The Horsehead Project:
THE HORSEHEAD TAKES ON ITS SHAPE AND POPULAR NAME

Isaac Roberts' Successful Failure.

A great tradition of amateur astronomy existed in 19th century England, with William Herschel as the most distinguished early example, followed by numerous wealthy amateur telescope makers and observers who made many more discoveries of interest to later astrophysicists than had many of the conservative professional astronomers, who often preferred to concentrate on refining orbital calculations and improving star maps using moderate-aperture refractors mounted in convenient observatories not ideally situated for the best conditions of seeing.

Pioneering Astrophotographer Dr. Isaac Roberts, courtesy of Mary Lea Shane Archives of Lick Observatory

The success of Lord Rosse's gargantuan 72-inch reflector telescope (completed in 1845) pointed the way toward the eventual adoption of the reflecting mirror over the expensive refracting lens, and presaged the work of latter- century astronomers, who would come to appreciate rural sites and the efficient power of large "light buckets".

The Welshman Isaac Roberts (1829-1904), a skilled builder turned amateur astronomer, forsook business at the height of his wealth, and used his resources to benefit science by becoming the leading (amateur) astrophotographer of the British Isles in the last two decades of the 19th century. Breaking with tradition, he ordered a superb and accurate 20-inch aperture reflector telescope from a leading manufacturer of observatory instruments, Sir Howard Grubb. Despite the excellence of the telescope, Roberts made numerous improvements until it was capable of perfectly tracking a star image at high power for several minutes without any manual guiding adjustments.

At first Roberts intended to produce a photographic survey of the entire northern sky, but he cancelled his project in 1887 when learning in Paris of the plans for the famous (and eventually abandoned) "Carte du Ciel", turning instead to a program of photographing nebulae. His October 1887 photograph of the Andromeda "nebula" (as galaxies were known before Hubble's time) preserved the first permanent image of its spiral shape, hard to detect in typical narrow-field observatory refractors.

In 1887, Roberts had met the next woman in the Horsehead story, the San-Francisco-born professional astronomer Dr. Dorothea Klumpke (1861-1942), then on the staff of the Paris Observatory, eventually to rise to the head of its mathematical department. She too was an astrophotographer, whom Roberts considered his superior in skill, and while the two were photographing the sky while floating above Paris in a balloon, the 72-year old amateur proposed marriage to the 40-year old professional!

After their marriage in 1901, Dorothy Klumpke Roberts devoted herself to assisting her husband's work at his private observatory in Sussex, where Roberts was now engaged in photographing Herschel's 52 nebulous regions. Roberts' plates of bright nebulae and star clusters may have won him acclaim and an honorary doctorate, but his photographs of the nebular regions would prove perplexing to observers like Edward E. Barnard, who had by now visually verified most of Herschel's claims.

Roberts employed 90-minute to 2-hour exposures, long enough for today's fast film, but too slow for the emulsion of the era.

Though pin-point stellar images down to the 15th magnitude were recorded: the faint nebulosity was missing. When he published his results in 1903, Roberts reported the sky to be clear in region after region, causing an irritated Edward Barnard to counter with a devastating rebuttal and to press forward with his own second photographic survey of the Milky Way.

One of the finest plates in Roberts' Herschel series was the Horsehead as shown in the photograph of Nebular Region no. 25, taken on January 25, 1900 (and published, as shown here, as a positive print in the Astrophysical Journal, included by permission.)

Though not all of the nebulosity in the cloud IC-434, discovered earlier at Harvard, was registered, the large scale of the photograph captured the structure of the Horsehead with improved clarity and edge definition compared to earlier attempts. When later published by his wife on the centennial of his death in 1929, this photograph -- shown as a negative -- would be one of the best in "Isaac Roberts' Atlas", brilliantly compiled by Dorothea Klumpke Roberts as an aid to observers of the '52 nebular regions'.

Using extreme amounts of image processing, the author prepared a highly enhanced 'closeup' of the Horsehead -- below, right -- showing that if Roberts had examined a contrast-boosted print, he might not have concluded, so easily, that the remarkable dark region 'bifurcating' the nebula was merely a discontinuity: for in this example, the Horsehead cloud is distinctly depicted as blocking the bright nebula "behind" it, as perceived along earth's line of sight.

