescopes...... 44
4.3 Star Charts . . . . . 46
4.4 Observing Procedure 50
4.5 Prism Observations. 59
Chapter 1
4.6 Nebular Filters . . 60
4.7 Astrophotography 65
5 Catalogue and Atlas ofPlanetary Nebulae 69 The Discovery of Planetary Nebulae
5.1 Introduction . 69
5.2 Source Material . 70
5.3 Extended Notes . 71
5.4 Cross Index ofPlanetary Nebulae . .105
5.5 Misclassified Planetary Nebulae . .117 1.1 Historical Perspective
5.6 Catalogue ofPlanetary Nebulae. . 121
Even before the invention of the telescope, observers of the sky recog
5.7 Atlas ofPlanetary Nebulae . 165
nized that amongthe "fixed stars" were also tiny, hazy patchesoflight called
Biblography 249 "nebulae" (little clouds). Such nebulae can be seen in the constellations of
Orion, Sagittarius, Cancer, and Andromeda, and by 1609, when Galileo first
Appendix A References to Catalogues and Lists of applied his diminutive "optick tube" to the heavens, perhaps eight or nine
Planetary Nebulae 251 were known.
Galileopresentedhisearlyobservationsinhisbook Sidereus Nuncius (The
Index 257 StarryMessenger), which was published in March 1610. Clearly, he had been
Subject Index .257 astonished by what he had seen:
Object Index .259 I haveobservedthenatureandmaterialoftheMilkyWay. Withtheaid
Name Index. .259 ofthetelescopethishasbeenexaminedsodirectlyandwithsuchocular
Planetary Nebulae Chart Index .261 certainty that all the disputes which have vexed philosophers through
so many ages have been resolved, and we are at last freed from wordy
debates about it. Thegalaxy is, infact, nothing but innumerablestars
grouped together in clusters. Upon whatever part of it the telescope
is directed, a vast crowd of stars is immediately presented to view.
Many are large and bright, while the number ofsmaller ones is quite
incalculable.
Having resolved the structure ofthe Milky Way, he examined the nebulae:
ButitisnotonlyintheMilkyWaythatwhitishcloudsareseen; several
patches ofsimilar aspect shinewithfaint light here and there through
out the aether, and ifthe telescope is turned upon any ofthem we are
confronted by a tight mass ofstars. And what is even more remark
able, thestarswhich have beencalled "nebulous" by everyastronomer
to this time turn out to be a group of very small stars arranged in a
wonderful manner. Although each star separatelyescapes our sight on
accountofitssmallness, ortheimmensedistancefrom us, the mingling
1
2 Chapter 1. The Discovery ofPlanetary Nebulae 1.2. The Observations ofWilliam Herschel 3
oftheir rays gives rise to that glow which was formerly believed to be coverer ofthe planet Uranus on 1781 March 13. In recognition ofthe latter,
somedenser part ofthe aether that was capableofreflecting rays from the first discovery ofa major planet ofthe Solar System in recorded history,
stars or from the Sun. he was elected into the Royal Society on 1781 December 7, and subsequently
awarded an annual stipend of £200 by King George III. This enabled him to
The Galilean view that all nebulae could ultimately be resolved into stars
give up music as a career and concentrate on astronomy.
was, of course, incorrect, but persisted for some time, even though some,
On the day of his election to the Royal Society his friend, Dr. William
such as that in the Sword ofOrion, refused to be brokendown intoindividual
Watson, sent him a copy ofthe new catalogue of nebulae recently published
components. Scant attention was paid to the nebulae for over a century and
by Messier and Mechain. This clearly stimulated his imagination and he
even then only because they were found to be a nuisance to comet-hunters.
