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96-513: NGC 3918 is a bright planetary nebula in the constellation Centaurus , nicknamed the " Blue Planetary " or " The Southerner ". It is the brightest of the far southern planetary nebulae. This nebula was discovered by Sir John Herschel in March 1834 and is easily visible through small telescopes. The round or even slightly oval diameter is telescopically between 8 and 10 arcsec , though deep images extends this to about 19 or 20 arcsec. More surprising

192-722: A violin concerto , an oboe concerto , and a harpsichord sonata . On 4 October 1767, he performed on the organ for the official opening of the Octagon Chapel. His sister Caroline arrived in England on 24 August 1772 to live with William in New King Street, Bath. The house they shared is now the location of the Herschel Museum of Astronomy . Herschel's brothers Dietrich (1755–1827), Alexander (1745–1821) and Jakob (1734–1792) also appeared as musicians of Bath. In 1780, Herschel

288-561: A 40-foot (12 m) focal length . The 40-foot telescope was, at that time, the largest scientific instrument that had been built. It was hailed as a triumph of "human perseverance and zeal for the sublimest science". In 1785 Herschel approached King George for money to cover the cost of building the 40-foot telescope. He received £4,000. Without royal patronage, the telescope could not have been created. As it was, it took five years, and went over budget. The Herschel home in Slough became

384-415: A blurred image. Because no one else was making mirrors of the size and magnification desired by Herschel, he determined to make his own. This was no small undertaking. He was assisted by his sister Caroline and other family members. Caroline Herschel described the pouring of a 30-foot-focal-length mirror: A day was set apart for casting, and the metal was in the furnace, but unfortunately it began to leak at

480-488: A disk. Herschel originally thought it was a comet or a stellar disc, which he believed he might actually resolve. He reported the sighting to Nevil Maskelyne the Astronomer Royal . He made many more observations of it, and afterwards Russian Academician Anders Lexell computed the orbit and found it to be probably planetary. Herschel agreed, determining that it must be a planet beyond the orbit of Saturn. He called

576-401: A dramatic rise in stellar luminosity, where the released energy is distributed over a much larger surface area, which in fact causes the average surface temperature to be lower. In stellar evolution terms, stars undergoing such increases in luminosity are known as asymptotic giant branch stars (AGB). During this phase, the star can lose 50–70% of its total mass from its stellar wind . For

672-443: A final stage of stellar evolution . Spectroscopic observations show that all planetary nebulae are expanding. This led to the idea that planetary nebulae were caused by a star's outer layers being thrown into space at the end of its life. Towards the end of the 20th century, technological improvements helped to further the study of planetary nebulae. Space telescopes allowed astronomers to study light wavelengths outside those that

768-488: A growing inner core of inert carbon and oxygen. Above it is a thin helium-burning shell, surrounded in turn by a hydrogen-burning shell. However, this new phase lasts only 20,000 years or so, a very short period compared to the entire lifetime of the star. The venting of atmosphere continues unabated into interstellar space, but when the outer surface of the exposed core reaches temperatures exceeding about 30,000 K, there are enough emitted ultraviolet photons to ionize

864-401: A higher content of copper. The mirrors had to be hand-polished, a painstaking process. A mirror was repeatedly put into the telescope and removed again to ensure that it was properly formed. When a mirror deformed or tarnished, it had to be removed, repolished and replaced in the apparatus. A huge rotating platform was built to support the telescope, enabling it to be repositioned by assistants as

960-400: A local mirror-builder and having obtained both tools and a level of expertise, started building his own reflecting telescopes . He would spend up to 16 hours a day grinding and polishing the speculum metal primary mirrors . He relied on the assistance of other family members, particularly his sister Caroline and his brother Alexander, a skilled mechanical craftsperson. He "began to look at

1056-724: A most capital speculum " of his own manufacture, in October 1779, Herschel began a systematic search for such stars among "every star in the Heavens", with new discoveries listed through 1792. He soon discovered many more binary and multiple stars than expected, and compiled them with careful measurements of their relative positions in two catalogues presented to the Royal Society in London in 1782 (269 double or multiple systems) and 1784 (434 systems). A third catalogue of discoveries made after 1783

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1152-538: A new 20-foot telescope came into service for William. During this time, William was attempting to observe and then record all of the observations. He had to run inside and let his eyes readjust to the artificial light before he could record anything, and then he would have to wait until his eyes were adjusted to the dark before he could observe again. Caroline became his recorder by sitting at a desk near an open window. William would shout out his observations and she would write them down along with any information he needed from

1248-474: A new residence on Windsor Road in Slough . Herschel lived the rest of his life in this residence, which came to be known as Observatory House . It was demolished in 1963. William Herschel's marriage in 1788 caused a lot of tension in the brother-sister relationship. Caroline has been referred to as a bitter, jealous woman who worshipped her brother and resented her sister-in-law for invading her domestic life. With

