Nebulae, star clusters, and galaxies are outside our solar system. They belong to the 'deep sky' and lead the observer to great distances and at the same time the view goes far into the past. The light of the most distant galaxies took billions of years to reach us. No less fascinating is our home galaxy, the Milky Way, offering many bright nebulae and star clusters. The book covers three important topics related to deep-sky objects: history, astrophysics, and observation. When beginners observe an object visually, not knowing anything about it, they will only perceive a faint spot of light - nothing really exciting. So, to get the right 'cosmic' feeling, the view should be enriched with stories about the object's discovery, distance, physical nature, or evolution. Supplied with this kind of information, deep-sky observing becomes a fascinating activity - braving the cold and darkness. Over time, advanced fields such as observation techniques or astrophotography come into play. The book informs the reader about all these topics and offers a comprehensive collection of interesting targets.
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To my wife Gisela
Orion Nebula (drawing by Wilhelm Tempel, 1877)
HISTORY OF DEEP-SKY OBSERVATION
Early Discoveries and the Messier Catalogue
The Epochal Work of William, Caroline, and John Herschel
First Ideas about Star Clusters and Nebulae
Lord Rosse’s Spiral Nebulae
Spectroscopy, Photography and the True Nature of Galaxies
OBJECT TYPES AND THEIR ASTROPHYSICAL NATURE
General Data of Deep-sky Objects
Celestial Position and Orientation
Brightness, Magnitude, and Size
Data Sources and Catalogues
Associations, Moving Groups, and Asterisms
Structure and Age
Distribution and Classification
Extragalactic Globular Clusters
Emission Nebulae, Supernova Remnants
Reflection and Dark Nebulae
Bipolar and Cometary Nebulae
Distribution and Physical Nature
Spectrum and Classification
The Milky Way
Brightness, Size, and Orientation
Diameter, Luminosity, Mass, and Rotation
Active Galactic Nuclei and Quasars
Pairs and Groups of Galaxies
Clusters of Galaxies
The Evolution of Galaxies and Clusters of Galaxies
PRACTICE OF OBSERVATION
Telescope and Equipment
Observation Site and Atmospheric Conditions
The Technique of Visual Observing
Subjectivity, Description, and Drawing
Imaging vs. Visual Observing
COLLECTION OF INTERESTING DEEP-SKY OBJECTS
Open and Globular Clusters
Galaxies, Quasars, and Galaxy Clusters
Source of Figures
People, when looking at the night sky, are fascinated by the Moon, planets and stars. The latter were historically arranged in constellations, like Orion or Cygnus. Perhaps one stumbles over conspicuous ensembles of stars, like the Hyades or Pleiades, located in Taurus. But the naked eye can discover more than that. There are the nebulous spots, located in Cancer, Perseus and Andromeda. Binoculars or a small telescope will easily show that the first two, known as Praesepe and χ Persei, consist of faint stars. However, the third ‘nebula’ stabbornly resists visual resolution – in any amateur telescope. Your view has fallen on a galaxy. These fascinating experiences may trigger a career as a deep-sky observer.
The term ‘deep sky’ refers to objects beyond our solar system. These objects include stars, star clusters, diffuse nebulae and galaxies. Deep-sky objects are in many ways’ attractive targets – for professional and amateur astronomers. They lead the observer to great distances, and at the same time the view goes far into the past. The light of the most distant objects (quasars) has taken billions of years to reach us. But also, nearby ones, located in the Milky Way, are fascinating. We speak of open clusters and galactic nebulae.
However, one class of deep-sky objects is ignored in this book: stars. Instead, the focus is on sources of light, showing a more or less extended structure. Consequently, star clusters, galactic nebulae and galaxies are called ‘non-stellar’ objects. They are interesting targets for both visual observing and photography.
The book covers three major subjects: history, astrophysics and observation. The first chapter treats the important role, clusters and nebulae played in the history of astronomy. This ranges from the early times of pure visual observing, over speculations about their nature to modern astrophysics. Only in the early twentieth century with the adavancment of observational techniques such as photography and the understanding of physical processes, could reveal the true nature of deep-sky objects. It has been a long road from the Greek astronomers like Hipparchus, via great observers like William Herschel or Lord Rosse, to eminent astrophysicists like Edwin Hubble. Thanks to such great scientists, we now know a lot about the creation and evolution of clusters, nebulae and galaxies, and their place in the hierarchical structure of the cosmos – solely by observing from Earth. It is a remarkable fact that the entire information exclusively comes from radiation of various kinds, emitted by remote, unreachable sources.
