LBN 437 · Gecko Nebula — Lacerta OB1, a Herbig star at the tip and galaxies behind the Hα veil

Lacerta is an unassuming corner of the autumn sky, which is precisely why it reveals something lost in brighter fields: a star-forming region that is almost finished. The silvery-gray, dusty shape at upper left is LBN 437, nicknamed the “Gecko” for its outline — a head with a pointed snout facing down and to the right. The pink band running diagonally across the image is hydrogen ionized by the hot stars of the Lacerta OB1 association; Sharpless cataloged the entire glowing field as Sh2-126 in 1959. Gaia parallaxes place the group at around 500 pc, or 1,600 light-years — the image covers about 110 × 70 light-years at that distance. North is left, east is down.

The engine sits at lower right. The brightest blue star in the field is 10 Lacertae, an O9 main-sequence star and one of the classic standards used to define spectral class O itself. Its ultraviolet light ionizes the hydrogen of Sh2-126 across several degrees; the bluish halo around it is dust scattering its light. Above and to the left lies 8 Lacertae, a close pair of B1 stars around which Gaia has identified a small moving group (UBC 159). Both belong to Lac OB1, one of the nearest OB associations to the Sun, where star formation has largely ended — leaving thinned-out cloud remnants such as the Gecko.

The snout is a stellar nursery. The Gecko's bright, compact tip is not a single star but the LkHα 233 group: a Herbig Ae star (LkHα 233, also V375 Lac) with a small bipolar reflection nebula, together with the emission-line stars LkHα 230 through 232 and an entire chain of Herbig–Haro objects (HH 398, HH 808 through 814) — shock fronts from jets that young stars launch into their surroundings. This is star formation on a small scale, along the edge of a cloud whose surface is being eroded by the association's radiation. The nearby B1.5 star HD 213976 supplies the light that makes the Gecko's head shine so brightly.

What a Herbig star is. Starting in 1960 at Lick Observatory, George Herbig cataloged stars that stood out through Hα emission (hence LkHα, Lick Hα) and lay within nebulae. A Herbig Ae/Be star is an intermediate-mass star — roughly two to eight solar masses, spectral class A or B — that has not yet reached the main sequence: it is still contracting, has yet to ignite hydrogen in its core, and is surrounded by a disk of gas and dust from which it continues to accrete material. The emission lines arise in this disk and the infalling gas; the infrared excess comes from warm dust within it. Herbig stars are the more massive siblings of T Tauri stars, which pass through the same stage with less than two solar masses; the phase lasts only a few million years, so these stars are always found in or beside their birth clouds. With its A7 type, LkHα 233 is a relatively low-mass example, and its bipolar nebula traces the disk in silhouette: starlight escapes only perpendicular to it, in two cones. Herbig–Haro objects are something else, named after Herbig and Guillermo Haro — not stars, but luminous shock fronts where a young star's jet hits surrounding gas at several hundred kilometers per second. Together, a Herbig star and a chain of HH objects mark a cloud in which stars are forming right now.

Why the Gecko is white — rather than dark, red or blue. The same dust cloud can take on four different appearances depending on its surroundings, and the Gecko illustrates which occurs and why. It would be dark if it lay against a bright background with no stars nearby: we would see only a silhouette, like the Horsehead's dark notch in Orion. Here, however, it lies in the middle of Lacerta OB1, surrounded by dozens of B stars, with HD 213976 right beside its snout — it is illuminated from outside. Only ionized gas glows red: ultraviolet light separates electrons from atoms, and recombination produces Hα. That requires an O star within reach; 10 Lac is two degrees away, and its radiation penetrates only to the cloud's edge (the red streamers LBN 442). The interior is cold, neutral and molecular — gas that does not glow, and dust that reflects light. A reflection nebula becomes blue like the Pleiades only under two conditions: the light source itself is blue, and the dust layer is thin enough that each photon scatters only once — scattering favors short wavelengths, so blue emerges. The Merope Nebula is a thin veil through which hot B stars shine. The Gecko is the opposite: a dense, optically thick cloud illuminated not by a single blue star but by the entire stellar field of the association and the Milky Way. In dense dust, light is scattered repeatedly and also absorbed — and absorption likewise preferentially affects blue light. The two effects largely cancel, leaving the spectrum of the light source: whitish to slightly bluish gray, the color of starlight that has passed through dust. High-latitude Galactic cirrus, illuminated only by the integrated light of the Milky Way, looks much the same. The snout appears brighter and warmer because HD 213976 and the LkHα 233 group lie directly in front of it — there the light source is close and the geometry more like that of a classic reflection nebula.

Two kinds of nebula in one image. The pink band is Hα, the recombination line of hydrogen; the silvery-gray veil is dust scattering starlight. Where the band meets the Gecko, both overlap, and the red streamers LBN 442 fray out along its lower edge — the ionization front where radiation from 10 Lac meets the denser cloud. SPCC color calibration sets the white point using the stars; the veil's neutral tone is therefore not imposed but what remains after a calibration based on stellar colors.

And then there are the galaxies. At a Galactic latitude of −15°, this field already looks well out of the Milky Way's plane, so the nebular layer is thin — and behind it, dozens of galaxies come into view. In the pink band at upper right lies the HDC 1194 group, including UGC 12062, UGC 12079 and several Zwicky galaxies, all at redshifts around 0.017, or roughly 70 Mpc, or 230 million light-years away. NGC 7330 is the brightest example at lower right, with further candidates along the upper right strip and even a spiral seen nearly edge-on from the flat-galaxy catalog in the left third of the image. The Gecko is 1,600 light-years away, the galaxies behind it a hundred and fifty thousand times farther — the same exposure captures both.

What this image is not. This is a purely broadband exposure without a narrowband filter: 26 five-minute sub-exposures from a single night. A color sensor without a filter records the Hα band only weakly — its prominence here comes from the processing, not the exposure time. A version with a true Hα layer would require a substantial number of dual-band exposures; two test frames exist, but are not part of this image.