NGC 7226
NGC 7226 in the field of Sh2-132 · a small cluster, a large Lion Nebula
The imaging plan was called NGC 7226, and that open cluster really is in the frame — but only just: a small, dense group of stars at the top edge, slightly right of center. The image belongs to a different object. Sh2-132, also LBN 473 and known as the Lion Nebula, is the large H II region that fills the center of the frame at roughly 1.5° across. And the striking cluster at the upper left is not NGC 7226 either but NGC 7235. The field is a collection: one star-forming region, five open clusters of very different ages, and two planetary nebulae that share nothing but the line of sight. North is to the left in this image, east at the bottom.
Sh2-132 lies on the border between Cepheus and Lacerta at a distance of about 3.5 kpc, some 11,000 light-years — the kinematic distance from Vasquez et al. (2010), with an uncertainty of ±1 kpc. Radio observations show two shells of ionized gas, together about 1,500 solar masses at an electron density around 20 particles per cubic centimeter. It is precisely this two-part structure that the image shows: at the lower left the red main shell, threaded with dust bands; at the upper right a cavity glowing turquoise; and a dark chamber between them.
The energy comes from two Wolf-Rayet stars. WR 153ab (HD 211853, magnitude 9.2) is a quadruple system of two binaries containing a WN6 Wolf-Rayet star and an O6 supergiant. It stands at the edge of the brightest cloud, together with the embedded cluster Teutsch 127 and the O7 star BD+55 2722, and is the main exciter of the red shell. WR 152 (HD 211564, magnitude 11.6) is a WN3 star: less luminous but hotter. Around it lies the turquoise cavity with its own ring nebula. Several O stars from the LS III survey, between O7 and O9, add to this. Their winds and ultraviolet light have blown the cavities clear and are compressing the gas at the edges; CO observations show molecular gas enclosing the ionized shells and interacting with them — the reservoir for the next generation of stars (star formation).
The star clusters in the field do not belong together. NGC 7226 (Collinder 446) probably lies behind the nebula at about 4 kpc, is roughly 470 million years old and weighs in at some 430 solar masses — an unremarkable cluster of middle age. NGC 7235 at the upper left is, at about 12 million years, very young, considerably heavier at some 1,500 solar masses, and sits at a distance similar to the nebula's. To the right lie NGC 7245 (about 400 million years, 3.2 kpc) and, right beside it, King 9, a cluster roughly three billion years old at twice the distance — a neighbor in the image, three kiloparsecs behind it in space. Berkeley 94 in the eastern part of the nebula, below center in the image, is at about 14 million years another child of the region. The values come from the Gaia-based Unified Cluster Catalogue.
Two small planetary nebulae lie at the edges: Minkowski 2-51 on the left, a bipolar nebula at only about 1.9 kpc and therefore far in front of Sh2-132, and Abell 79 at the lower right, a pale bubble 2′ across at an uncertain distance. Both are the end stages of Sun-like stars — the counterpart to the young, massive stars that make the Lion Nebula glow.
The image was taken through a dual-band filter that passes only two lines: Hα at 656 nm and [O III] at 501 nm. On the color sensor Hα lands in the red channel, [O III] in green and blue. The deep red is therefore the glow of hydrogen, and the turquoise cavity around WR 152 is directly visible light from doubly ionized oxygen — the hot WN3 star supplies the hard ultraviolet radiation needed for it. Star colors survive this filter only as a remnant: hot stars emit relatively more continuum in the [O III] window and appear bluish, cool ones reddish. The image is thus a map of two emission lines in photographic color, not a natural-color photograph.
Where in the sky
The rectangle is this image's real field of view, with its measured rotation — not a symbol. Constellation figures and star positions come from SIMBAD.
7 entries sit within 30° of each other here — click a field to jump to it.
Crops
Sh2-132
The Lion Nebula as a whole: two shells, two colors
1.94° × 1.88°·22h 18m 41.60s +55° 46′ 12.9″
About 360 × 370 light-years at 3.5 kpc, north to the left. The crop shows the two-part structure that the radio maps see as well: at the lower left the bright red main shell around WR 153ab, at the upper right the turquoise cavity around WR 152. Between them lies the dark chamber with the dark-cloud complex LDN 1150/1154, whose edges glow as red arcs – ionization fronts where the ultraviolet light of the hot stars meets denser gas.