1344-457: A planet, that is to say, of equal brightness all over, round or somewhat oval, and about as well defined in outline as the disk of the planets, of a light strong enough to be visible with an ordinary telescope of only one foot, yet they have only the appearance of a star of about ninth magnitude. He assigned these to Class IV of his catalogue of "nebulae", eventually listing 78 "planetary nebulae", most of which are in fact galaxies. Herschel used

1440-557: A planetary nebula (i.e., a 4% distance solution). The cases of NGC 2818 and NGC 2348 in Messier 46 , exhibit mismatched velocities between the planetary nebulae and the clusters, which indicates they are line-of-sight coincidences. A subsample of tentative cases that may potentially be cluster/PN pairs includes Abell 8 and Bica 6, and He 2-86 and NGC 4463. Theoretical models predict that planetary nebulae can form from main-sequence stars of between one and eight solar masses, which puts

1536-606: A reference book. Caroline began to make astronomical discoveries in her own right, particularly comets . In 1783, William built her a small Newtonian reflector telescope, with a handle to make a vertical sweep of the sky. Between 1783 and 1787, she made an independent discovery of M110 (NGC 205), which is the second companion of the Andromeda Galaxy . During the years 1786–1797, she discovered or observed eight comets. She found fourteen new nebulae and, at her brother's suggestion, updated and corrected Flamsteed's work detailing

1632-478: A relatively short time, typically from 100 to 600 million years. The distances to planetary nebulae are generally poorly determined, but the Gaia mission is now measuring direct parallactic distances between their central stars and neighboring stars. It is also possible to determine distances to nearby planetary nebula by measuring their expansion rates. High resolution observations taken several years apart will show

1728-407: A scramble of "labourers and workmen, smiths and carpenters". A 40-foot telescope tube had to be cast of iron. The tube was large enough to walk through. Mirror blanks were poured from Speculum metal , a mix of copper and tin . They were almost four feet (1.2 m) in diameter and weighed 1,000 pounds (450 kg). When the first disk deformed due to its weight, a second thicker one was made with

1824-433: A single concave mirror rather than a convex lens. This avoids chromatic aberration. The concave mirror gathered more light than a lens, reflecting it onto a flat mirror at the end of the telescope for viewing. A smaller mirror could provide greater magnification and a larger field of view than a convex lens. Newton's first mirror was 1.3 inches in diameter; such mirrors were rarely more than 3 inches in diameter. Because of

1920-568: A sweep progressed. A platform near the top of the tube enabled the viewer to look down into the tube and view the resulting image. In 1789, shortly after this instrument was operational, Herschel discovered a new moon of Saturn : Mimas , only 250 miles (400 km) in diameter. Discovery of a second moon ( Enceladus ) followed, within the first month of observation. The 40-foot (12-metre) telescope proved very cumbersome, and in spite of its size, not very effective at showing clearer images. Herschel's technological innovations had taken him to

2016-426: A telescope. An essential part of constructing and maintaining telescopes was the grinding and polishing of their mirrors. This had to be done repeatedly, whenever the mirrors deformed or tarnished during use. The only way to test the accuracy of a mirror was to use it. The largest and most famous of Herschel's telescopes was a reflecting telescope with a 49 1 ⁄ 2 -inch-diameter (1.26 m) primary mirror and

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2112-495: A very perfect metal was found in the mould, which had cracked in the cooling. Herschel is reported to have cast, ground, and polished more than four hundred mirrors for telescopes, varying in size from 6 to 48 inches in diameter. Herschel and his assistants built and sold at least sixty complete telescopes of various sizes. Commissions for the making and selling of mirrors and telescopes provided Herschel with an additional source of income. The King of Spain reportedly paid £3,150 for

2208-445: Is known as the main sequence , which can last for tens of millions to billions of years, depending on the mass. When the hydrogen in the core starts to run out, nuclear fusion generates less energy and gravity starts compressing the core, causing a rise in temperature to about 100 million K. Such high core temperatures then make the star's cooler outer layers expand to create much larger red giant stars. This end phase causes

2304-409: Is the beautiful rich blue colour that looks much like the coloured images of Neptune taken by Voyager 2 in 1989. Spectroscopy reveals NGC 3918 is approaching us at 17±3.0 kilometres per second, while the nebulosity is expanding at around 24 kilometres per second. The central star is 14.6 visible light magnitude, and remains invisible to optical observers, as it is obscured by the sheer brightness of