The second chapter presents the various types and subtypes of deep-sky objects and discusses their astrophysical nature, based on essential quantities like distance, brightness, or size. The relevant classification schemes, data sources, and catalouges are mentioned too. A special focus is on galaxies, the building blocks of our universe. They consist of stars and interstellar matter – and a supermassive black hole in the centre. Galaxies come in various forms and tend to build pairs, groups and clusters. The largest aggregates are superclusters, marking the endpoint of cosmic hierarchy.
The third chapter is dedicated to the practice of observation. This covers instrumental factors (e.g. telescope, eyepieces, filters) and important quantities (e.g. contrast, field of view, magnification). Moreover, the methods and conditions for successful observing are discussed, like viewing techniques, based on the functions of the eye, or atmospheric conditions at the observing site. Also important are object selection and finding methods. Not only visual observing and drawing is treated, but also the important field of astrophotography.
The final chapter presents a selection of interesting deep-sky objects (along with their data), covering the relevant types. Not only easy targets for smaller telescopes are chosen, but also tough ones for large apertures or digital cameras. Following these lines, one gets a deep impression of cosmic hierarchy.
Regarding the writing of this book, I would like to thank Peter Morris (formerly at the Science Museum, London) for interesting discussions. Helpful support came from two colleagues at the Webb Deep-Sky Society, Owen Brazell and Stewart Moore. They have carefully checked my English text. Finally, I have to thank my German friend Stefan Binnewies for using some of his professional astrophotos. Though presented in black and white, they have lost none of their extraordinary beauty.
Wolfgang Steinicke, April 2019
For millennia, humans must have looked up at the sky. In the pristine darkness of prehistoric times, the Milky Way with its striking mix of bright and dark clouds would have left a profound impression on those early observers. They probably resorted to mythical explanations such as the Ancient Greek story about the origin of the shimmering band. It is based on the legend that Zeus had his son Heracles, whom the mortal Alcmene had given him, to drink at the breast of his sleeping wife Hera. This was to equip Heracles with divine powers. The young hero sucked so strongly that Hera woke up. Abruptly, she pushed away the strange baby and a ray of her milk spilled over the sky. The Greek coined the name γαλαξίας (galaxías), which means ‘milky nebula of stars’.
Beside the luminous Milky Way, other ‘milky’ spots or clouds are visible with the naked eye. There is no doubt that some of them – now identified as nebulae or star clusters – were already known in prehistoric times, although there is no record. The two Magellanic Clouds are particularly prominent in the southern sky. Alas, there are no historical representations, like rock drawings by native Australians.
The first records about extended (non-stellar) objects are due to the Greek natural philosophers. The cluster below Sirius, known as M 41, was seen by Aristotle about 325 BC, Preasepe (M 44) in Cancer by Aratus about 260 BC, and the Double Cluster in Perseus (χ Persei) by Hipparchus about 130 BC. Later, in about 130 AD, Ptolemy added M 7 in lower Scorpius. He catalogued such objects in his famous book, the Almagest.
Ptolemy’s Cluster M 7 in Scorpius is the most southern Messier object.
The next deep-sky object recorded is due to the Persian astronomer Abd ar-Rahman Al-Sufi: of the Andromeda Nebula (M 31), seen before 964 from Isfahan and plotted as ‘little cloud’ in his famous Book of the Fixed Stars. Al-Sufi also described the ο Velorum Cluster and the Large Magellanic Cloud (LMC), called ‘white ox’ by him. However, the Small Magellanic Cloud (SMC) was too far south for Al-Sufi to see. This conspicuous object was first mentioned by Amerigo Vespucci (together with the large cloud). The Florentine explorer saw it on his cruise in 1501, 20 years earlier than the Portugese Fernão de Magalhães.
The ο Velorum Cluster (IC 2391), visible to the naked eye, was first mentioned by the Persian astronomer Al-Sufi.