resolved to look for more-he was to discover some 2300 within a period of
Even now, there is a certain prestige attached to the discovery of a new
just seven years! With this significant database ofobservational material, he
comet and in the eighteenth century it was a sure way of achieving recogni
began to classify the objects he found and placed them in eight categories:
tion and reward. A comet-hunter of particular renown was the Frenchman,
Charles Messier, born in 1730. In 1750 he went to Paris to become ap
I Bright nebulae
prenticed to the prominent astronomer Delisle; he soon became a proficient II Faint nebulae
observer and eventually discovered 13 comets. Telescopically, comets often III Very faint nebulae
appear as hazy patches oflight, distinguished from the nebulae only by their IV Planetary nebulae
V Very large nebulae
movement across the sky. Messier found that his attempts to increase his
VI Very compressed, rich clusters ofstars
tally ofdiscoveries were continually frustrated by the realization that objects VII Compressed clusters ofsmall and large stars
he believed to be comets were, in fact, fixed among the stars. For his own VIII Coarsely scattered clusters ofstars
benefit and the benefit ofother observers, he decided to catalogue these ob
Herschel gave the name "planetary nebulae" to the fourth category of
jects, laterwiththe assistanceofPierre Mechain, and betweenthem 110were
objects because their small, well defined, sometimes greenish discs reminded
eventually recorded. The 27th entry in the catalogue, found on 1764 July 12,
himofthe appearanceofthe planet Uranus. He seems to have felt that these
was the first discovered member of the class ofobjects later to be known as
characteristics identified them as being different from the other types to the
"planetary nebulae."
extent that perhaps they were not even true nebulae. On the other hand, if
theywerestarsoflargeenoughdiameterforthedisctobedetectable, hecould
1.2 The Observations ofWilliam Herschel
not account for their comparative faintness. At one point he considered the
possibilitythattheywereinfact starclustersrenderednebulousinappearance
ThediscoveriesofMessier and Mechaingreatlyincreased the known num
becauseofthefaintnessofthecomponentsandthehighdegreeofcompression;
ber ofnebulae and excited the interest of amateur astronomer William Her
he speculated that they were in the process ofcoming together under gravity
schel, livinginEngland. Bornin1738intheGermancityofHanover, Herschel
to "unite into a new body," adding "Perhaps the extraordinary and sudden
became a musician in the regimental band ofthe Foot-Guards, joininghis fa
blaze ofa new star in Cassiopeia's chair of 1572 might possibly be ofsuch a
ther and brother Jacob. In 1757 at Hastenbeck, he faced battle and found
nature."
that insuchcircumstancesthesafetyofmusicians was not a military priority.
On the night of1790 November 13 he discovered a new planetary nebula
He managed to obtain a dischargeand went to Hamburg, where heand Jacob (now known as NGC 1514) which caused him to completelyrethink his ideas
(whohadbeeninhidingtoavoid conscription) tookaboatfor England. After on these objects. His original observation reads as follows:
two years, Jacob returned home to Hanover but William stayed on, pursuing
Amostsingular phenomenon! A star ofabout the8thmagnitude with
his love ofmusic. Eventually, heset up home in Bath, and in 1772wasjoined
a faint luminous atmosphere, ofcircular form, and about 3 minutes in
by his sister Caroline who herself became an astronomer of no small repute.
diameter. The star is perfectly in the center, and the atmosphere is so
On 1773 May 10, at the age of 35, he found his interest in astronomy after delicate, faint andequalthroughout that therecanbe nosurmiseofits
buying a copy ofFerguson's Astronomy, a popular text ofthe time. consisting ofstars; nor can there be a doubt ofthe evident connection
It is possiblethat neither beforenorsincehas a simplebook purchasehad between the atmosphere and the star. Another star, not much less in
sucha dramaticeffect onthe directionofastronomy. With hiseverincreasing brightnessandinthesamefield withthe above, was perfectlyfree from
enthusiasm he became the leading telescope-maker of his time and the dis- any such appearance.