2400-576: The BBC television programme Stargazing Live built a replica of the 20-foot telescope using Herschel's original plans but modern materials. It is to be considered a close modern approximation rather than an exact replica. A modern glass mirror was used, the frame uses metal scaffolding and the tube is a sewer pipe. The telescope was shown on the programme in January 2013 and stands on the Art, Design, and Technology campus of

2496-578: The Ring Nebula , "a very dull nebula, but perfectly outlined; as large as Jupiter and looks like a fading planet". The nature of these objects remained unclear. In 1782, William Herschel , discoverer of Uranus, found the Saturn Nebula (NGC 7009) and described it as "A curious nebula, or what else to call it I do not know". He later described these objects as seeming to be planets "of the starry kind". As noted by Darquier before him, Herschel found that

2592-933: The Royal Guelphic Order in 1816. He was the first President of the Royal Astronomical Society when it was founded in 1820. He died in August 1822, and his work was continued by his only son, John Herschel . Herschel was born in the Electorate of Hanover in Germany, then part of the Holy Roman Empire , one of ten children of Isaak Herschel and his wife, Anna Ilse Moritzen, of German Lutheran ancestry. His ancestors came from Pirna , in Saxony . Theories that they were Protestants from Bohemia have been questioned by Hamel as

2688-446: The Sun will form a planetary nebula at the end of its life cycle. They are relatively short-lived phenomena, lasting perhaps a few tens of millennia, compared to considerably longer phases of stellar evolution . Once all of the red giant's atmosphere has been dissipated, energetic ultraviolet radiation from the exposed hot luminous core, called a planetary nebula nucleus (P.N.N.), ionizes

2784-572: The asymptotic giant branch phase, they create heavier elements via nuclear fusion which are eventually expelled by strong stellar winds . Planetary nebulae usually contain larger proportions of elements such as carbon , nitrogen and oxygen , and these are recycled into the interstellar medium via these powerful winds. In this way, planetary nebulae greatly enrich the Milky Way and their nebulae with these heavier elements – collectively known by astronomers as metals and specifically referred to by

2880-406: The metallicity parameter Z . Subsequent generations of stars formed from such nebulae also tend to have higher metallicities. Although these metals are present in stars in relatively tiny amounts, they have marked effects on stellar evolution and fusion reactions. When stars formed earlier in the universe they theoretically contained smaller quantities of heavier elements. Known examples are

2976-467: The 1990s, Hubble Space Telescope images revealed that many planetary nebulae have extremely complex and varied morphologies. About one-fifth are roughly spherical, but the majority are not spherically symmetric. The mechanisms that produce such a wide variety of shapes and features are not yet well understood, but binary central stars , stellar winds and magnetic fields may play a role. The first planetary nebula discovered (though not yet termed as such)

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3072-419: The 500.7 nm emission line and others. These spectral lines, which can only be seen in very low-density gases, are called forbidden lines . Spectroscopic observations thus showed that nebulae were made of extremely rarefied gas. The central stars of planetary nebulae are very hot. Only when a star has exhausted most of its nuclear fuel can it collapse to a small size. Planetary nebulae are understood as

3168-461: The AGB. As the gases expand, the central star undergoes a two-stage evolution, first growing hotter as it continues to contract and hydrogen fusion reactions occur in the shell around the core and then slowly cooling when the hydrogen shell is exhausted through fusion and mass loss. In the second phase, it radiates away its energy and fusion reactions cease, as the central star is not heavy enough to generate

3264-620: The Astronomer Royal to announce the discovery of her second comet, and wrote to Joseph Banks upon the discovery of her third and fourth comets. The Catalogue of stars taken from Mr Flamsteed's observations contained an index of more than 560 stars that had not been previously included. Caroline Herschel was honoured by the Royal Astronomical Society for this work in 1828. Caroline also continued to serve as William Herschel's assistant, often taking notes while he observed at

3360-599: The Bath Philosophical Society. Herschel became an active member, and through Watson would greatly enlarge his circle of contacts. A few years later, in 1785, Herschel was elected an international member of the American Philosophical Society in Philadelphia. Herschel's early observational work soon focused on the search for pairs of stars that were very close together visually. Astronomers of

3456-505: The Earth's atmosphere transmits. Infrared and ultraviolet studies of planetary nebulae allowed much more accurate determinations of nebular temperatures , densities and elemental abundances. Charge-coupled device technology allowed much fainter spectral lines to be measured accurately than had previously been possible. The Hubble Space Telescope also showed that while many nebulae appear to have simple and regular structures when observed from

3552-452: The French astronomer Antoine Darquier de Pellepoix described in his observations of the Ring Nebula , "very dim but perfectly outlined; it is as large as Jupiter and resembles a fading planet". Though the modern interpretation is different, the old term is still used. All planetary nebulae form at the end of the life of a star of intermediate mass, about 1-8 solar masses. It is expected that