No other deep-sky object was recorded until the invention of the telescope in 1608. On 24 November 1610, Nicolas Claude Fabri de Peiresc discovered the Orion Nebula (M 42) with a small Galilean telescope. Independently, Johann Baptist Cysat saw the object a year later. The Andromeda Nebula was rediscovered on 15 December 1612 by the German astronomer Simon Marius, not knowing of Al-Sufi’s work. Whilst, in later years, some nebulous objects appeared in the telescope as star clusters, both the Andromeda Nebula and Orion Nebula could not be resolved.
Real progress was made by Giovanni Battista Hodierna’s observations, made in about 1654 with a small refractor. He discovered 12 deep-sky objects, among them the Lagoon Nebula (M 8) in Sagittarius and the Triangulum Nebula (M 33); the first galaxy, exclusively found with the aid of the telescope. Larger telescopes were built in the seventeenth century and the list of objects grew. In 1665, the German astronomer Abraham Ihle discovered the first globular cluster, M 22 in Sagittarius. Then the famous English astronomer Edmond Halley entered the scene. His target was the southern sky, surveyed from the island of St Helena. There he discovered the brightest globular cluster, ω Centauri, in 1677. While measuring star positions at Greenwich in 1690, another Englishman, the first Astronomer Royal, John Flamsteed, found the open cluster NGC 2244 around the star 12 Monocerotis.
The first half of the eighteenth century brought about a slow increase in the number of objects found, but in the second half the rate of discovery became inflationary. 42 nebulae and clusters were discovered until Messier entered the stage in 1758. In 1714, Halley saw the bright globular cluster M 13 in Hercules. Another spectacular object was found by the Englisch amateur John Bevis in 1731: M 1, the Crab Nebula in Taurus. And in 1749, the Andromeda Nebula got a companion: M 32, found by Guillaume Legentil. The main discoverers of that period where Nicolas-Louis de Lacaille (27 objects) and Jean-Philippe Loys de Chéseaux (8 objects). The latter observed from Paris, finding the Omega Nebula (M 17) in Sagittarius (1745). Following in Halley’s footsteps, Lacaille surveyed the southern sky from Cape Town in 1751, using a refractor of only 12 mm aperture. His discovery of the bright galaxy M 83 in Hydra with such a tiny instrument was exceptional.
The ‘grand design’ spiral galaxy M 83 in Hydra.
When the French astronomer Charles Messier, at the end of August 1758, was searching for the reappearing comet Halley with a 3.5-inch refractor, he noticed a nebulous spot 1.2° northwest of the star ζ Tauri. He first regarded it as the object sought but noted: ‘Nebula without stars above the southern horn of the bull. It is of pale white light and has an oval shape, like the flame of a candle.’ The true comet, however, was about 10° southeast. Later observations revealed no motion. Thus, the obscure object was shown to be a nebula, like that in Orion or Andromeda. To avoid future confusion, the famous comet hunter documented similar cases. A first list of 1771 contained 45 nebulae and star clusters, sorted by discovery date. Messier soon realized that his alleged ‘comet Halley’ was already discovered by Bevis in 1731. Thus, the 8.4 mag bright and 4' large object became the first entry of the list: M 1. The last one is M 45, the Pleiades in Taurus. Though this open cluster, and also Praesepe (M 44), can never be confused with a ‘comet’, Messier included them for the sake of completeness. 18 of the 45 objects were discovered by him, among them the Trifid Nebula (M 20) in Sagittarius and the Dumbbell Nebula (M 27) in Vulpecula.
The Crab Nebula M 1 in Taurus was visited by Saturn in the year 2003.
In 1780, Messier published an update of his catalogue, now containing 70 entries (M 70 is a globular cluster in Sagittarius). Among the new discoveries is the famous Ring Nebula in Lyra (M 57). Messier saw the bright planetary on 31 January 1779. On 13 October 1773, he found the Whirlpool Nebula (M 51) in Canes Venatici, which got its name later after the detection of spiral structure by Lord Rosse.
Messier’s final catalogue, listing 103 objects, eventually appeared in 1781. It contains discoveries made until April that year. 18 were contributed by a new observer, Pierre Méchain, a close friend. Among them was M 97, the Owl Nebula in Ursa Major, seen in February 1781. Messier himself added seven discoveries. Among the new objects were the galaxies M 81 and M 82 in Ursa Major. The popular pair was found by the Berlin astronomer Johann Elert Bode in 1774, thus the name Bode’s Nebulae.