The lion shape is an accident of perspective. Physically this is a bubble being inflated from within: stellar winds and radiation pressure clear out the interior, the displaced gas gathers in the bright rims, and where it meets the surrounding molecular cloud it is compressed. The embedded cluster Teutsch 127 and the young cluster Berkeley 94 at the lower edge show that the region is still forming stars.
WR 153ab
The engine room: WR 153ab, Teutsch 127 and the brightest cloud
36.8′ × 37.6′·22h 19m 33.94s +56° 08′ 41.8″
About 115 × 125 light-years. The brightest part of the nebula is also its engine room. In the upper part of the crop, to the right of center, stands WR 153ab (HD 211853, magnitude 9.2), a quadruple system of two binaries with a WN6 Wolf-Rayet star and an O6 supergiant; right beside it the embedded cluster Teutsch 127 with the O7 star BD+55 2722. At the center lies the radio H II region G102.9−0.7 with the O9 star LS III +55 40 and a dense group of catalogued young stellar objects – stars are still forming here.
The dark shreds cutting across the bright gas are dust in the foreground (LDN 1161), not a hole in the nebula: they cover the glow. Their ragged edges show that the radiation of the O stars is in the process of dissolving them.
WR 152
The turquoise cavity around WR 152
52.6′ × 49.0′·22h 16m 09.52s +55° 26′ 50.6″
About 165 × 155 light-years. The turquoise cavity is the ring nebula around WR 152 (HD 211564, magnitude 11.6), a Wolf-Rayet star of the early type WN3 – rarer and hotter than the WN6 type of WR 153ab, but less luminous. Around it stands the compact cluster SBB 3.
The color here is not an interpretation: the dual-band filter separates [O III] (green and blue) from Hα (red). Where the ultraviolet radiation is hard enough to ionize oxygen twice, [O III] dominates – and that is only the case close to a very hot star. The fine bright threads inside are the torn-open shell itself; the dark clumps at the left edge are dense globules that the ionization front has not yet dissolved. Toward the lower left the cavity passes into the red main shell around WR 153ab without a sharp boundary.
Dark clouds
The dark chamber: LDN 1150 and LDN 1154
49.5′ × 40.0′·22h 15m 48.89s +56° 07′ 28.0″
About 155 × 130 light-years. Between the two shells lies a dark chamber, and it is not empty: this is where the dark clouds LDN 1150 and LDN 1154 (TGU H621) sit, together with cold clumps of molecular gas from the Planck catalogue. The red arc at the upper edge is an ionization front: the illuminated skin of the cloud, behind which the gas stays dense and dark.
The dark trunk in the lower half – with a broadened head and two bright blue stars at its other end – is a pillar of denser gas that is withstanding the front: the same mechanism as the Elephant's Trunk in IC 1396, here in miniature. That it is dust in the foreground and not merely missing glow is betrayed by the star density, which decreases toward the middle of the chamber.
NGC 7235
NGC 7235: the young cluster in the corner
9.7′ × 11.8′·22h 12m 27.78s +57° 15′ 58.5″
Roughly 28 × 37 light-years at 3.3 kpc. NGC 7235 is a very young open star cluster: Gaia-based analyses give about 12 million years, some 1,500 solar masses and a distance similar to the nebula's. The brightest members are B stars. The dual-band filter lets only a remnant of the star colors through, but the direction is right: hot stars appear bluish, cool ones reddish – the reddish stars in the field are evolved giants or foreground stars.
NGC 7235 may belong to the same wave of star formation as Sh2-132 – distance and age sequence fit. Not to be confused with NGC 7226, which gave the imaging plan its name: that one stands small and inconspicuous at the upper edge of the frame, slightly right of center, and at roughly 470 million years is an entirely different kind of cluster.
NGC 7245
NGC 7245 and King 9: neighbors only in projection
39.4′ × 25.2′·22h 15m 42.60s +54° 11′ 57.9″
Roughly 47 × 48 light-years at 3.2 kpc. Two star clusters in one crop: NGC 7245 at the center, about 400 million years old and weighing some 1,600 solar masses, and below and to the left of it King 9. It looks like an appendage, but at roughly 6.3 kpc it stands at twice the distance and, at about three billion years, is one of the old clusters of the galactic disk.