3648-450: The Herschel telescopes revealed that many objects called nebulae in the Messier catalogue were actually clusters of stars. On 13 March 1781 while making observations he made note of a new object in the constellation of Gemini. This would, after several weeks of verification and consultation with other astronomers, be confirmed to be a new planet, eventually given the name of Uranus . This was

3744-526: The Philosophy of Musical Sounds (1749), he took up Smith's A Compleat System of Opticks (1738), which described techniques of telescope construction. He also read James Ferguson 's Astronomy explained upon Sir Isaac Newton's principles and made easy to those who have not studied mathematics (1756) and William Emerson 's The elements of trigonometry (1749), The elements of optics (1768) and The principles of mechanics (1754). Herschel took lessons from

3840-470: The Sun. The huge variety of the shapes is partially the projection effect—the same nebula when viewed under different angles will appear different. Nevertheless, the reason for the huge variety of physical shapes is not fully understood. Gravitational interactions with companion stars if the central stars are binary stars may be one cause. Another possibility is that planets disrupt the flow of material away from

3936-734: The arrival of Mary, Caroline lost her managerial and social responsibilities in the household, and with them much of her status. Caroline destroyed her journals between the years 1788 to 1798, so her feelings during this period are not entirely known. According to her memoir, Caroline then moved to separate lodgings, but continued to work as her brother's assistant. When her brother and his family were away from their home, she would often return to take care of it for them. In later life, Caroline and Lady Herschel exchanged affectionate letters. Caroline continued her astronomical work after William's death in 1822. She worked to verify and confirm his findings as well as putting together catalogues of nebulae. Towards

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4032-414: The core temperatures required for carbon and oxygen to fuse. During the first phase, the central star maintains constant luminosity, while at the same time it grows ever hotter, eventually reaching temperatures around 100,000 K. In the second phase, it cools so much that it does not give off enough ultraviolet radiation to ionize the increasingly distant gas cloud. The star becomes a white dwarf , and

4128-544: The death of their father, William suggested that Caroline join him in Bath, England. In 1772, Caroline was first introduced to astronomy by her brother. Caroline spent many hours polishing the mirrors of high performance telescopes so that the amount of light captured was maximized. She also copied astronomical catalogues and other publications for William. After William accepted the office of King's Astronomer to George III, Caroline became his constant assistant. In October 1783,

4224-412: The disk resembled a planet but it was too faint to be one. In 1785, Herschel wrote to Jérôme Lalande : These are celestial bodies of which as yet we have no clear idea and which are perhaps of a type quite different from those that we are familiar with in the heavens. I have already found four that have a visible diameter of between 15 and 30 seconds. These bodies appear to have a disk that is rather like

4320-451: The ejected atmosphere, causing the gas to shine as a planetary nebula. After a star passes through the asymptotic giant branch (AGB) phase, the short planetary nebula phase of stellar evolution begins as gases blow away from the central star at speeds of a few kilometers per second. The central star is the remnant of its AGB progenitor, an electron-degenerate carbon-oxygen core that has lost most of its hydrogen envelope due to mass loss on

4416-562: The ejected material. Absorbed ultraviolet light then energizes the shell of nebulous gas around the central star, causing it to appear as a brightly coloured planetary nebula. Planetary nebulae probably play a crucial role in the chemical evolution of the Milky Way by expelling elements into the interstellar medium from stars where those elements were created. Planetary nebulae are observed in more distant galaxies , yielding useful information about their chemical abundances. Starting from

4512-532: The end of her life, she arranged two-and-a-half thousand nebulae and star clusters into zones of similar polar distances. She did this so that her nephew, John, could re-examine them systematically. Eventually, this list was enlarged and renamed the New General Catalogue . In 1828, she was awarded the Gold Medal of the Royal Astronomical Society for her work. The most common type of telescope at that time

4608-440: The end of the lives of intermediate and low mass stars between 0.8 M ⊙ to 8.0 M ⊙ . Progenitor stars that form planetary nebulae will spend most of their lifetimes converting their hydrogen into helium in the star's core by nuclear fusion at about 15 million K . This generates energy in the core, which creates outward pressure that balances the crushing inward pressures of gravity. This state of equilibrium

4704-498: The era expected that changes over time in the apparent separation and relative location of these stars would provide evidence for both the proper motion of stars and, by means of parallax shifts in their separation, for the distance of stars from the Earth. The latter was a method first suggested by Galileo Galilei . From the back garden of his house in New King Street, Bath, and using a 6.2-inch aperture (160 mm), 7-foot-focal-length (2.1 m) (f/13) Newtonian telescope "with

