Sharpless 73 – from remote

In 2025, I received a high-quality LRGB dataset of Sh2-73 from Hellas Sky on the island of Crete. The data were made freely available to participants in the Hellas Sky Summer Contest, providing an excellent opportunity to process professional-grade astronomical images acquired under the exceptionally dark skies of southern Greece. The deep LRGB dataset offered outstanding image quality, allowing the nebula’s delicate structures and the rich surrounding star field to be reproduced with remarkable detail and natural color.

Sh2-73 is a very faint, dusty nebula in the constellation Hercules, now understood to be part of the high‑latitude “galactic cirrus” rather than a classic bright emission nebula.

Sh2-73 appears in the Sharpless catalog of nebulae, where it was originally listed as an emission nebula, but that classification turned out to be incorrect. It is also known under other designations, notably LBN 105 (or sometimes LBN 105–106) and MBM 40 in catalogues of faint reflection and dark clouds. The nebula lies in Hercules near the western side of the constellation, close to the borders with Serpens Caput and Corona Borealis, in a region of sky that looks quite sparse in bright stars. Its central coordinates are around right ascension 16h 07–11m and declination +21° 52′, giving it a high galactic latitude of about 45°, well above the plane of the Milky Way.

On the sky, Sh2-73 covers an area larger than the full Moon, with a quoted apparent diameter of roughly 75 arcminutes, though its edges are very ill‑defined because of the extremely low surface brightness. Distance estimates place the cloud among the nearer interstellar structures: on the order of 200 parsecs (about 650 light‑years) from the Sun, which makes it one of the closest known high‑latitude dust clouds. Mass estimates for the associated MBM 40 cloud suggest some tens of solar masses of gas and dust, roughly in the range 20–40 times the mass of the Sun.

The key to understanding Sh2-73 is that it is not shining on its own, nor is it being lit by a single nearby hot star. Instead, it is a cold molecular and dusty cloud that reflects and scatters the combined light of the entire Milky Way, the so‑called integrated radiation field from the galactic disk. Nebulae of this type are known as Integrated Flux Nebulae (IFN), a term popularized by astrophotographer Steve Mandel, and Sh2-73 is often cited as a textbook example of this class. In professional parlance, such structures are commonly grouped under the label galactic cirrus, an analogy with thin, wispy clouds in Earth’s atmosphere.

Because there is no strong ionizing radiation source nearby, there is no H II region and no obvious star‑formation activity inside the cloud at present. The dust instead acts like a faint fog, slightly brightening the sky background and subtly reddening the light of more distant galaxies that appear through it. Careful imaging of Sh2-73 reveals numerous background galaxies and even quasars whose colors and brightness are affected by this foreground dust, and deep observations have identified several dozen quasars in fields overlapping the nebula.

Visually, Sh2-73 is extremely challenging: it is essentially invisible to the eye even in large amateur telescopes, because its surface brightness is far below what the human eye can pick up against the natural sky glow. Long‑exposure wide‑field astrophotography under dark skies is required to record it, and imagers often devote many hours to bring out the delicate filaments from the background noise. On processed images, the nebula appears as a very faint, looping or hairpin‑shaped veil of light, sometimes described as resembling a bent pin or the mouth of a catfish.

For observers in mid‑northern latitudes, Sh2-73 is best placed from spring through early autumn, roughly March to September, when Hercules is high in the evening sky. From your location in northern Germany, it culminates well above the horizon on spring and summer nights, but photographing it still demands very dark conditions, fast optics, and aggressive image processing to tease out the IFN from sky background and light pollution

The data the calibration, registration, and final processing were performed in PixInsight. The result was as follows:

The image also contains numerous background galaxies and quasars, adding considerable depth far beyond the stars and nebulae of our own Milky Way. The two most distant identified quasars in the field are:

• SDSS J160829.81+212321.6 with a redshift of z=4.155 and an apparent magnitude of 21.79 [SDSS 16]
• SDSS J161042.57+222328.6 with a redshift of z=3.691 and an apparent magnitude of 21.09 [SDSS 16]

The images were taken with the following equipment:

• Date: 2025
• Location: Crete, Greece
• Telescope/Lens: Planwave DR 350
• Focal length [mm]: 1050
• Focal ratio: 3
• Mount: 10Micron GM3000
• Camera: QHY 600M Pro
• Filter: LRGB
• Exposure time [min]: 579:288:231:237
• Resolution: 1.49″/px (bin 2)