Herschel's Ghosts

by Mel Bartels

Visual discovery of the Integrated Flux Nebulae (galactic cirrus) and InterStellar Medium (ISM)

The task isn't to see what's never been seen before, it is instead to think about what nobody has yet thought about that which everyone sees (adapted from Erwin Schrodinger).

“Nothing ever becomes real ‘til it is experienced.” - John Keats

Check out my article in the April issue of Sky and Telescope Magazine, "Herschel's Ghosts". I share my visual discovery of Integrated Flux Nebulae and my investigation into little known observations of earlier visual observers extending all the way back to William Herschel.

See Integrated Flux Nebulae in a small telescope? Impossible! Surely it is imagination or extraneous light or Zodiacal Light. That's what I thought until first light in my 6 inch [15cm] F2.8 Richest Field Telescope in 2013. After convincing myself that the Pleiades Bubble and Andromeda Shelf were aspects of the Milky Way and not extraneous light or Zodiacal Light (the Zodiacal Light is very broad, very gradually brightening and devoid of detail), I presumed that the faint patches of light I was seeing were part of the ISM (InterStellar Medium). One night I tried the impossible, to see and sketch an IFN. To my utter surprise, the IFN in the M81/82 was bright and detailed. I embarked on researching and discovering more IFN and ISM. My catalog includes more than 50 IFN.

Dust and gas in the plane or disk of the Milky Way that resides between the stars is called the InterStellar Medium (ISM). Faint streamers of gas and dust that extend far above and below the galactic plane all the way to the galactic pole is called galactic cirrus or Integrated Flux Nebulae, since the dust glows dimly from the integrated light of countless millions of Milky Way stars.

I'm creating a catalog of my visual IFN and ISM discoveries that includes sketches and descriptions - everything you need to see these faint glows for yourself except the dark skies!

Here is the PDF version
Here is the Excel version

Why am I able to see IFN and ISM? First because I look for them, secondly because my Richest Field Telescope design has higher luminance or etendue. Luminance or etendue is the flux or light throughput of a telescope, measured in aperture times area (cm^2deg^2). Think of my 6 inch [15cm] F2.8 with its 4.3 degree field at low power. It fills my eye's pupil with 6 inches of aperture across 100 degrees of apparent field. I can zoom in closer, magnifying dim objects to a greater extent, yet have room to fit the entire object into the field of view so that I can see its edges. All this combines to make detection much easier. IFN can be seen in common telescopes, but in many cases, their detection is more difficult. For more on luminance, see my telescope optimizer.

Want to track down IFN? Discover new IFN? Alan Sandage analyzed the brightest portions of the IFN to be 24.5 mag/arcsec^2. If you observe from 21.4 mag/arcsec^2 skies, then you have a 6% contrast ratio, sufficient to see the IFN and dimmer detail when magnified to fit a very wide angle apparent field eyepiece. My observations suggest that only the brightest portions of the IFN can be seen when the sky is at 21.2 mag/arcsec^2. IFN visibility depends critically on very dark, contrasty, transparent skies.

Arndt Latussek describes the fascinating history of personal discovery followed by community disbelief with William Herschel in the late 1700's, Father Johann Hagen, Director of the Vatican Observatory in the 1920's and Marcel de Kérolyr in the 1930's. Other observers include Hermann Goldschmitt who discovered the Pleiades Bubble in 1863, Baron Renaud de Terwangne who observed IFN from 1934-1954 and Walter Scott Houston who mentioned the Pleiades Bubble in an early Sky and Telescope Gleanings article.

Herschel compiled a catalog from 1783 to 1811 of 52 nebulous regions where he observed the sky's 'floor' to be brighter than adjoining areas. His catalog included the Horsehead Nebula (discovery commonly attributed to Willamina Fleming, one of Pickering's 'computers' in 1888 and popularized by E.E.Barnard's astrophotography in 1894), the North American Nebula, Barnard's Loop and the IFN near M81 and M82. He came to believe by the end of his life that the sky was filled with nebulous material. Neither his son John Herschel, nor John Dreyer (author of the New General Catalog), chose to include Herschel’s nebulosities in their catalogs. Thus Herschel’s diffuse areas became historical and observational ghosts.