4 Chapter 1. The Discovery ofPlanetaryNebulae 1.4. Huggins and "Nebulium" 5
This observation, together with his conclusions, was published in a paper t In their experiments they had made observations ofthe spectra of
apparen . h' d d "b . ht r" t
called "NebulousStars, properlyso called," in the Philosophical Transactions . descent substances and found that t IS pro uce ng me spec ra,
mcan . . f . l' . th
of the Royal Society in 1791. Herschel reasoned that, since the star was so and moreover, it was possibleto match upthe pOSItions0 cehrtam mes:n. e
exactly centrally situated in the nebula, it was unlikely to be just a chance trum against specific elements. It soon became clear t at the pOSItIOns
superposition and they were therefore part of the same system at the same sopfetche darklines found byFraunhoferwe:e aIsom. d'Ica~I.ve 0fkno~.ieemten s.
distance. For the nebula to have been composed of unresolved stars they
Soon this amazing discovery, by whIch the chemIcal composItIOn of ob
would have had to have beenso faint incomparisonwith the centralstarthat co~ld
jects be determined by messages encoded in light,.was applied to the
such a situation was beyond belief. He concluded that:
heavens. By visual observation ofthe solar sp~ctru~, Klr~hhoffwas abl: to
We therefore have a central body which is not a star, or have a star identifythe elements sodium, magnesium, calcmm, Iron, nIckel, copper, zmc,
which is involved in a shining fluid ofa nature totally unknown to us.
and barium. Another noteworthy discovery around this time was that ofthe
Preferring to accept the latter explanation, Herschel went on to develop spectral lines ofhydrogen, this by Angstrom in 1862: . .
the implications ofhis conclusion: In more general terms, Kirchhoffwas able to denve the basIC laws whIch
Butwhatafieldofnoveltyishereopenedtoourconceptions! A shining determine the type ofspectrum observed in any particular circumstances:
fluid of a brightness sufficient to reach from the remote regions of a
1. Continuous spectrum-produced by an incandescent solid or liquid
star ofthe 8th, 9th, 10th, 11th and 12th magnitude, and ofan extent
so considerable as to take up 3, 4, 5 or 6 minutes in diameter...We or very dense gas.
may now explain that very extensive nebulosity, expanded over more
2. Emission (bright line) spectrum-produced bya luminous but rar
than sixty degrees ofthe heavens, about the constellation ofOrion; a
efied gas.
luminous matter accounting much better for it than clustering ofstars
at a distance.
3. Absorption (dark line) spectrum-produced by the presence of a
gas between the observer and a hotter source of continuous spectrum
1.3 The Discovery ofSpectroscopy light.
The nature of Herschel's "shining fluid" remained mysterious until the
1.4 Huggins and "Nebulium"
middle of the nineteenth century, when the spectroscope was first applied
to celestial objects. This became possible following the work of Joseph von
ThediscoveriesmadebyKirchhoffstimulatedtheimaginationofmanysci
Fraunhofer who, in 1814, found that if sunlight was passed through a long,
entists but in particular the English amateur astronomer William Huggins.
narrow slit in front of a prism, the spectrum produced was crossed by a
Huggins, like many of the Victorian "gentleman scientists," was a man of
number ofdark lines. Eventually, he was able to map as many as 574 against
wide-ranging interests and occupied himselfwith chemistry, physics, and mi
the colored band. Fraunhofer did not know the cause of the lines, nor was
croscopical research, as well as astronomy. However, around 1853 he decided
he the first to observe them, since Wollaston had also seensome as indefinite
to concentrate on observational astronomy and in 1856 built an observatory
shadings in 1802, but he had apparently dismissed them as vague divisions
attached to his house at Thlse Hill, south ofLondon. This he equipped with
between the colors. However, Fraunhofer's systematic observations were of
a 12.7-cm Dollond equatorially-mounted refractor and an 8.25-cm transit
such a quality and precision that the lines are still called "Fraunhofer lines"
circle. In 1858 he bought a fine 20.3-cm Alvan Clark lens from the Rev. W.
in some textbooks, and the alphabetical sequence he devised to identify the
R. Dawes, and had it mounted by the well-known telescope makers, Cookeof
more prominent is still used (e.g., the "D" line ofsodium). York.