4800-401: The expanding gas cloud becomes invisible to us, ending the planetary nebula phase of evolution. For a typical planetary nebula, about 10,000 years passes between its formation and recombination of the resulting plasma . Planetary nebulae may play a very important role in galactic evolution. Newly born stars consist almost entirely of hydrogen and helium , but as stars evolve through

4896-504: The expansion of the nebula perpendicular to the line of sight, while spectroscopic observations of the Doppler shift will reveal the velocity of expansion in the line of sight. Comparing the angular expansion with the derived velocity of expansion will reveal the distance to the nebula. The issue of how such a diverse range of nebular shapes can be produced is a debatable topic. It is theorised that interactions between material moving away from

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4992-458: The first planet to be discovered since antiquity, and Herschel became famous overnight. As a result of this discovery, George III appointed him Court Astronomer. He was elected a Fellow of the Royal Society and grants were provided for the construction of new telescopes. Herschel pioneered the use of astronomical spectrophotometry , using prisms and temperature measuring equipment to measure

5088-571: The ground, the very high optical resolution achievable by telescopes above the Earth's atmosphere reveals extremely complex structures. Under the Morgan-Keenan spectral classification scheme, planetary nebulae are classified as Type- P , although this notation is seldom used in practice. Stars greater than 8  solar masses (M ⊙ ) will probably end their lives in dramatic supernovae explosions, while planetary nebulae seemingly only occur at

5184-431: The highest densities, sometimes as high as 10 particles per cm . As nebulae age, their expansion causes their density to decrease. The masses of planetary nebulae range from 0.1 to 1  solar masses . Radiation from the central star heats the gases to temperatures of about 10,000  K . The gas temperature in central regions is usually much higher than at the periphery reaching 16,000–25,000 K. The volume in

5280-647: The last Twenty-five Years, in the relative Situation of Double-stars; with an Investigation of the Cause to which they are owing . In all, Herschel discovered over 800 confirmed double or multiple star systems, almost all of them physical rather than optical pairs. His theoretical and observational work provided the foundation for modern binary star astronomy; new catalogues adding to his work were not published until after 1820 by Friedrich Wilhelm Struve , James South and John Herschel . In March 1781, during his search for double stars, Herschel noticed an object appearing as

5376-465: The limits of what was possible with the technology of his day. The 40-foot would not be improved upon until the Victorians developed techniques for the precision engineering of large, high-quality mirrors. William Herschel was disappointed with it. Most of Herschel's observations were done with a smaller 18.5-inch (47 cm), 20-foot-focal-length (6.1 m) reflector. Nonetheless, the 40-foot caught

5472-455: The line at 500.7 nm was due to a familiar element in unfamiliar conditions. Physicists showed in the 1920s that in gas at extremely low densities, electrons can occupy excited metastable energy levels in atoms and ions that would otherwise be de-excited by collisions that would occur at higher densities. Electron transitions from these levels in nitrogen and oxygen ions ( O , O (a.k.a. O  iii ), and N ) give rise to

5568-404: The majority of them belong to just three types: spherical, elliptical and bipolar. Bipolar nebulae are concentrated in the galactic plane , probably produced by relatively young massive progenitor stars; and bipolars in the galactic bulge appear to prefer orienting their orbital axes parallel to the galactic plane. On the other hand, spherical nebulae are probably produced by old stars similar to

5664-422: The metal poor Population II stars. (See Stellar population .) Identification of stellar metallicity content is found by spectroscopy . A typical planetary nebula is roughly one light year across, and consists of extremely rarefied gas, with a density generally from 100 to 10,000 particles per cm . (The Earth's atmosphere, by comparison, contains 2.5 × 10 particles per cm .) Young planetary nebulae have

5760-424: The moment when ready for pouring, and both my brothers and the caster with his men were obliged to run out at opposite doors, for the stone flooring (which ought to have been taken up) flew about in all directions, as high as the ceiling. My poor brother fell, exhausted with heat and exertion, on a heap of brickbats. Before the second casting was attempted, everything which could ensure success had been attended to, and

5856-425: The more massive asymptotic giant branch stars that form planetary nebulae, whose progenitors exceed about 0.6M ⊙ , their cores will continue to contract. When temperatures reach about 100 million K, the available helium nuclei fuse into carbon and oxygen , so that the star again resumes radiating energy, temporarily stopping the core's contraction. This new helium burning phase (fusion of helium nuclei) forms

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5952-512: The new planet the "Georgian star" (Georgium sidus) after King George III , which also brought him favour; the name did not stick. In France, where reference to the British king was to be avoided if possible, the planet was known as "Herschel" until the name "Uranus" was universally adopted. The same year, Herschel was awarded the Copley Medal and elected a Fellow of the Royal Society . In 1782, he