By the date of publication in 1929 of the Atlas it was finally conceded that the photographs inadequately represented the diffuse nebular clouds, so the plates were presented as aids to researchers for accurately obtaining the appropriate star-fields for studying Herschel's nebular regions. A visual study of them in the dark skies provided by World War One blackouts was conducted by Father Johann Hagen of the Vatican Observatory, and finally confirmed by eye all of Herschel's claims of nebulosity, though there was a certain underlying inference -- perhaps an understandable 'family bias' -- that perhaps photography wasn't adequate to the challenge! In this regard, Roberts' Atlas curiously represents an intersection of the old and the new: photography employed to try to confirm the by-now almost "ancient" observations of Herschel, but used to promote further (obsolete and by definition subjective) visual study, which could only help to assert the claims of an eyepiece-observer, not preserve absolute evidence.

Edward Barnard SEES the Horsehead

Having profited from his experiences with the Willard lens in the 1890s at Lick on Mt. Harmilton, Barnard was now employing exposure times of 5 or 6 hours to register the faintest possible images, showing for the first time dim clouds of nebulosity that could not be visually perceived even in the large reflectors coming into general use. Though a pioneer in astrophotography, Barnard realized that certain details were still missing in the images on his plates, surmising that a general celestial background glow was wiping out fine shades of contrast.

An obsession for thirty years of his career was the study of the mysterious dark nebulae, for which, he said, the spectroscope was of no use, since there was practically no palpable light that could clearly register a spectrum on the instruments and films of the time. Barnard's alternative procedure was to photograph the dark regions, and then to examine the same areas visually with the narrow-field, high-power refractors to determine if the regions of apparent 'vacancy' were filled with any kind of opaque medium that could be detected by his sensitive eye.

By 1913 Barnard was convinced that most of the dark regions were not "holes" but areas blocked by non-luminous clouds.

Conservative, cautious, and sober (unlike Lowell, W. H. Pickering, and other reckless promoters of far-fetched theories based on fuzzy, fleeting glimpses through relatively small telescopes), Barnard refused to make claims that he had not carefully substantiated himself; thus, to the end, he would not assert that all possible dark spots he had not personally examined were obscuring clouds.

The World's Largest Refractor Telescope:
The Yerkes 40-inch, early 20th century -
public domain image from 1922 book by David Todd

After studying his voluminous publications in context, I am convinced that Barnard was neither inconsistent nor unconvinced of the reality of dark clouds, but probably over-qualified his claims to avoid being an object of criticism by other scientists, since the prevailing climate at Lick and Yerkes was less than charitable to flamboyant astronomers, whose mistakes could be gleefully ridiculed in private. (And, academically Barnard was an "overachiever" with only a bachelor's degree in mathematics, not a distinguished professor like his superiors Simon Newcomb and E. S. Holden. His scientific hypotheses, while given in earnest and supported by the force of his own personal passion, were nevertheless expressed with less of the magisterial certainty that oozes from the pronouncements of, say, Holden or Roberts.)

In 1913, Barnard wrote in the Astrophysical Journal of the Milky Way's "so-called 'black holes'" that "Some of them are so definite that, possibly, they suggest not vacancies, but rather some kind of obscuring body lying in the Milky Way, or between us and it, which cuts out the light from the stars." As proof, he supplied in part his own photograph of the object we now call the Horsehead, "a very conspiciuous black notch which is very sharply defined." He complains that Dr. Isaac Roberts had "dismissed" the object as an "embayment" in the nebulously, but found from his own photograph that it was not a hole or rift, but instead that "the almost complete outline of a dark object is shown projected against the bright nebulosity." His 4-1/2 hour exposure, reproduced as a negative print, demonstrated that it was not a "perforation" but "clearly a dark body" obscuring the background glow. The dramatic increase in detail, compared to earlier versions, makes the distinct shape of the dark nebula appear closer to modern photographs or digital images (in part, no doubt, due to Barnard's skill in optimizing a copy of his original plate, which was not -- oddly -- included in his published Atlas.)