The neighborhood is pure projection, and even through the dual-band filter the residual color hints at it: NGC 7245 is rich in hot, bluish-looking stars, King 9 shows redder, older giants – and the dust along six kiloparsecs of path does its part. That an open cluster survives three billion years is the exception; King 9 owes it to its mass.
M 2-51
Minkowski 2-51: a planetary nebula in the foreground
18.5′ × 18.3′·22h 15m 47.89s +57° 27′ 57.1″
About 31 × 32 light-years at 1.9 kpc; the nebula itself measures only 47″ × 38″, roughly 1.4 light-years. Minkowski 2-51 is a bipolar planetary nebula of magnitude 13.6: the ejected envelope of a star of a few solar masses that is in the process of becoming a white dwarf. It stands at a distance of about 1,900 pc and thus well in front of Sh2-132 – all they have in common is the line of sight.
The reddish cloud above it does not belong to the nebula but is an offshoot of the Hα veil that covers the whole field. The greenish core of the nebula is [O III], the reddish rim Hα and [N II] – the usual stratification of a planetary nebula, here compressed into a few pixels.
Abell 79
Abell 79: the bubble at the edge of the frame
18.0′ × 17.0′·22h 26m 14.19s +54° 49′ 40.6″
About 17′ on a side; the bubble itself is roughly 2′ × 1.5′. Abell 79 (PN G102.9−02.3) was found by George Abell in 1955 on the plates of the Palomar Sky Survey. The ring is expanding at about 13 km/s; the magnitude 17 central star is a cool dwarf that cannot account for the observed ionization on its own – it needs a hot, invisible companion. The distance is uncertain, with literature values ranging from about 3,300 to 4,800 light-years.
The green-white core is [O III], the red rim Hα – the same stratification as in many old planetary nebulae. The reddish veils to the left of it are the outermost edge of Sh2-132, running out here at the southeastern edge of the frame.
NGC 7226
NGC 7226: the namesake at the edge
26.5′ × 13.6′·22h 10m 25.11s +55° 22′ 52.1″
Roughly 95 × 50 light-years at 4 kpc. This is the cluster the imaging plan was named after: NGC 7226, also Collinder 446, a compact open star cluster of only 2.3′ catalogue diameter. Gaia-based analyses give about 470 million years and some 430 solar masses; the photometric distance is 4 kpc, while the parallaxes suggest even more. It therefore probably stands behind Sh2-132, not within it.
In the image it sits at the upper, western edge – the field is centered on the nebula, not on the cluster. Its age shows: no gas remnants, no bright O star, a dense core of a few dozen stars. The comparison with NGC 7235 in the other corner of the frame is what this crop is really good for: two open clusters, one twelve million, the other nearly half a billion years old.
Acquisition and processing
Taken on the night of 1 to 2 September 2026 in Friedrichsdorf with a ZWO ASI2600MC Air on a William Optics RedCat 71 at 349 mm focal length, gain 100, bin 1 and around −10 °C, through a dual-band filter (Hα + [O III]). The final stack contains 17 individual exposures of 300 s each, together 5,100 s or 1 h 25 min. Eight frames sorted out manually beforehand were deliberately left out of account.
Calibration used a MasterBias, a MasterFlat and a MasterDark bias-corrected and scaled to 300 s. WBPP registered and integrated all 17 retained frames with LocalNormalization; the run ended without warnings, registration errors or further exclusions.
The linear master was then processed along the logged PiMagic expert path, including background correction, photometric color calibration and separate treatment of the noise, detail and star layers. AstroMagic produced the layered Photoshop file from it; tonality and color were finalized in Photoshop. The gallery uses the 5863 × 4109 pixel AVIF export with Display P3 color space and an HDR gain map.
The image is plate-solved (PixInsight ImageSolver, 1672 stars, RMS 1.3″): 2.22″/px, field of view 3.62° × 2.53°, north to the left. The object markers in the image and in the crops are computed from this solution; positions come from SIMBAD.