6048-580: The parallax caused by the Earth's orbit. He waited until 1802 (in Catalogue of 500 new Nebulae, nebulous Stars, planetary Nebulae, and Clusters of Stars; with Remarks on the Construction of the Heavens ) to announce the hypothesis that the two stars were "binary sidereal systems" orbiting under mutual gravitational attraction , a hypothesis he confirmed in 1803 in his Account of the Changes that have happened, during

6144-462: The plane of the Milky Way , with the greatest concentration near the Galactic Center . Only about 20% of planetary nebulae are spherically symmetric (for example, see Abell 39 ). A wide variety of shapes exist with some very complex forms seen. Planetary nebulae are classified by different authors into: stellar, disk, ring, irregular, helical, bipolar , quadrupolar, and other types, although

6240-567: The planets and the stars" in May 1773 and on 1 March 1774 began an astronomical journal by noting his observations of Saturn's rings and the Great Orion Nebula (M42). The English Astronomer Royal Nevil Maskelyne visited the Herschels while they were at Walcot (which they left on 29 September 1777). By 1779, Herschel had also made the acquaintance of Sir William Watson , who invited him to join

6336-458: The poor reflectivity of mirrors made of speculum metal , Herschel eliminated the small diagonal mirror of a standard newtonian reflector from his design and tilted his primary mirror so he could view the formed image directly. This "front view" design has come to be called the Herschelian telescope . The creation of larger, symmetrical mirrors was extremely difficult. Any flaw would result in

6432-512: The position of stars. She also rediscovered Comet Encke in 1795. Caroline Herschel's eight comets were published between 28 August 1782 to 5 February 1787. Five of her comets were published in Philosophical Transactions of the Royal Society . William was even summoned to Windsor Castle to demonstrate Caroline's comet to the royal family. William recorded this phenomenon himself, terming it "My Sister's Comet." She wrote letters to

6528-440: The potential discovery of planetary nebulae in globular clusters in the galaxy M31 . However, there is currently only one case of a planetary nebula discovered in an open cluster that is agreed upon by independent researchers. That case pertains to the planetary nebula PHR 1315-6555 and the open cluster Andrews-Lindsay 1. Indeed, through cluster membership, PHR 1315-6555 possesses among the most precise distances established for

6624-614: The previously cited Catalogue of 500 New Nebulae ... (1802). He arranged his discoveries under eight "classes": (I) bright nebulae, (II) faint nebulae, (III) very faint nebulae, (IV) planetary nebulae, (V) very large nebulae, (VI) very compressed and rich clusters of stars, (VII) compressed clusters of small and large [faint and bright] stars, and (VIII) coarsely scattered clusters of stars. Herschel's discoveries were supplemented by those of Caroline Herschel (11 objects) and his son John Herschel (1754 objects) and published by him as General Catalogue of Nebulae and Clusters in 1864. This catalogue

6720-491: The progenitor star's age at greater than 40 million years. Although there are a few hundred known open clusters within that age range, a variety of reasons limit the chances of finding a planetary nebula within. For one reason, the planetary nebula phase for more massive stars is on the order of millennia, which is a blink of the eye in astronomic terms. Also, partly because of their small total mass, open clusters have relatively poor gravitational cohesion and tend to disperse after

6816-461: The public imagination. It inspired scientists and writers including Erasmus Darwin and William Blake , and impressed foreign tourists and French dignitaries. King George was pleased. Herschel discovered that unfilled telescope apertures can be used to obtain high angular resolution, something which became the essential basis for interferometric imaging in astronomy (in particular aperture masking interferometry and hypertelescopes ). In 2012,

6912-618: The results derived from the two methods. This may be explained by the presence of small temperature fluctuations within planetary nebulae. The discrepancies may be too large to be caused by temperature effects, and some hypothesize the existence of cold knots containing very little hydrogen to explain the observations. However, such knots have yet to be observed. William Herschel Frederick William Herschel KH , FRS ( / ˈ h ɜːr ʃ əl / HUR -shəl ; German : Friedrich Wilhelm Herschel [ˈfʁiːdʁɪç ˈvɪlhɛlm ˈhɛʁʃl̩] ; 15 November 1738 – 25 August 1822)

7008-521: The star as the nebula forms. It has been determined that the more massive stars produce more irregularly shaped nebulae. In January 2005, astronomers announced the first detection of magnetic fields around the central stars of two planetary nebulae, and hypothesized that the fields might be partly or wholly responsible for their remarkable shapes. Planetary nebulae have been detected as members in four Galactic globular clusters : Messier 15 , Messier 22 , NGC 6441 and Palomar 6 . Evidence also points to