The Horsehead (B.33 in the photographer's Catalog of 182 Dark Markings):
Bruce Telescope, by E. E. Barnard, 1913 - Courtesy of the Astrophysical Journal

Following his procedures in studying other black spots, Barnard trained the great 40-inch Yerkes refractor (shown above) on the region. His report accompanying an enlarged closeup of the Horsehead made with the Bruce telescope states, "On the night of November 4, 1913, with good conditions of seeing and fair transparency, I examined this object with the 40-inch telescope and a power of 460."

This may seem to today's amateurs, working with small-aperture scopes, an extraordinarily high power to employ on faint nebulousity, but he nonetheless obtained a reasonably efficient exit pupil of over 2mm with the great refractor working at that magnification, permitting enough light for Barnard to scan for details. Though the precise sharp outline in the photographs could not be seen, Barnard nonetheless found in the dark cloud's location "that the spot is certainly not clear sky, for the field was dull, apparently indicating the presence of some material substance at this point. To me the observation would confirm the supposition of an obscuring medium," he concluded. Thus, in contradiction to statements published elsewhere, Barnard did see and visually confirm the essential character of the Horsehead nebula: it is a cloud of obscuring dark matter, and certainly not (as Roberts asserted) merely a "rift" or discontinuity in a bright nebula.

The Horsehead and the New Astrophysics

Though Max Wolf was able to publish by 1909 his observation of the Balmer series of hydrogen spectral lines in the nebulosity near the Horsehead region, the only procedure that Barnard could devise to prove the existence and positions of the clouds was outlined in his 1913 paper: he provided a detailed set of precise transit measurements of stars inside and outside of a dark marking in Sagittarius, so that other astronomers could help obtain parallax measurements to determine if the stars were on the Earth's side of a supposed dark cloud blocking out the Milky Way.

Herschel's pioneering visual observations of faint nebulosity in "52 regions" had been confirmed, photographically by Barnard and visually by Father Hagen. W. H. Pickering's early and prescient insight that the curious phenomenon -- now known as the Horsehead -- discerned by Mrs. Fleming on his 1887 photograph might consist of dark obscuring matter had been explored further by Wolf and Barnard, the latter producing convincing arguments and evidence. Now it was up to the new generation of astrophysicists finally to unravel the mystery of the dark nebulae, and specifically the composition of the the Horsehead.

In the years 1898 through 1900, the young new director of Lick Observatory, James E. Keeler, modified and improved the old Crossley 36-inch reflecting telescope, built in 1879 by a British amateur astronomer named A. A. Common and donated by its owner Edward Crossley to the Observatory in 1895. At first the frail contraption was utterly unworkable, but thanks to Keeler's determination and skills, and the almost total re-building of the instrument in the first years of the century, the Crossley became capable of surpassing the performance of the Harvard 60-inch reflector, due to the clear California skies and the genius of Heber Doust Curtis (1872-1942).

Heber D. Curtis, controlling the Crossley 36-inch reflector at Mt. Hamilton:
courtesy of the Mary Lea Shane Archives of Lick Observatory


The brilliant, classically-educated Curtis turned from Latin literature to mathematics and astronomy, and began working at Lick Observatory in 1898. James Keeler had photographed some 104 known nebulae (plus many new spiral galaxies as they are now termed) and by 1918, Curtis had extended the list to include some 762 nebulae and clusters, with estimates that as many as 500,000 such spiral objects were theoretically within reach of the instrument. His extensive study on the dark lanes seen in nearly-edge-on spirals convinced him that the nebulae were spiral galaxies like our own system, farther off in space than previously had been imagined: the dark occulting lanes in the spirals were surely similar to the rifts in the Milky Way. Since no spirals could be photographed in those regions by the powerful Crossley telescope, he concluded the bright and dark Milky Way clouds were blocking the light from distant galaxies. Curtis' position on the matter was brilliantly espoused in the 1920 "Great Debate" with Harlow Shapley, then working at Mt. Wilson Observatory, who was misled by some spurious measurements of proper motions within spiral nebulae, and believed therefore that they were relatively small, and close to our galaxy.