A century passed with no mention of Herschel's nebulous objects. Perhaps no one managed to build telescopes equal to Herschel's.

During the early part of the 20th century, Herschel’s catalog was surveyed visually by the Austrian-American Jesuit astronomer, Johann Hagen, Directory of the Vatican Observatory in Rome. In 1893, while observing double stars he incidentally noticed ‘cosmic clouds’ using the 6 inch F15 refractor. He eventually claimed to have observed all 52 of Herschel’s nebulae; results that met with derision because his clouds were not photographically detectable. Hagen continued to compile his faint clouds up until his death, coming to believe that faint clouds cover much of the night sky. Hagen was a true visual observer, favoring the visual record over photographs, saying that many hours of long exposures couldn’t match watch he could see in minutes. He advised astronomers to put away their cameras and look through the telescope, lamenting that visual skills were a dying art.

Robert’s widow, the leading French astronomer Dorothea Klumpke Roberts, collaborated with Hagen, perhaps to resuscitate her husband’s reputation, which had suffered with his harsh criticism of Herschel. Her photographs received attention and applause. Even Edward Hubble, a sharp critic of Hagen, in a turnabout, suggested more photographs. With Hagen’s death in 1930, interest in Herschel’s and Hagen’s observations ceased. The astronomical world lost this fascinating thread reaching back to Herschel, one of humanity’s greatest visual observers.

The French amateur Marcel de Kérolyr, one of the greatest astrophotographers of all time, well known violinist, whose home the American dancer Isadora Duncan left only to suffer her fatal accident, but unknown today, became involved, successfully photographing some of Herschel objects and publishing visual observations of most of them. His astrophotos were published by Jean Giono, author of the wonderful The Man Who Planted Trees.

Baron R. de Terwangne was an accomplished double star, lunar and planetary observer. His planetary sketches are quite beautiful. He began observing extremely faint nebulous regions in 1929, later publishing a map that illustrated his astonishing observations that the IFN form streamers perpendicular to the galactic plane.

References

Andreo, Rogelio Bernal, RBA Premium Astrophotography. http://www.deepskycolors.com/
Baron Renaud de Terwangne's observations and comments. http://articles.adsabs.harvard.edu//full/1946C&T....62..272D/0000272.000.html
Bartels, Mel, Observing Dark Nebulae. http://www.bbastrodesigns.com/dneb/Observing Dark Nebulae.html
Bartels, Mel, Reflecting Telescope Optimizer Suite. http://www.bbastrodesigns.com/telescopeCriteriaCalc.html
GaBany, R.J., Integrated Flux Nebulae Surround Our Galaxy. http://www.cosmotography.com/images/galactic_cirrus.html
DragonFly Telescope Array comment about galactic cirrus. https://jgroub.wordpress.com/2017/04/15/april-14-2017-the-dragonfly-telephoto-array-galactic-formation-and-dark-matter/
Hommage a Mercel Bonnemain de Kerolyr, https://www.youtube.com/watch?v=UGGPi0WtEbY (2015)
Hughes, Stephen, Catchers of the Light, the Forgotten Lives of the Men and Women Who First Photographed the Heavens (2015)
Kato, Hisayoshi, Flickr stream, https://m.flickr.com/#/photos/hiroc/6754827529/
Latussek, Arndt, William Herschel’s Fifty-Two Fields of Extensive Diffused Nebulosity – A Revision. Journal of Astronomical History and Heritage, 11(3), 235-246 (2008) http://www.narit.or.th/en/files/2008JAHHvol11/2008JAHH...11..235L.pdf
Lombry, Thierry, Les cirrus et les nebuleuses de flus integre (IFN) http://www.astrosurf.com/luxorion/univers-cirrus-ifn.htm
Mandel-Wilson, Unexplored Nebulae Project. http://www.galaxyimages.com/UNP1.html (2005)
Microsoft, World Wide Telescope. http://www.worldwidetelescope.org/webclient/ (e.g., the Plank thermal map)
Plantureax, Serge, La Peur Du Noir, http://plantureux.fr/wp-content/uploads/2013/10/20.05.2005-Afraid-of-the-Dark.pdf