This very wide celestial field in Ursa Major presents a dense and varied collection of extragalactic and galactic structures.

Galaxies
The region is dominated by the M81 Group, a gravitationally interacting assembly of galaxies located approximately 12 million light-years away. More precisely, measurements using Cepheid variable stars observed by the Hubble Space Telescope place the group at a distance of 11.8 ±1.1 million light-years. Within this group, Messier 81 (NGC 3031) is a grand design spiral galaxy, while Messier 82 (NGC 3034) is an irregular starburst galaxy undergoing intense star formation triggered by tidal forces from M81. A third member, NGC 3077, appears distorted with evidence of recent star-forming activity due to these same gravitational influences. In the background, the more distant galaxy NGC 2985 is visible as a nearly face-on spiral located roughly 70 million light-years away. This places it nearly six times farther than the M81 group, with modern estimates confirming its distance at 70.1 ± 4.1 million light-years. Characterized by a prominent bulge and tightly wound spiral arms, it is the principal member of its own distant galaxy group and possesses an active galactic nucleus (a LINER), powered by a supermassive black hole estimated to be 160 million times the mass of our Sun.

.

M81 and M82

Hydrogen-Beta

You might have noticed something odd about M82 in the image. The blueish region corresponds to Hydrogen-Beta (Hβ) emission. Normally, we would expect to find Hβ in strong star formation regions, shock fronts and planetary nebulae, etc. In our data, we detected minimal signal, except in the region of M82. As with Hα, Hβ emissions trace the ionized gas powered by high energy sources, but it is generally much dimmer than its Hα counterpart. However, its observed strength is likely heavily influenced by the large amounts of dust within M82. The ratio of the observed flux of Hα to Hβ, known as the Balmer decrement, increases in regions of dense interstellar dust that absorbs and scatters blue light more effectively than red light. This might explain why the signal is so faint overall. In the raw data we see a two notable features. First, is a bright compact knot probably representing a warm region of ionized gas located where the superwind punches out of the disc. Second, we observe a chain of at least 3 inline, point-like condensations along the major boundary of the disc. These structures appear to lie along on the upper X-ray lobe shown as in Chandra observations. This spatial correlation is expected based on observations of X-ray and optical emission lines where hot X-ray emitting plasma and cooler ionized gas trace the same structures. While our data are low in resolution and signal-to-noise, the morphology we observe is consistent with this known behaviour.

Hβ in M82

Raw continuum subtracted Hβ signal of M82

IFN and Vulcan Nebula

Superimposed on this field are faint structures within our own galaxy. These include the high-latitude galactic cirrus clouds of the Integrated Flux Nebula (IFN), which are illuminated by the combined light of the Milky Way, rather than a single star. This more intense central region is sometimes referred to as MW2 - The Angel Nebula (APOD 2005-09-29). A deeper analysis reveals that the nebulosity in this field is a composite structure. While much of it is true Integrated Flux Nebula within our own galaxy, comparisons with radio-telescope data show that some of the filamentary structures are spatially coincident with tidal streams of neutral hydrogen gas stripped from the M81 group itself. The image thus likely captures a rare superposition where foreground galactic dust is visually blended with the faint, distant gas of an interacting galaxy group.

Also present is the Vulcan Nebula (WPS 46), a large, diffuse reflection / emission nebula associated with high-latitude dust. Like the IFN, it is part of the foreground interstellar medium and is likely ionized by its central or surrounding stars, but this is a point of ongoing investigation. While there is a central star, its properties do not fully explain the nebula's illumination, making it an object of some mystery. The mystery surrounds its central star, TYC 4376-968-1. In a typical reflection or emission nebula, the central star is a very hot, young, blue star whose intense ultraviolet radiation is powerful enough to ionize the surrounding gas, causing it to glow. However, analysis of TYC 4376-968-1 reveals that its spectral type does not suggest it is capable of producing enough high-energy photons to fully account for the observed brightness and ionization of the surrounding nebula. It implies that either the properties of the star are not fully understood, or there are other sources of energy contributing to the nebula's glow, such as shocks from stellar winds or illumination from other nearby stars. More research required.

Hydrogen-alpha

With over 700hrs of Hα, we were also able to detect the tail end of the Ursa Major arc. The UMa Arc was initially detected in 1997 through a two-degree section observed in the Hα optical spectral line. However, the true scale of the structure remained unknown for years. The breakthrough came when researchers, investigating the H-alpha filament, discovered a colossal, 30-degree-long arc of ultraviolet emission in archival data from NASA's Galaxy Evolution Explorer (GALEX) satellite. It was this UV data that revealed the small filament was merely one piece of a vast, near-perfectly circular structure.

The Hα emission, along with ultraviolet emissions, is consistent with the hypothesis that the UMa Arc is a shock or pressure front from a supernova explosion. Based on its size and expansion characteristics, this supernova is estimated to have occurred approximately 100,000 years ago at a distance of about 600 light-years from Earth. Models of radiative shocks further support this, as the predicted Hα surface brightness compares well with the 0.5 Rayleigh value measured in 1997. Even with ~700hrs of data the structure was extremely faint and required significant processing to visualize and isolate.

Intermediate UMa arc processing.

Stacking

The original stack was 10125x8795 pixels. Each contributor stacked their respective files which were registered and stacked by channel using Astro Pixel Processor. Stacking images with disparate focal lengths and ratios presents some challenges, but after much trial and error there was a result.

Unprocessed luminance plate.

Below we visualize the pixel-wise contributions to illustrate the non-uniformity of the overall field. The image shows the density of integration from the various contributors is concentrated in the central region, but all pixels have at least ~200 hours in Hα.

Collaboration Details

The data was collected by a team of 23 individual contributors (not everyone chose to be listed). The 1650 hrs in the final image is a subset of ~1900 total hours of data collected. Data collection was performed between January and May 2025, doing our best to avoid the moon. Though, given the weather we had this spring it was not always possible. As evidenced by the list of equipment, there was no restriction on the data that was accepted, therefore the integration time across the final image is non-uniform. The individual stacks were registered and combined within a large FoV (~495 x 330 arcminutes) based on @Nick Fritz's RedCat51, 6200mm data. Nick also performed the final stacking and registration. Adon provided the processed image, visualizations, and description. Credit goes to ip in the Starfront Observatories Discord Server for coordinating everyone's effort and to Jason for proposing the project. Thank you to all contributors.

https://articles.adsabs.harvard.edu/pdf/1994ApJ...427..628F
https://www.aanda.org/articles/aa/pdf/2020/04/aa37975-20.pdf
https://web.archive.org/web/20170323234644/http://www.galaxyimages.com/UNP1.html
https://simbad.u-strasbg.fr/simbad/sim-basic?Ident=TYC+4376-968-1
https://www.astro.wisc.edu/research/research-areas/galactic-astronomy/wham/wham-sky-survey/
https://iopscience.iop.org/article/10.1086/322097/fulltext/201076.text.html
https://ned.ipac.caltech.edu/
https://steevebody.com/portfolio/m81-82-uma-arc-the-vulcan-nebula/