Curtis was somewhat firmer than Barnard in putting forth the argument that all of the dark nebulae were obscuring clouds, having been convinced since his early study of the "Trifid" nebula (M-20) that such dim lanes were not vacancies -- not, surely, "holes in the Heavens" as had been claimed by Herschel followed by other prominent figures for a century.

In the monumental 1918 Lick Observatory publication of Curtis' photographs and data of nebular dimensions, positions, and spectra, one of the three Horsehead photographs made by the Crossley during the month of January, 1918, is included, an exposure of almost 5-1/2 hours.

As Lick astronomers have often stated, a run on the Crossley is virtually the ultimate test of the operator's technical skills; Curtis' competence is evident in his ability to keep the unwieldy instrument precisely on the field to secure untrailed long exposures on the old Seed plates, estimated by Lick's photographic investigator Eugene Harland to be about six to eight times slower than the Kodak 103aE emulsion used by Harland for his magnificent 1980 Lick Observatory Horsehead photograph.

The reproduction of the Curtis picture above -- included here by permission of Lick Observatory -- is from the half-tone print in the 1918 Lick Publication volume, and lacks some of the contrast on the original plates that I was fortunate to be able to examine. The brighter star images are somewhat distorted in a Crossley photograph, due to reflections from the internal supporting structures, but the original large-scale 1918 plate of the Horsehead demonstrates a definite advance over Barnard's enlarged 1913 picture in depicting the sharp outline and rim of faint light around the "horse's head".

According to Gene Harland, the only hypersensitization employed at the time was a brief "pre-flash" of skylight, so the result achieved was impressive, though not entirely conveyed by the rather dim reproduction, which diminishes the original's clearer registration of the bright nebula IC-434.

Big Science and the Horsehead

John Charles Duncan (1882-1967) was the photographer of this and many other beautiful portraits of galaxies and nebulae made just after the inauguration of the Hooker.

Lowell Observatory staff, 1905: John C. Duncan (right) Left to right: Henry Hussey, Wrexie Leonard (Lowell's secretary), V. M. Slipher, Percival Lowell, Carl Lampland, John C. Duncan. Photograph courtesy of the Lowell Observatory.

Obtaining a Lawrence College Fellowship at Lowell Observatory, John C. Duncan spent the years 1905-6 in Flagstaff beginning the photographic search for the trans- Neptunian planet that would later be found by Tombaugh in 1930. After receiving his Ph.D., Duncan taught at Harvard, returning to Lowell in 1912 for a second photographic planet search. His career settled into the teaching of astronomy, first for 35 years at Wellesley College and then for 12 years is a Visiting Professor at the University of Arizona until his final retirement in 1962, acknowledged as the developer of important astronomical instruction aids still in use years later. From 1920 through 1949, Duncan continued to be a visiting astronomer at Mt. Wilson, returning during summers to obtain photographs of deep-sky objects, his finest achievement being the confirmation of C. O. Lampland's discovery of the expansion of the Crab nebula.

The Hooker 100 inch telescope:
at the time of Duncan's Horsehead -
public domain image from 1922 book by David Todd

Duncan reported that he had consulted with Edward Barnard choosing specific dark objects to photograph with the powerful 100-inch Hooker. In the early years of the telescope's operation, Duncan published reproductions of his plates of bright and dark nebulae in a series of articles in the Astrophysical Journal; the Horsehead was one of the first such objects discussed.

The scale of the original 1920 negative was several times greater than the plate used in the Crossley, and the almost eightfold increase in light-gathering area of the larger telescope's mirror enabled subtleties in the nebulosity of IC-434 and the 'trunk' of the Horsehead cloud to be registered as never before. On a positive print from the original plate we obtained from the Carnegie Institution (but which was "lost" by Kalmbach Publishing, as recounted in our companion article about this research), the effect of the protrusion of the Horsehead nebulosity against the dull background is almost palpably three-dimensional. It is a pity that the character of this photograph is only suggested by the poor photocopy reproduction we have as our only example of the achievement of Duncan, included here, below.