When Messier – always hunting for comets – inspected the region of Virgo and Coma Berenices in 1781, he noticed a remarkable accumulation of nebulae; a few of which had already been catalogued by him. He wrote: ‘The constellation Virgo and especially the northern wing is one of the constellations which enclose the most nebulae. The catalogue contains 13 which have been determined. All these nebulae appear to be without stars and can be seen only in a good sky and near meridian passage. Most of these nebulae have been pointed out to me by M. Méchain.’ Messier had found the Virgo Cluster, the nearest cluster of galaxies.
In the final catalogue, 41 of the 103 objects must be credited to Messier and 19 to Méchain, followed by Hodierna (8), Koehler and de Chéseaux (6 each). The rest is due to 18 different observers. In the twentieth century, the original Messier catalogue was enhanced to 110 entries. The additional objects are based on unpublished observations of Méchain (M 104–109) and Messier (M 110); the latter is the second companion galaxy (NGC 205) of the Andromeda Nebula.
There is no doubt, the Messier catalogue collects many of the finest deep-sky objects of the northern sky. Looking at the modern version with 110 entries, we have 40 galaxies, 29 globular clusters, 27 open clusters, 7 diffuse nebulae (including the supernova remnant M 1) and 4 planetaries. What about the remaining seven? Three objects are special (all found by Messier): M 24 is a large star cloud in Sagittarius, M 40 in Ursa Major is merely an optical pair of stars and M 73 in Aquarius is a small random ensemble of four stars. Moreover, there are four ‘lost objects’: M 47, M 48, M 91 and M 102. Due to poor information in the orginal catalogue, they could not be identified with existing objects. However, recent investigations brought the answer: M 47 and M 48 are star clusters (Puppis, Hydra), M 91 and M 102 are galaxies (Coma Berenices, Draco).
Messier’s object M 73 in Aquarius (found in 1780) is only a random ensemble of four stars (10 –11 mag, diameter 1.5').
Messier’s work was continued by William Herschel – no doubt, the greatest visual observer of all time. His surveys of the northern sky, made from small English villages near Windsor Castle and lasting over 30 years, rocketed the number of known deep-sky objects from 110 to about 2,500!
Astronomical giants: William Herschel (1738–1822), his sister Caroline Herschel (1750–1848), and his son John Herschel (1792–1871).
The German born astronomer started his observational career about 1774 and his first target was double stars. Detecting unexpected cases like Polaris and Castor, his ambition for the deep-sky was awakened. However, he never neglected to observe solar system objects. Herschel performed so called ‘star reviews’, mainly a search for double stars, using a fine 6.2-inch Newtonian reflector and the best available data, in form of Flamsteed’s British Catalogue of about 3,000 stars and the corresponding Atlas Coelestis. Equipped with these tools, Herschel systematically inspected bright stars and their vicinity. Until the end of September 1783, when the third review was finished, he had discovered about 700 double or multiple stars.
An epochal by-product of this campaign was the discovery of Uranus on 13 March 1781, then in Taurus. This brought him great recognition by his colleagues – and, of course, King George III, an admirer of astronomy. As a consequence, Herschel moved from Bath to Datchet in the Windsor area in 1782 to become the King’s personal astromomer. In the following years, He often was visited by members of the royal court, eager to look through his unrivalled telescopes – stealing from the master precious observing time.
Occasionally, Herschel encountered nebulae and star clusters in that period. His first find was the open cluster NGC 2232 in Monocerotis, seen on 5 December 1779; 10 more non-stellar objects should follow until September 1783. The most spectacular was the planetary nebula NGC 7009 in Aquarius, discovered on 7 September 1782 and later called Saturn Nebula by Lord Rosse due to its peculiar appearance.
Willam Herschel’s first deep-sky discovery was made with a 6.2-inch reflector in 1779: the open cluster NGC 2232 in Monoceros.