Inthe1850'sa numberofscientistswereinvolvedinresearchontheFraun Huggins' early observations were undirected and he recorded details of
hofer lines, and of particular importance is the work ofAnders Angstrom in stellar occultations and features on the planets. However, when he read of
Swedenand Kirchhoffand Bunsenin Germany. Angstrom actuallyanticipat Kirchhoff's work in 1862 he decided that the most useful scientific work he
ed much of the work of the German scientists but his memoir of 1853 was could employ himself with would be the application of spectroscopy to the
published in Swedish and was overlooked by most ofthe scientific communi stars. With the assistance of his friend and neighbor, William A. Miller,
ty. It was not until 1859 and 1860 that Kirchhoff and Bunsen published the pr?fessor ofchemistry at King's College, he built a spectroscope suitable for
results oftheir work and the true significance ofthe Fraunhofer lines became this Purpose and published his first report in the Proceedings of the Royal
6 Chapter 1. The Discovery ofPlanetary Nebulae 1.5. Early 20th Century Studies 7
Society the following year. A more comprehensive report was produced in 1.5 Early 20th Century Studies
1864 and he concluded that:
Ofcourse, the larger and totally unrelated diffuse nebulae also exhibited
...in plan of structure the stars, or at least the brightest of them,
a similar spectrum to the planetary nebulae but they were differentiated on
resemble the Sun. Their light, like that of the Sun, emanates from
the basis of their morphology, as explained by Heber D. Curtis in a classic
intensely white-hot matter, and passes through an atmosphere of ab
treatiseon planetary nebulae in Lick Observatory Publications, Volume XIII,
sorbent vapours. With this unity of general plan of structure there
exists a great diversity amongst the individual stars. Star differs from 1918:
star in chemical constitution. Greater differencesofform and structure could scarcely exist than ob
tain between the small, clear-cut planetaries, and the enormous, ten
Soon, Huggins turnedhis telescopeandspectroscopetowards the nebulae
uous, highly-irregular and cloud-like diffuse nebulosities. Many small
and in particular, towards the planetarynebulaNGC 6543 inDraco. Helate;
masses ofdiffuse nebulosity are associated with stars more or less cen
described his remarkable observation in the Nineteenth Century Review of
June 1897: . trally placed within the nebula, but there is never in such cases, any
resemblance to thetypicalplanetaryform, nor is thereany doubt as to
I looked into the spectroscope. No spectrum such as I expected. A
the group ofnebulae with which such objects should be classified.
single bright line only! At first I suspected some displacement ofthe
prism, and that I was looking at a reflection of the illuminated slit In 1918, less than 100 planetary nebulae were known and it was apparent
from oneofitsfaces. This thoughtwasscarcelymorethan momentary; that they were comparatively rare objects. Curtis concluded that they were
either the manifestation of some genuinely exotic evolutionary process, or a
then the true interpretation flashed upon me. The light ofthe nebula
phase instellar evolutionofrelatively briefduration which many stars passed
was monochromatic, and so, unlike any other light I had as yet sub
through. Althoughthelatterisnowknowntobethecase, Curtis, onthebasis
jected to prismatic examination, could not be extended out to form a
ofthe limited data available to him, was unable to accept this possibility:
complete spectrum. After passing through the two prisms it remained
concentrated into a single bright line, having a width corresponding to ...it would seem much more reasonable to regard them as merely spo
the width ofthe slit, and occupying in the instrument at that part of radic cases ofstellar evolution, presumably ofcataclysmic origin.
the spectrum to which its light belongs in refrangibility. Alittle closer
Interestingly, the observationswhich ultimatelyproved Curtiswrongwere
looking showed two other bright lines on the side towards the blue, all
also published in the same work. W.W. Campbell and J.H. Moore had made
the three lines being separated by intervals ofrelatively dark.