The Horsehead, photographed by John C. Duncan on 13 November 1920 with 100-inch Hooker Telescope, Mt. Wilson: courtesy of Ron Brashear, curator of historical photographs, Observatories of the Carnegie Institution

Duncan found evidence of at least four types of nebulosity in the field near Zeta Orionis, and astutely speculated that the fringe of IC-434 represented the boundary of masses of different types of cloudy matter that were in relative motion, an issue to be studied in past decades by means of radio astronomy. The photograph long continued to be useful to scientists, as recently as 1982: Antony A. Stark and John Bally of the Bell Laboratories wrote that Horsehead photographs of just the right exposure, such as Duncan's, could optically depict the waviness of the edge on the cloud IC-434 that could be measured by radio astronomy in studying the spectrum of molecular carbonic monoxide emission.

According to Dr. Allan R. Sandage of the Observatories of the Carnegie Institute (who kindly consented to be interviewed as part of our research program), the superb resolution, accuracy, small central obstruction, and large scale of image provided by the Hooker telescope, and the then-dark Mt. Wilson skies, accounted for the quality of those photographs taken in the era between the two World Wars.

The real astrophotographic genius of Mt. Wilson -- averred Sandage -- was Walter Baade; but Duncan was a most welcome visitor at each return to the mountain. Sandage fondly recollected the retired elderly astronomer arriving for a sojourn, and delightedly accepting the offer of a guiding run for old-times-sake while Dr. Sandage was photographing RR Lyrae variables in M-15.

WHO Named the Horsehead?

As an amateur astronomer, I suppose I was first attracted to the object, decades ago, by its apparent uncanny resemblance to a horse's head -- if you hold a picture at 90 degrees to the alignment that it naturally appears to have, in the sky, when the constellation of Orion is overhead in the northern hemisphere. The colloquial names of deep-sky objects have a magnetic appeal to the amateur observer; thus we try, again and again, to study or photograph "The Trifid" or "Thor's Helmet" while perhaps ignoring any number of other worthy objects that bear only a "dull" scientific numbering.

I was reluctant, however, to put much emphasis in this research on the acquisition of the popular name "Horsehead" in a scientific article -- until I received unexpected interest from professional astronomers! I was invited to give an illustrated lecture about my findings in the Archives to staff and graduate students of Lick Observatory back in 1990. Begging them to "be gentle to an amateur who is emphatically not an astrophysicist!", I proceeded with some timidity to broach the last subject, the naming of the object known to scientists as Barnard No. 33, but to the general public as "The Horsehead." Many eyes in the audience perked up, and I was delighted to see special interest -- and amusement -- from the professional experts before me, especially displayed by the venerable Albert E. Whitford, former Director of Lick, then a vigorous 94 years of age!

So, I seem to have tapped into a vague, lingering curiosity about an almost inconsequential -- but fascinating -- aspect of the object, even held by professional astronomers. Who, indeed, had first named it the "Horsehead"? No one seemed to know...

I had searched to resolve this fact in the Lick Observatory archives, studying both professional journals and amateur publications alike in the era of interest, and found not one reference to the term "Horsehead'' applied to the object up to the mid-1920s. I did find and study, of course, Barnard's 1919 Catalog of 182 Dark Markings, from which the dark cloud received its accepted scientific name "B33." I located German language descriptions by Max Wolf of a "bucht" (bay) but no "pferdkopf" (horsehead). Isaac Roberts called it "an embayment free from nebulosity" in 1903, and Barnard variously described it as a black spot, dark body, or black notch.

Curtis labelled it in 1918 a "Dark Nebula" and described it as "a dark bay which juts into and bifurcates" the nebulosity of IC-434; I have no doubt that he does not avoid colloquialisms, because Curtis uses several of the accepted common names for other nebulae in the same volume.

In Duncan's 1921 paper it is a "Dark Nebula", and to Edwin Hubble in 1922 it was "the remarkable Bay nebula" (the use of the word 'bay' referring not to a horse, I am led to believe by all the evidence, but back to the earliest conception of the dark body causing an embayment in the nebular stream IC-434, per Roberts' description: like a discretely-contoured ocean bay jutting sharply into the smoother, shallower fractal curves of a coastline.)