During the star reviews, Herschel also found time to look-up deep-sky objects, listed in his copy of Messier’s catalogue. He observed 68 of the 103 Mnumbers, using selfmade metal-mirror (speculum) reflectors of up to 12 inches aperture. Herschel was proud that his instruments were superior to that of Messier or any other astronomer of the time. The Frenchman had described many objects as ‘nebula without stars’, i.e. he was unable to resolve them in his 3.5-inch refractor. As the leading maker of large reflectors, Herschel wanted a competition – and Messier’s objects were the ideal targets. Concerning resolution, he was successful in many cases where Messier had failed. Herschel also discovered ‘garnet stars’, showing a deep red colour. The most famous is μ Cephei, first seen on 27 September 1782 and known as Herschel’s Garnet Star. Another field of research was the monitoring of variable stars.
Some small, round nebulae turned out to be spherical aggregations of large numbers of stars, called ‘globular clusters’ by him. They offer a certain range of concentration. In compact cases, like M 80 in Scorpius, he could only perceive faint stars in the outer region. But there were also loose exemplars, like M 55 in Sagittarius, where even the core appeared resolved. Other nebulae, like M 31 in Andromeda, showed no stars even with the highest magnification. It should be noted that Herschel used powers up to incredible values of about 5000, which caused incredulous amazement among contemporary astronomers. During his Messier survey, Herschel encountered also mixed cases, where stars are embedded in nebulous matter. For instance, the Orion Nebula has a close quartet of stars in its centre (θ Orionis), called ‘Trapezium’ by him.
The successful observations of Messier objects and the unexpected discovery of some new ones inspired Herschel in 1783 to change the observational field from chasing after double stars to the search for nebulae and star clusters.
However, there was a second stimulus, coming from his able sister Caroline. In late August 1782, she started own observations with a small refractor, given to her by William. However, this was not voluntary: ‘I found to be trained for an assistant astronomer.’ She should scan the sky for all kinds of objects (expecting known stars, clusters and nebulae from the Messier catalogue) – or perhaps comets. However, her brother was always near to assist or check promising finds. Until the end of October 1783, Caroline had used 65 nights. Mostly, William was observing too, working for the star review, inspecting Messier objects, visiting Uranus or other planets. Caroline’s refractor was replaced in May 1783 by the ‘small sweeper’, a fine 4.2-inch Newtonian reflector with 2 feet focal length and 2.2° field of view. In 1793, she could even upgrade to a 9.6-inch (‘large sweeper’).
Finally, Caroline has viewed 52 Messier objects. But there was still greater yield: 9 new open clusters, a galaxy and – what brought the greatest honor – 8 comets! Her first deep-sky discovery was the open cluster NGC 2360 in Canis Major, proudly noting ‘Messier has it not’. On 23 September 1783, the galaxy NGC 253 in Sculptor was found. Watching Caroline’s success, William was impressed – and a bit jealous. It convinced him that there still was much to discover in the virgin field of nebulae and clusters. Obviously, the former observations by astronomers, leading to the Messier catalogue, had missed many objects. Thus, he summarized the plan for a comprehensive sky survey with the aid of a superior telescope.
Sketches of the nebula NGC 253 in Sculptor made in 1783 by Caroline (left) and William Herschel.
On 23 October 1783, Herschel started his great campaign to sweep the entire sky, visible from southern England, for new nebulae and star clusters. At the pretty dark site (Datchet), the declination limit was about –30°. The instrument, constructed for the ambitious task, was a Newtonian reflector of 18.7 inches aperture and 20 feet focal length, held by a wooden altazimuthal mounting. It became operational in September 1783 and was large enough to see even faint nebulae, but not too cumbersome to allow effective visual observing. The standard eyepiece had a magnification of 157, offering 15' field of view.
William Herschel’s standard telescope for sweeping: the 18.7-inch reflector.
Herschel developed a sophisticated method called ‘sweeping’. The reflector was fixed in azimuth to the south meridian. At the chosen elevation, the sky passed through the field of view from east to west due to the Earth’s rotation. By slowly swinging the tube up and down by about 2° over many hours, a large rectangular area of the sky was covered. In October 1786, the Newtonian design was given up by removing the secondary mirror. By slightly tilting the main mirror, the focus went to the edge of the tube opening, where the eyepiece was now placed. The new design, called ‘front view’, meant he could see objects about half a magnitude fainter.