7104-542: The star at different speeds gives rise to most observed shapes. However, some astronomers postulate that close binary central stars might be responsible for the more complex and extreme planetary nebulae. Several have been shown to exhibit strong magnetic fields, and their interactions with ionized gas could explain some planetary nebulae shapes. There are two main methods of determining metal abundances in nebulae. These rely on recombination lines and collisionally excited lines. Large discrepancies are sometimes seen between

7200-524: The surname Herschel already occurred a century earlier in the very same area in which the family lived. Herschel's father was an oboist in the Hanover Military Band. In 1755 the Hanoverian Guards regiment, in whose band Wilhelm and his brother Jakob were engaged as oboists, was ordered to England. At the time the crowns of Great Britain and Hanover were united under King George II . As

7296-562: The surrounding nebula. The distance is estimated at 1.5 kpc (4 900 light-years ). Planetary nebula The term "planetary nebula" is a misnomer because they are unrelated to planets . The term originates from the planet-like round shape of these nebulae observed by astronomers through early telescopes. The first usage may have occurred during the 1780s with the English astronomer William Herschel who described these nebulae as resembling planets; however, as early as January 1779,

7392-470: The telescope. For her work as William's assistant, she was granted an annual salary of £50 by George III. Her appointment made her the first female in England to be honoured with a government position. It also made her the first woman to be given a salary as an astronomer. In June 1785, owing to damp conditions, William and Caroline moved to Clay Hall in Old Windsor . On 3 April 1786, the Herschels moved to

7488-440: The term "planetary nebulae" for these objects. The origin of this term not known. The label "planetary nebula" became ingrained in the terminology used by astronomers to categorize these types of nebulae, and is still in use by astronomers today. The nature of planetary nebulae remained unknown until the first spectroscopic observations were made in the mid-19th century. Using a prism to disperse their light, William Huggins

7584-513: The threat of war with France loomed, the Hanoverian Guards were recalled from England to defend Hanover. After they were defeated at the Battle of Hastenbeck , Herschel's father Isaak sent his two sons to seek refuge in England in late 1757. Although his older brother Jakob had received his dismissal from the Hanoverian Guards, Wilhelm was accused of desertion (for which he was pardoned by George III in 1782). Wilhelm, nineteen years old at this time,

7680-455: The vicinity of the central star is often filled with a very hot (coronal) gas having the temperature of about 1,000,000 K. This gas originates from the surface of the central star in the form of the fast stellar wind. Nebulae may be described as matter bounded or radiation bounded . In the former case, there is not enough matter in the nebula to absorb all the UV photons emitted by the star, and

7776-462: The visible nebula is fully ionized. In the latter case, there are not enough UV photons being emitted by the central star to ionize all the surrounding gas, and an ionization front propagates outward into the circumstellar envelope of neutral atoms. About 3000 planetary nebulae are now known to exist in our galaxy, out of 200 billion stars. Their very short lifetime compared to total stellar lifetime accounts for their rarity. They are found mostly near

7872-456: The wavelength distribution of stellar spectra. In the course of these investigations, Herschel discovered infrared radiation . Other work included an improved determination of the rotation period of Mars , the discovery that the Martian polar caps vary seasonally, the discovery of Titania and Oberon (moons of Uranus) and Enceladus and Mimas (moons of Saturn ). Herschel was made a Knight of

7968-580: Was a German-British astronomer and composer . He frequently collaborated with his younger sister and fellow astronomer Caroline Herschel . Born in the Electorate of Hanover , William Herschel followed his father into the military band of Hanover, before emigrating to Britain in 1757 at the age of nineteen. Herschel constructed his first large telescope in 1774, after which he spent nine years carrying out sky surveys to investigate double stars. Herschel published catalogues of nebulae in 1802 (2,500 objects) and in 1820 (5,000 objects). The resolving power of

8064-582: Was a quick student of the English language. In England, he went by the English rendition of his name, Frederick William Herschel. In addition to the oboe, he played the violin and harpsichord and later the organ . He composed numerous musical works, including 24 symphonies and many concertos, as well as some church music. Six of his symphonies were recorded in April 2002 by the London Mozart Players , conducted by Matthias Bamert (Chandos 10048). He

8160-995: Was appointed "The King's Astronomer" (not to be confused with the Astronomer Royal ). On 1 August 1782 Herschel and his sister Caroline moved to Datchet (then in Buckinghamshire but now in Berkshire ). There, he continued his work as an astronomer and telescope maker. He achieved an international reputation for their manufacture, profitably selling over 60 completed reflectors to British and Continental astronomers. From 1782 to 1802, and most intensively from 1783 to 1790, Herschel conducted systematic surveys in search of "deep-sky" or non-stellar objects with two 20-foot-focal-length (610 cm), 12-and-18.7-inch-aperture (30 and 47 cm) telescopes (in combination with his favoured 6-inch-aperture instrument). Excluding duplicated and "lost" entries, Herschel ultimately discovered over 2,400 objects defined by him as nebulae . (At that time, nebula