Furthermore, many of the earliest published photographs of the object that I found, searching the leading world-wide popular and scientific press as preserved in the Mary Lea Shane Archives, Lick Observatory, and UC/Santa Cruz science library, were printed -- up to the time of Barnard's photograph of 1913 -- with the bright nebular stream IC-434 arrayed vertically: the dark nebula B33 therefore appears in an arrangement that, in effect, portrays the "neck" of the perceived horselike body on its side, perhaps not immediately standing out as an equine object in the perception of observers who were not conditioned to recognize the nebula's shape as having an uncanny resemblance to a horse's head: it really only appears so when the photographic image is rotated 'correctly', as in Barnard's magnified Bruce telescope picture.

The first published instance I could find of the name "Horsehead" was, significantly, in Duncan's own 1926 textbook on astronomy, as the caption to his 1920 Mt. Wilson photograph of the object. I questioned Dr. George Hebig, a former director of Lick Observatory and Professor at the Institute for Astronomy at the University of Hawaii at Manoa, who recommended the list by Cederblad of all papers on nebulae published up to the mid-1940s: all but one of the papers up to the time of Duncan's book were in the Lick collection, and none used the term "horsehead" even though most of the familiar common names for other nebulae were encountered. Dr. Herbig surmised that John C. Duncan himself may likely have originated the name. It is obvious from my examination of all available published pictures up to his 1920 effort (obtained as close to the original negatives as was possible), and from seeing unpublished exposures in the Lick Observatory plate vault, that Duncan's picture may be the earliest one that really shows the unmistakable equine appearing torso in large scale and full clarity.

While researching other matters, Dr. Donald E. Osterbrock, Professor of Astronomy and Astrophysics at the University of California in Santa Cruz located a 1923 letter from Mt. Wilson staff astronomer Frederick H. Seares (1873-1964) to George Ellery Hale. Seares edited the publication of the Mt. Wilson Observatory, and was responding to an article by Hale on Barnard's dark objects, offering a few suggestions and corrections. Knowing of my own particular interest in the object, Dr. Osterbrock was kind enough to send me a photocopy of it; and Mr. Ron Brashear, the Archivist for the Observatories of the Carnegie Institute, has graciously permitted me to indicate from this private correspondence that Seares refers to "the Horsehead" in a context which suggests that both men were certainly familiar with the term.

Had Duncan's photograph been passed around the Mt. Wilson staff until someone came up with the name, or was it an obvious appellation long used privately but not yet published? None of the professional astronomers I have asked has been able to supply an absolutely definitive answer.

Yet the colloquial name, associated with this important body, has served its purpose well. Donald Osterbrock pointed out to me that the Horsehead was being used in early publicity for the Hubble Space Telescope, in a colorful and dramatic image printed in Parade Magazine, showing the dark nebula positioned in the cursor of the Hubble guide camera. The object's public "face" is now nearly ubiquitous, having come to serve as an icon of the mysteries of deep space.

In the final part of our study, we'11 explore the methods of attaining profound dark adaptation to see the Horsehead, discuss the issue of oxygen-depletion in diminishing visual sensitivity, and demonstrate how the simplest instruments may be used to see and photograph the Horsehead.

Notes:

The abbreviated version of the modern Horsehead history, contained in this article, did not list references, and the ones I had for it are now lost. Dr. David Malin has kindly supplied the following information for those readers who might be interested. His related papers and article are:

1. Herbig-Haro Objects in the Vicinity of NGC 2023
D.F. Malin, K. Ogura, J.R. Walsh. Mon. Not. R. astr. Soc. 227 361-372, 1987.

2. The Extended Red Emission Filaments of NGC 2023
A.N. Witt and D.F. Malin. Ap. J. Lett 347 L25-L27, 1989.

3. "In the Shadow of the Horsehead Nebula". Sky and Telescope 74, 253-257, September 1987

Dr. Malin also added, "I think you are a bit harsh on Roberts. Certainly he made wild remarks about what he was photographing but he [was] not unique in that. And he married well! I visited his grave in Flaybrick Cemetery, Birkenhead, near Liverpool in England recently."

Click for Part Three: Horsehead Astrophysics, & Amateur Viewing of the The Nebula by Telescope and Binocular

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