Herschel perfomed no less than 1112 sweeps in 616 nights. No minute was left out when there was a chance for clear sky. During the day, he was mainly employed with telescope making. Obviously, this great man of exemplary persistence and power did not need much sleep. The result was overwhelming: from 1783 to 1802, Herschel discovered about 2,500 nebulae and star clusters. Moreover, in 1787, he found two moons of Uranus with the 18.7-inch: Titania and Oberon.
Of course, this enormous work was impossible without careful help – and the right person was nearby: William’s talented sister Caroline, perhaps one of the most undervalued persons in the history of astronomy. She was responsible for important tasks on the sweep campaign (with the exception of the observing itself, along with certain technical matters). Caroline planned the observations (selecting sky area and possible targets), documented them in real time (sitting in a room near to the telescope) and reduced the data on the next day. By recording time and elevation for each object crossing the meridian, she calculated relative positions to nearby Flamsteed stars. For this, she was well trained in mathematics by her brother. The new objects were numbered and later classified. Caroline also calculated coordinates: right ascension and ‘polar distance’ (= 90° – declination). For instance, a star at the local zenith (51° declination) has 39° polar distance.
Caroline finally produced the three Herschel catalogues of nebulae and star clusters, published 1786, 1789 and 1802 in the Philosophical Transactions of the Royal Society. They contain 1000, 1000 and 500 entries, respectively. In addition, based on William’s observations, she revised Flamsteed’s British Catalogue. The result was published in 1789 by the Royal Society under her name. Finally, she compiled the (unpublished) Zone Catalogue, arrangeing all 2,500 Herschel objects in zones of polar distance. This work was done for her nephew John. John Herschel wanted it for his ambituous task to confirm and enhance his father’s data by a new observing campaign. In 1828 Caroline’s work was honoured with the Gold Medal of the Royal Astronomical Society. This institution was founded in 1820 in London and William Herschel became its first President (two years before his death).
The many documents and records, written by William and Caroline Herschel, are archived at the Royal Society and the Royal Astronomical Society. From them the sweep campaign was reconstructed by the author. This includes the observing program, telescope configuration, sweeping method, data recording, reduction and compilation up to the printed catalogues and later publications. One major result is Herschel’s sky coverage. Due to the overlapping sweep areas, 90% of the sky, visible from the Windsor area, was theoretically observed. However, the sweep pattern (by the two perpendicular motions) is not area-covering. The resulting gaps in each sweep area led to an effective sky coverage of 66%. This explains too, why Herschel has missed some objects. Due to catalogue identities, he actually discovered 2,367 deep-sky objects (with a brightness of 12.1 mag). However, the sky offers 3,585 objects, accessible with the 18.7-inch reflector. Thus 1,218 objects were missed, which gives a success rate of 66%. This exactly corresponds to the observed sky, resulting from his not area-covering sweep method. Thus, Herschel worked with maximum effectivity! Moreover, given the rough mounting, the data quality and position accuracy are astonishingly high.
In Herschel’s catalogues, galaxies are clearly dominating (much more than in the Messier catalogue). We have 89% galaxies, 7% open clusters; the rest are diffuse/planetary nebulae and globular clusters. The brightest object is NGC 2264 (4.1 mag), an open cluster in Monoceros; the faintest NGC 2843 (15.5 mag), a galaxy in Cancer. The latter again demonstrates the power of the reflector and, of course, Herschel’s outstanding observational skill.
The 15.5 mag galaxy NGC 2843 in Cancer is the faintest object in Herschel’s catalogue. It was found in 1787.
Between 1785 and 1789, Herschel, now living at Slough, constructed a ‘front-view’ reflector of 48 inches aperture and 40 feet focal length. This enormous task absorbed much of his time, needed for further sweeps. It was the largest telescope in the world for about 50 years (beaten in 1845 by Lord Rosse’s 72-inch reflector). Due to its cumbersome structure and unexpected optical defects, it was neither used for sweeping nor the study of nebulae. However, Enceladus and Mimas, the 6th and 7th moon of Saturn, were discovered in 1789 with the ’40 foot’. It was soon clear that the huge instrument could not fulfil the high expectations of its maker (and the financier, King George III). In 1840 it was eventually dismantled by John Herschel.