high-resolution spectroscopic studies of43 planetary nebulae and found evi
The riddle ofthe nebulae was solved. The answer, which had come to
dence ofinternal motions in 23. The spectral lines were broadened, in a few
us in the light itself, read: Not an aggregation ofstars, but a luminous
cases to the extent that they were split; the lines were also tilted. Campbell
gas. Stars after the order of our own Sun, and ofthe brighter stars,
would give a different spectrum; the light of this nebula had clearly and Moore suggested that this phenomenon was the result ofthe rotation of
been emitted by a luminous gas. With an excess of caution, at the the nebulae; however, within a few years, C.D. Perrine and H. Zanstra had
moment I did not venture to go further than to point out that we had shown that it could more simply be explained by nebular expansion. Having
heretodowithbodiesofanorderquitedifferentfrom that ofthestars. made measurements of the size of the nebulae and their expansion rates, it
A few days later, Huggins observed the Andromeda Nebula and found then became possible to estimate their ages, which turned out to be in the
a continuous spectrum, correctly concluding that this object was formed of order of a few tens of thousands of years, clearly indicating that the plane
innumerable faint stars. Over the. next four years, he spectroscopically ex tary nebulaeareobjectsofcomparativelyrecent formation. Since it would be
amined about seventy nebulae, finding that about a third exhibited a "bright reasonable to assume that as the nebulae expand the density ofthe gas falls
line" spectrum. In each case, the brightest line was in the green part ofthe an~ surfac~ brightness diminishes, it was apparent that within a fairly short
spectrum, ata wavelengthof500.7nm; thislinewasknownasthe "ChiefNeb penod oftime they would fade into oblivion.
ularLine" buttheelementwhichitsexistenceproclaimedremaineda mystery. Discoveries of planetary nebulae grew slowly; the catalogue produced by
It was presumed that it must be some previously unknown substance, called Vorontsov-Velyaminovin 1934, for example, listedjust 134objects. However
som . 'fi '
"nebulium." This was to be proved incorrect, but not for another 60 years. e slgm cant advances were made in understanding the astrophysics of
p~anetary
Meanwhile, the spectroscope remained a highly effective tool for the classifi nebulae. Of particular importance was the work of Ira S. Bowen
~uth~ ~alifornia
cation ofnebulae, to the extent that ofthe 67 planetary nebulae listed in the Institute of Technology, who in 1927 was able to identify
New General Catalogue (NGC) of 1888, 57 are still accepted as such today. ggms mysterious "nebulium" lines.
8 Chapter 1. The Discovery ofPlanetaryNebulae 1.7. The "P&K" 9
Itis well known that spectrallinesarisewhenelectronsinanatom "jump" 1.5-meter and 2.5-meter reflectors. This first list was soon followed by lists
(Le., make a "transition") from oneenergy levelto another. Sometransitions in 1947 (56 objects) and 1948 (55 objects).
are more probable than others and clearly, those transitions which are most Duringthe early 1960's, several catalogues ofnote were produced; in par
probable will give rise to the strongest lines. Bowen knew that there was ticular those ofB.A. Vorontsov-Velyaminov in the Soviet Union and Karl G.
no place for a new element in the Periodic Table so the unidentified lines Henize'in the U.S.A. In 1961, the latter produced a short list of 7 objects
at 500.7 nm and 495.9 nm must have been a known element for which the (He 1- ) found in the Cygnus-Vulpecul~r~g~ondurin? the co.urse ofobtai~ng
transitions were improbable under terrestrial laboratory conditions. Such objective-prism plates to compare the hmItmg magmtude WIth that obtamed
lines are called "forbidden lines." Having given further consideration to the during his greatly more extensive southern Ha survey. The southern survey
matter, Bowen realized that the energy differences between the two energy (He 2- ), eventually published in 1967, listed 468 objects, many ofwhich are
levels calculated for doubly-ionized oxygen corresponded exactly to the fre planetary nebulae.
quencies ofthe nebulium lines and therefore they were the forbidden lines of Undoubtedly the greatest impetus to the discovery of planetary nebu
doubly-ionized oxygen [0 III]. lae, however, came from the publication ofthe National Geographic Society
The fact that [0 III] lines were so difficult to reproduce in the laboratory Palomar Observatory Sky Survey (P.O.S.S.). This was carried out using a
yet dominated the spectraofplanetary nebulae indicated that the conditions 1.2-meter Schmidt telescope and covered the entire northern sky down to
which exist in these objects are quite remarkable. In the laboratory the Declination -33°.
excited level is heavily depopulated by electron collision, greatly reducing The P.O.S.S. consists of 935 pairs of plates-one blue sensitive (Kodak
the intensity ofany [0 III] emission. On the other hand, in an extremely low 103a-0 plates) and one red-sensitive (Kodak 103a-E +red plexiglass)---each
densityenvironment, suchas canbefound in planetarynebulae, de-excitation covering a field of 6° x 6° at a scale of 1 mm = 67". The limiting stellar
by this means is much less important. magnitude is 21 in the blue and 20 in the red.
