A deep look at Polaris and the surrounding matter respectively the galactic cirrus (IFN). Although this region of the sky is always visible from the northern hemisphere all year round, there are just a small number of images here on AstroBin. The image data I already gathered in May this year. It was now presented as "Astrophoto of the week (AdW)" on Astronomie.de, so I can finally show it here. It's probably the most deepest image shown here :-) 

For this AdW, Peter Riepe (many thanks to him for the great research!) created a very detailed, scientifically researched text. With his permission I translated the text into English - it contains a whole treasure of information. It's really worth to read it! 

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Who points his field of view at Polaris? Probably the fewest astrophotographers. But Frank, member of the „VdS Expert Group Astrophotography“, did it. Why, he describes now: "My goal was to make the numerous surrounding elements visible. For this purpose I gathered about 13 hours of image material with the small photo lens in four nights. The imaging quality of the lens is quite good, so I can work here with fully opened aperture. To make the nebulae visible, the image had to be made star-free first, then the nebulae parts had to be processed and finally the stars had to be inserted again (very carefully). Polaris is a challenge, of course. For image processing I used PixInsight and Topaz Denoise AI and tried to do as little denoising of the image material as possible. Due to Corona, I took the images with my mobile setup (actually intended for travel) directly from my backyard."

The image is from 02/07/12/17 May 2021 and was taken in the region "Bergisches Land" (i.e. the region east of the Rhine at about the level of Cologne). A telephoto lens Samyang 1:2.0/135 mm ED UMC was used, then a color camera of the type ZWO ASI294MC. At -10°C and open aperture 396 x 120 seconds were exposed without binning, that is 13 hours and 12 minutes. Only a UV/IR cut (Baader) was used as filter. The corresponding mount was a Sky-Watcher Star Adventurer on a wooden tripod from Berlebach. The field of view measures 7.0° x 4.8° - so very wide angle - with north at 13:15.

What does our presented AdW show? The area around Polaris is usually perceived as dark, but it is not - with long exposure times. The centrally located pole star (lat.: Polaris) and its scattered neighboring stars are stuck in an abundance of filamentary nebulae - this is the so-called "galactic cirrus". The name cirrus was chosen by astronomers, because a similarity with terrestrial cirrus clouds becomes recognizable. These nebulae show all dynamic forms. Here a clear movement takes place - of course only recognizable in long periods of time. The nebula colors vary from bluish to gray to yellowish-brown, depending on the light wavelengths falling on these faint reflection nebulae. The stellar colors turn out rather subtle, without significant increase in color saturation. After all, that is up to the image author himself (and his personal taste). About the stars themselves:

(1655/1586), HD 221525, spectral type A7IV, V = 5.56 mag
(1176/1614), HD 6319, spectral type K2III, V = 6.19 mag
(411/1222), HD 22701, spectral type F5IV, V ~ 5.7 mag
(1826/542), λ UMi, spectral type M1III, V = 6.38 mag
(1138/229), HD 66368, spectral type A0, V = 7.13 mag
(484/191), HD 51802, spectral type M2III, V = 5.07 mag

Why do I describe the stars, their spectral types and their magnitudes over and over again? This last mentioned M2 star glows rich orange according to its spectral type, but here it comes out rather pale. In the upper left corner of the image two galaxies are just visible (zoom in). They are the interacting pair NGC 2276 and NGC 2300, a nice sc-spiral and an elliptical galaxy. Their distance is about 100 million lightyears. Because of the short focal length, both appear very small. However, both are 6' apart, with NGC 2276 having an extent of 2.3'. In combination with the surrounding galactic cirrus this would be a subject for long focal lengths.

What can be said about the height of the galactic cirrus above the Milky Way plane? There I make again a small detour back to the polar star. This A2 star is first of all a double star and at the same time the nearest Cepheid. Polaris A - the main component - is 2.02 mag bright (V), the companion Polaris B in 18" distance comes to 8.2 mag. It has long been known that Polaris A itself is a spectroscopic binary with a period of 30 years. With the Hubble Space Telescope an optical separation could be achieved: Polaris Aa and Polaris Ab had a distance of 0.18" in 2007, and 0.15" in 2009. The distance of Polaris is derived from the measured parallax of 7.54 milliarcseconds (van Leeuwen, 2013) at about 132 pc (430 lightyears). In the AdW, the galactic cirrus around Polaris appears slightly bluish. This may indicate that the star and nebula are adjacent. The MBM nebulae in Pegasus (also galactic cirrus with intermingled molecular clouds) have a similar height above the galactic plane, so: it fits. If you look at the direct surroundings of Polaris, you might have a suspicion: Isn't Polaris in a star cluster? It looks like it. The astronomers' opinions are quite different. Turner (2009) made a Hertzsprung-Russel diagram for stars in 3° vicinity of Polaris and came to the conclusion that such a star cluster exists. Van Leeuwen (2013) believed he could bury the star cluster story after his own measurements. Problem: Polaris makes such photometries collossally difficult. And which professional astronomer accepts this?