John continued the observations of his father in two campains. The first covers the period 1825–33 at Slough. He used a front-view reflector with an aperture of 18¼ inches and 20 feet focal length, built in the style of William and completed in 1820. John’s primary intention was not finding new objects but the re-examination of the three Herschel catalogues. All discoveries should be confirmed, giving better positions and descriptions. To realise this project, he compiled ‘working lists’ to direct his sweeps, making the observations more effective. They are based on Caroline’s unpublished Zone Catalogue. However, during 428 sweeps, many new nebulae and clusters were found. The observational result (Slough catalogue) is published in the Philosophical Transactions of the Royal Society for 1833.
The second campaign was a systematic survey of the southern sky, made 1834–38 at Feldhausen, South Africa (now a part of Cape Town). The southern mission, again using the 18¼-inch, was much different from the Slough campaign, inspecting mainly terra incognita between –2.5° and –90° declination. Only Halley, Lacaille and James Dunlop (Australia) had already observed parts of the southern sky, though with much smaller instruments and not in a comprehensive manner.
John Herschel’s 18¼-inch reflector, erected at Feldhausen, near the Table Mountain.
At the rural site, John performed 382 sweeps in 349 nights. The result, known as the Cape catalogue, was not published until 1847. It contains 1,733 entries. The objects are mainly galaxies (56%) and open clusters (30%). Counting the independent deep-sky objects we get 1,649, of which 1,125 were new. There are two reasons for the discovery of the large number of open clusters, firstly the Milky Way is much more prominent in the southern sky and, secondly, we have the Magellanic Clouds, offering a bunch of individual nebulae and clusters. As 638 possible deep-sky objects that he could have seen were missed, John Herschel’s success rate is 72%. As in the case of his father, the sweeping method does not lead to a complete coverage. The mean visual magnitude is 12.2 mag. John also identified clusters of galaxies. For instance, he recognized the crowded regions in Centaurus and Fornax. What makes the younger Herschel unique is the fact that he is the only observer in history who visually surveyed the entire sky with a large telescope. In 1864, he eventually published a catalogue of all known nebulae and star clusters, the General Catalogue (GC), listing 5,079 objects.
John Herschel’s GC paved the way for the most important catalogue of non-stellar deep-sky objects, the New General Catalogue (NGC), published in 1888 by the Danish astronomer John Louis Emil Dreyer. The Danish astronomer had mainly worked in Ireland and England. For some years he was scientific assistant of Lawrence Parsons at Birr Castle before he became Director of the Armagh Observatory. Dreyer also is the author of the monumental Scientific Papers of Sir William Herschel, published 1912 in Oxford.
The Danish astronomer John Louis Emil Dreyer (1852–1926) is the creator of the famous New General Catalogue (NGC), first published in 1888.
With its 7,840 entries, the NGC is the last comprehensive visual catalogue of non-stellar objects covering the whole sky. Despite all modern successors, which usually feature a particular type of object (galaxies, open clusters etc.), it is still the most popular catalogue and the NGC number is the primary designation for bright, large deep-sky objects, both in amateur and professional astronomy. In 1895 and 1908, Dreyer published two supplements (IC I and II) with a total of 5,386 objects, today collectively referred to as the Index Catalogue (IC). The IC II is entirely based on photographs. The Dreyer catalogues are abbreviated as NGC/IC.
Pierre-Simone de Laplace and Immanuel Kant had thought about the formation of cosmic bodies by gravity, based on Newton’s revolutionary theory of 1687, explaining all mechanical processes on Earth and beyond. Laplace and Kant theorised, the solar system should have formed out of spinning nebulous matter, which became oblate by the centrifugal force and eventually condensed into Sun and planets. Saturn, showing a central body surrounded by a ring, was seen as a proof – an object in the making.
However, not only was the question of evolution of heavenly bodies tackled. Another challenge concerns the distance and hierarchy of cosmic objects like the Andromeda Nebula. The first important contribution to the issue was made by Kant, who interpreted these objects as independent ‘Milky Ways’. In 1754, he wrote: ‘We have seen with astonishment figures in the heavens, which are nothing but those systems of fixed stars, limited to a common plan, such Milky Ways, if I may express myself in this way, which represent elliptical forms, seen by the eye in various positions and appearing faint due to its infinite distances.’
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