Once this phenomenon was understood it became possible to identify a Using these plates a number of investigators, notably Lubos Kohoutek
large number of other lines and from these, the chemical composition and and George Abell, have found a considerable number of new nebulae. In
physical processes occurring in planetary nebulae. the case of Abell, 86 objects were recorded (although a small number have
AnotherinterestingdevelopmentatthistimewasthediscoverybyZanstra, been found to be misclassifications). Even in very recent years, searches of
in 1931, ofa method for determining the temperature ofa planetary nebula the P.O.S.S. plates have proved fruitful; for example, Weinberger reported 12
nucleus (or indeed, any star involved in a low density nebula). This enables new planetary nebulae in 1977 and Dengl et al. found 5 in 1980. A complete
one to "count" the number ofionizing photons emitted by the star, the tem reference list ofplanetary nebulae is given in Appendix A.
perature being found by comparing this with the number ofphotons emitted
at visual wavelengths. "Zanstra temperatures" are often quoted in the pro
1.7 The "P&K"
fessional literature.
Until 1967, astronomers studying planetary nebulae were hampered by
1.6 Modern Discoveries ofPlanetary Nebulae the lack of a "general catalogue" to act as a database. Observations, find
er charts, and individual listings were scattered through many publications
Although there have been many great advances in understanding the as produced over a long period oftime. Short lists, such as the two by Merrill
trophysics of planetary nebulae since 1945-some ofwhich will be discussed which contained only 2 objects each, could easily be overlooked. This major
in later sections-perhaps the post-war years might be considered as charac gap in the literature was corrected with the publication of the Catalogue of
terized by an ever-increasing rate of planetary nebula discoveries. Galactic Planetary Nebulae (CGPN) by Academia, the publishing house of
In 1946, Rudolf Minkowski published his first catalogue of new emission theCzechoslovakAcademyofSciences. ThecataloguewascompiledbyLubos
line objects, found on objective-prism survey plates obtained by W.C. Miller PerekandLubosKohoutek, and contains 1036entries, coveringalldiscoveries
with a 25.4 cm telescope. He subsequently classified 80 of these objects as up to 1965. Despite quite a number of misclassifications, the "P&K"-as it
planetary nebulae on the basis of more detailed spectra or their appearance is often known-remains the standard reference catalogue.
on direct photographs obtained at the Newtonian focus ofthe Mount Wilson This catalogue introduced a new system ofidentifying objects which has
been extended to cover all subsequent discoveries and has now been adopted
10 Chapter 1. The Discovery ofPlanetary Nebulae
1.9. Extragalactic PlanetaryNebulae 11
as thestandardwayoflistingplanetarynebulae. However, althoughtheP&K
As far as future discoveries are concerned, it seems likely that more new
numbering is the preferred listing system, it is generally used in conjunction
lanetary nebulae will be found on the Southern Sky Survey plates and also
with personal designations rather than in placeofthem. Thesystem is based ~n
the forthcoming Second Palomar Observatory Sky Survey, which is being
on galactic co-ordinates rather than the traditional R.A. and Dec. The order
repared using the improved emulsions which have become available since
oflistingisinone-degreestripsofgalacticlongitudeand from the northto the
rhe original was published in the mid-1950's. However, it does seem certain
south galactic pole inside each strip; the number is formed by truncating the
that mostofthefuture discoveries will beofobjectsoflowsurface brightness.
galactic longitude and latitude to integer degrees and then allocating serial
Further discoveries might also be expected through objective-prism surveys
numbers from north to south within each lOx 1° compartment. New objects
specifically for this purpose or photographic surveys of small sections ofthe
are slotted into the system by assigning the next free serial number in the
Milky Way at high resolution.
