John Herschel Observed NGC 6818 July 1826, July 1828 and August 1831

Summary: John Herschel observed NGC 6818 July 1828, August 1830 and August 1831, over three decades after its Aug. 8, 1787, discovery by his father.

NGC 6818, known popularly as Little Gem Nebula, lies at a distance of roughly 6,000 light years; its diameter is about 0.5 light-year; March 10, 1997, composite image by NASA/ESA Hubble Space Telescope’s Wide Field and Planetary Camera 2, with long exposure time at Hbeta (blue) filter giving blue appearance to the nebula’s central binary star: ESA/NASA/Space Telescope Science Institute (STScI), Public Domain, via Wikimedia Commons

Sir John Herschel observed NGC 6818 July 1828, August 1830 and August 1831, with his first observation taking place 38 years 11-plus months after his father’s Aug. 8, 1787, discovery of the planetary nebula in Sagittarius the Archer constellation.

German-British astronomer and composer Sir Frederick William Herschel (Nov. 15, 1738-Aug. 25, 1822) discovered NGC 6818, also known as the Little Gem Nebula, six years four-plus months after his March 13, 1781, discovery of the solar system’s seventh planet, Uranus. Sir William’s discovery of NGC 6818 appears as entry IV-51 in his Catalogue of a Second Thousand of New Nebulae and Clusters of Stars, published in 1789. The planetary nebula’s designation reflects its occurrence as number 51 under “IV. Fourth claſs. Planetary nebulae.”

Sir John made his first observation of NGC 6818 July 31, 1826, during Sweep 34 of deep-sky objects. His second observation took place July 19, 1828, during Sweep 163. Sir John’s third observation happened Aug. 10, 1831, during Sweep 365.

Sir John collated his three observations under entry no. 2047 in his Observations of Nebulae and Clusters of Stars, published in 1833. He entered the observations in order of right ascension, the celestial equivalent of terrestrial longitude. Accordingly, the observation from Sweep 365 (Aug. 10, 1831), which occurred chronologically as the last of his three observations, appears first. The right ascension is calculated at 19 hours 34 minutes 20.3 degrees. The declination is given at 104 degrees 32 minutes 37 seconds.

Sir John’s description for his Aug. 10, 1831, observation reads: “Planetary nebula. B; R; equals a * 9 m in its light; diam = 10” by estimation. Perfectly round; there is no central vacuity (power 320). The light, though not hazy, is turbid, not like stars, but a kind of curdled appearance, very singular. Has two stars very near. The nearer, A, 15.16 m; pos 309°.5, 312°.8, 313°.0; the first measure taken with 320, and better than the others. The further, B, 15.16 m; pos 82°.1, 78°.0 (with 320). Dist of A 30”, of B 50”. (See fig 46.).”

His description transcribes as: “Planetary nebula. Bright; round; equals a star of the ninth magnitude in its light; diameter equals 10 seconds by estimation. Perfectly round; there is no central vacuity (power 320). The light, though not hazy, is turbid, not like stars, but a kind of curdled appearance, very singular. Has two stars very near. The nearer, A 15.16 magnitude; angle of position 309°.5, 312°.8, 313°.0; the first measure taken with 320, and better than the others. The further, B, 15.16 magnitude, angle of position 82°.1, 78°.0 (with 320). Distance of A 30 seconds, of B 50 seconds. (See Fig 46.).”

Sir John’s observations from Sweep 163 (July 19, 1828) appear next. Right ascension is 19 hours 34 minutes 21.3 seconds. Declination is 104 degrees 33 minutes 29 seconds.

Sir John’s description for the July 19, 1828, observation notes: “Exactly R; = * 8 m; disc = 10” or 12” in diam; has 2 stars 12 m near. Pos of A = 307°.0, of B = 81°.5. Central measures. Light equable, and disc very lucid, yet a little (not hazy, but) as if boiling at the edges, with a suspicion that it may not be well defined.”

His description transcribes as: “Exactly round; equals a star of the eighth magnitude; disc equals 10 seconds or 12 seconds in diameter; has two stars of the 12th magnitude near. Angle of position of A equals 307°.0, of B equals 81°.5. Central measures. Light equable, and disc very lucid, yet a little (not hazy, but) as if boiling at the edges, with a suspicion that it may not be well defined.”

Sir John’s observations from Sweep 34 (July 31, 1826), which chronologically occurred as the first of the trio, appears last. The right ascension is calculated at 19 hours 34 minutes 22.7 seconds. The declination is 104 degrees 32 minutes 54 seconds.