8256-574: Was appointed director of the Bath orchestra, with his sister often appearing as soprano soloist. Herschel's reading in natural philosophy during the 1770s not only indicates his personal interests, but also suggests an intention to be upwardly mobile, both socially and professionally. He was well-positioned to engage with eighteenth-century "philosophical Gentleman" or philomaths , of wide-ranging logical and practical tastes. Herschel's intellectual curiosity and interest in music eventually led him to astronomy. After reading Robert Smith 's Harmonics, or

8352-662: Was head of the Durham Militia band from 1760 to 1761. Herschel moved to Sunderland in 1761; Charles Avison engaged him as the first violin and soloist for his Newcastle orchestra, where he played for one season. In "Sunderland in the County of Durh: apprill [ sic ] 20th 1761" he wrote his Symphony No. 8 in C Minor. He visited the home of Sir Ralph Milbanke at Halnaby Hall near Darlington in 1760, where he wrote two symphonies, as well as giving performances himself. After Newcastle, he moved to Leeds and Halifax where he

8448-403: Was hypothesized that the line might be due to an unknown element, which was named nebulium . A similar idea had led to the discovery of helium through analysis of the Sun 's spectrum in 1868. While helium was isolated on Earth soon after its discovery in the spectrum of the Sun, "nebulium" was not. In the early 20th century, Henry Norris Russell proposed that, rather than being a new element,

8544-547: Was later edited by John Dreyer , supplemented with discoveries by many other 19th-century astronomers, and published in 1888 as the New General Catalogue (abbreviated NGC) of 7,840 deep-sky objects. The NGC numbering is still the most commonly used identifying label for these celestial landmarks. Herschel's discoveries later compiled in the New General Catalogue include NGC 12 , NGC 13 , NGC 14 , NGC 16 , NGC 23 , NGC 24 , NGC 1357 , and NGC 7457 . Following

8640-484: Was one of the earliest astronomers to study the optical spectra of astronomical objects. On August 29, 1864, Huggins was the first to analyze the spectrum of a planetary nebula when he observed Cat's Eye Nebula . His observations of stars had shown that their spectra consisted of a continuum of radiation with many dark lines superimposed. He found that many nebulous objects such as the Andromeda Nebula (as it

8736-463: Was published in 1821 (145 systems). The Rev. John Michell of Thornhill published work in 1767 on the distribution of double stars, and in 1783 on "dark stars", that may have influenced Herschel. After Michell's death in 1793, Herschel bought a ten-foot-long, 30-inch reflecting telescope from Michell's estate . In 1797, Herschel measured many of the systems again, and discovered changes in their relative positions that could not be attributed to

8832-490: Was the Dumbbell Nebula in the constellation of Vulpecula . It was observed by Charles Messier on July 12, 1764 and listed as M27 in his catalogue of nebulous objects. To early observers with low-resolution telescopes, M27 and subsequently discovered planetary nebulae resembled the giant planets like Uranus . As early as January 1779, the French astronomer Antoine Darquier de Pellepoix described in his observations of

8928-526: Was the refracting telescope , which involved the refraction of light through a tube using a convex glass lens . This design was subject to chromatic aberration , a distortion of an image due to the failure of light of different component wavelengths to converge. Optician John Dollond (1706–1761) tried to correct for this distortion by combining two separate lenses, but it was still difficult to achieve good resolution for far distant light sources. Reflector telescopes , invented by Isaac Newton in 1668, used

9024-554: Was the first organist at St John the Baptist church (now Halifax Minster ). In 1766, Herschel became organist of the Octagon Chapel, Bath , a fashionable chapel in a well-known spa, in which city he was also Director of Public Concerts. He was appointed as the organist in 1766 and gave his introductory concert on 1 January 1767. As the organ was still incomplete, he showed off his versatility by performing his own compositions including

9120-448: Was the generic term for any visually diffuse astronomical object, including galaxies beyond the Milky Way , until galaxies were confirmed as extragalactic systems by Edwin Hubble in 1924. ) Herschel published his discoveries as three catalogues: Catalogue of One Thousand New Nebulae and Clusters of Stars (1786), Catalogue of a Second Thousand New Nebulae and Clusters of Stars (1789) and

9216-482: Was then known) had spectra that were quite similar. However, when Huggins looked at the Cat's Eye Nebula, he found a very different spectrum. Rather than a strong continuum with absorption lines superimposed, the Cat's Eye Nebula and other similar objects showed a number of emission lines . Brightest of these was at a wavelength of 500.7  nanometres , which did not correspond with a line of any known element. At first, it

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