At this point I (Peter Riepe) have the rare opportunity to discuss the faint reflection nebulae in detail. Therefore it will be a somewhat longer text today. In 2013 Dr. Arndt Lattußeck gave an impressive lecture at the "Bochum Autumn Meeting" and presented in detailed pictures what reflection nebulae, galactic cirrus and the so called "IFN" are all about (see below). Old papers came up, which actually already existed at the beginning of the last century on the subject of faint nebulae. The best known work was done by the Austrian astronomer Johann Georg Hagen. He discovered such faint nebulae visually in the 1920s - indeed: visually! At that time, light pollution was apparently not yet as it is today. Since then, these nebulae are called "Hagen's Clouds" and are clearly brighter and larger connected regions of the galactic cirrus.

Lattußeck was also able to reproduce some of Hagen's observations himself - by binoculars, and confirm brighter "Hagen Clouds" under dark French skies. In Hagen's time, these faint formations were not photographically detectable, because the unsensitized, at most orthochromatic films of the 1930s were not suitable for this purpose, because they were completely insensitive in the red range. Panchromatic, red-sensitive materials came later. When the wide-angle POSS images finally became available, "light came into the darkness" in the truest sense of the word. Johann Georg Hagen - at that time strongly attacked from many sides by his professional colleagues - was right: The faint nebulae exist. Often enough he has represented his observations with commitment before the scientific public - but again and again "rebuffed". Today, Hagen is considered rehabilitated.

All faint clouds are reflection nebulae. Two Americans (Mandel and Wilson) photographed the northern sky far from the Milky Way and found strange nebular structures, therefore believed in new discoveries. They called the nebulae "Integrated Flux Nebulae" (IFN) and gave the more conspicuous ones rather prosaic names, e.g. "angel nebula", "animal track nebula" or "volcano nebula", as they like to do it in the USA (and we like it too...). But Mandel & Wilson obviously never followed the German literature about Hagen's clouds, because on their website about IFN there is not the slightest reference to the good Austrian Hagen - well, why didn't Hagen publish in English language? Self-blame, eh?

In the amateur field the term "IFN" appears again and again. What did Mandel & Wilson actually want to say with it? The name "Integrated Flux Nebulae" is chosen very unfortunate, because "flux" in astrophysics means the radiation flux in general. Even in the Simbad database the term "flux" is used incorrectly, namely for the indication of object brightnesses (instead of magnitude). "Integrated flux" is thus all wavelengths, not only optical, but also UV and shorter, and IR and longer. But it should be obvious to everyone that gamma, X-ray, or radio wavelengths do not contribute to the visualization of IFN, only the optical wavelengths of light. Mandel and Wilson actually write: "... nebulae that are illuminated not by a single star (as most nebulae in the plane of the Galaxy are) but by the energy from the integrated flux of all the stars in the Milky Way". Well - dear people: Integrated Light Nebulae ILN instead of IFN would have been more correct.

So the term IFN should be used with caution - better: galactic cirrus. And the misunderstanding gets even bigger, because some astrophotographers understand such low-light nebulae just by the little word "flux"... that's wrong. And wrong is also what I have read several times on foreign websites: Galactic cirrus is not (!) extragalactic nebular clouds, but galactic nebulae. The word "galactic" means "belonging to our galaxy (the Milky Way)". What can be seen in the immediate vicinity of the galaxies M 81/M 82 is only to a small part galactic cirrus, which lies between us and M 81. Much of these nebulae around M 81/82 are arc-shaped interaction relics from earlier times when the two galaxies came very close.

Notes: Using today's AdW, Frank was able to achieve a remarkable result. 13 h 12 min exposure time at f-stop 2 corresponds to the same photon gain as if about 53 h would be exposed at f-stop 4. One can only say: Chapeau! In addition, the image processing was done very neatly, although the star colors ... well, everything has already been said.

The AdW team thanks for the appealing image and congratulates Frank to the Astro Photo of the Week!

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