particular compartment. For example, the planetary nebula Pu1, reported
by A. Purgathofer in 1978 (ref. 1), lies at galactic co-ordinates I= 181?7, b=
+1?3; in the P&K system this becomes PK181+1°1. The same astronomer 1.9 Extragalactic Planetary Nebulae
found Pu2 in 1980 (ref. 2); this lies at galactic co-ordinates I = 173?5, b=
+3?2 and would be designated as PK173+3°1. Thesmallsizeandcomparativefaintness ofplanetarynebulaemakes them
The Catalogue of Galactic Planetary Nebulae contains much valuable po a difficultsubject for studyat thegreat distancesofeventhe nearby galaxies.
sitional and other data and is particularly useful for its selection of finder Despite this, considerable progress has been made in this field during recent
charts. All but two of the objects in the catalogue are represented, but the years, althoughinfact thefirst "extragalactic" planetarynebulacould besaid
chartsarenotahomogeneoussetandvarygreatlyinscale, depth, andquality. to be that discovered by Pease in the globular cluster M 15 in 1928.
The importance of the extragalactic planetary nebulae is largely that,
beingessentiallyat thesamedistance, theirparameterscanbecomparedwith
more precisionthan those in the Galaxy, whosedistances are not well known.
1.8 Recent Planetary Nebula Discoveries
In addition, their chemical composition reflects to some extent the chemistry
ofthe parent galaxy, so it can shed light on this aspect ofastrophysics.
Recently, many discoveries ofnew planetary nebulae have occurred in the
Obviously, extragalactic planetary nebulae are not likely to be recognized
southern hemisphere, having been found on the blue ESO-SERC Southern
bytheir morphology---evenin the closest galaxies the largest are not likelyto
Sky Survey plates, obtained with the 1.2-meter U.K. Schmidt Telescope at
be larger than 3"-therefore, spectroscopic techniques must be the primary
Siding Spring, Australia. This survey has used Kodak IIIa-J plates plus a
detection method. Candidates must be stellar (or virtually so), with strong
Schott GG 395 filter; this is a particularly useful combination for recording
emission lines including [0 III] and [0 II], and little or no continuum.
planetary nebulae as the [0 III] lines at 500.7 nm and 495.9 nm, and the H,B
TheMagellanic Clouds, inthesouthernhemisphere, arethenearestgalax
line at 486.1 nm are all covered by the passband.
iesinwhicha large numberofplanetarynebulaecan beidentifiedandstudied
Lists of new planetary nebulae discovered on the Southern Sky Survey
in detail. As a result of several surveys since the mid-1950's, a total of 51
plates have been published by, among others, A.J. Longmore (17 objects),
planetarynebulaehave beendiscovered in theSMC and 137inthe LMC; esti
and A.J. Longmore and S.B. Tritton (11 objects). In addition, Holmberg et
mates ofthe total number in the Clouds are about 300 and 1000respectively.
al. have published several extensive lists of new objects of all kinds, which
Spectroscopicanalysisofthe MagellanicClouds planetarynebulaeshowsthat
include planetary nebulae, based on the ESO(B) Atlas.
the helium and carbon abundance is comparable to that in the Milky Way
These and other new discoveries of planetary nebulae are being included
?ut.oxygen, nitrogen, and neonare significantly underabundant. Overall, the
in the new Strasbourg-ESO Catalogue ofPlanetary Nebulae (SEC) and great 1ll.dIcations are of metal deficiency in the interstellar medium ofthe Clouds,
care has been taken by the authors, Agnes Acker and her co-workers, to WIth the SMC havingslightlylower abundances than the LMC. Ifthis is cor
eliminate objects which have previously been misclassified. The SEC seems ~ect, it may be concludedthat the Cloudshave beenchemicallyenrichedonly
destinedtoreplaceP&Kasthestandardplanetarynebulacatalogue, although ~n the last few thousand million years, perhaps as a result of a burst ofstar
the numbering system will remain the same. A full account ofthe proposed ?rmationhavingoccurred (ref. 4). The brightest LMC planetarynebulae are
l
scope ofthe SEC was given by Acker in 1987 (ref. 3). Isted in Table 1-1.