Sir John’s description for the July 31, 1826, observation states: “Exactly R; diam estimated at 8”; the light equable and equals a * 6.7 m. It is exactly like a planet and two satellites. Distance of A, the nearer, 20”, Pos 45° n p; of B, 25”, 20° n f.”

His description transcribes as: “Exactly round; diameter estimated at 8 seconds; the light equable and equals a star of 6.7 magnitude. It is exactly like a planet and two satellites. Distance of A, the nearer, 20 seconds, angle of position 45 degrees north preceding; of B, 25 seconds, 20 degrees north following.”

NGC 6818 lies in Sagittarius the Archer constellation. The Little Gem Nebula is positioned about “40’ north preceding” of Barnard’s Galaxy (NGC 6822), according to American astronomer Edward Emerson “E.E.” Barnard (Dec. 16, 1857-Feb. 6, 1923).

The takeaway for Sir John Herschel’s observations of NGC 6818 in July 1826, July 1828 and August 1831 is that he added three carefully detailed observations to the astronomical record of the Little Gem Nebula, discovered by his father Aug. 8, 1787.

The Little Gem Nebula NGC 6818 is positioned 40 minutes north preceding of Barnard's Galaxy NGC 6822, the barred irregular galaxy in constellation Sagittarius extensively surveyed by American extragalactic astronomer and observational cosmologist Edwin Hubble (Nov. 20, 1889-Sept. 28, 1953): The Dark Side Observatory @thedarksideobservatory, via Facebook Oct. 18, 2018

Acknowledgment

My special thanks to talented artists and photographers/concerned organizations who make their fine images available on the internet.

Image credits:

NGC 6818, known popularly as Little Gem Nebula, lies at a distance of roughly 6,000 light years; its diameter is about 0.5 light-year; March 10, 1997, composite image by NASA/ESA Hubble Space Telescope’s Wide Field and Planetary Camera 2, with long exposure time at Hbeta (blue) filter giving blue appearance to the nebula’s central binary star: ESA/NASA/Space Telescope Science Institute (STScI), Public Domain, via Wikimedia Commons @ https://commons.wikimedia.org/wiki/File:Ngc6818.jpg

The Little Gem Nebula NGC 6818 is positioned 40 minutes north preceding of Barnard's Galaxy NGC 6822, the barred irregular galaxy in constellation Sagittarius extensively surveyed by American extragalactic astronomer and observational cosmologist Edwin Hubble (Nov. 20, 1889-Sept. 28, 1953): The Dark Side Observatory @thedarksideobservatory, via Facebook Oct. 18, 2018, @ https://www.facebook.com/thedarksideobservatory/photos/a.238332840024480/445907089267053/

For further information:

The Dark Side Observatory @thedarksideobservatory. “Here is a short wide-field view of NGC 6822 (Barnard's Galaxy) and the blue dot in the lower right corner in the planetary nebula NGC 6818. Barnard's Galaxy is about 1.6 million light-years away in the constellation Sagittarius and is one of the closer galaxies to the Milky Way. Tech Specs: Sky-Watcher Esprit 120mm ED Triplet APO Refractor, Celestron CGEM-DX mount, Canon 6D stock camera, ISO 3200, 14 x 60 second exposures with dark/bias frames, guided using a ZWO ASI290MC and Orion 60mm guide scope. Image date: August 9, 2018. Location: The Dark Side Observatory, Weatherly, PA, USA..” Facebook. Oct. 18, 2018.
Available @ https://www.facebook.com/thedarksideobservatory/photos/a.238332840024480/445907089267053/

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Available @ https://ia800501.us.archive.org/14/items/newgeneralcatalo00dreyrich/newgeneralcatalo00dreyrich.pdf
Available via HathiTrust @ https://hdl.handle.net/2027/osu.32435078053089?urlappend=%3Bseq=192

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Available @ https://ia800501.us.archive.org/14/items/newgeneralcatalo00dreyrich/newgeneralcatalo00dreyrich.pdf
Available via HathiTrust @ https://hdl.handle.net/2027/osu.32435078053089

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Available via Biodiversity Heritage Library (Natural History Museum Library, London) @ https://biodiversitylibrary.org/page/54093290
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Available via Biodiversity Heritage Library (Smithsonian Libraries) @ https://biodiversitylibrary.org/page/49130796
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