Crescent, Soap Bubble and the ring around WR 134 · Three Wolf-Rayet stars, P Cygni and the light of oxygen

Three Wolf-Rayet stars, one luminous blue variable, two planetary nebulae, a distant star-forming bubble and four open clusters — all in a field of 3.5° × 2.3° south of Sadr. The best-known resident is the Crescent Nebula NGC 6888 (Caldwell 27) at the upper left. But the image is deliberately not centered on it: it shows what stands around it, and that is a neighborhood where the most massive stars of the Milky Way live close together. North is to the left in this image, east is down.

The main subject is color. It was taken through a dual-band filter that passes only Hα (red) and [O III] (turquoise). Hydrogen glows almost everywhere in this part of Cygnus; the red sea is the backdrop of Cygnus X. Doubly ionized oxygen, by contrast, needs three times the energy: photons above 35 eV or hard shock fronts. Turquoise therefore appears only where an exceptionally hot star sits or where a wind of thousands of kilometers per second drives into its surroundings — the Crescent, the ring around WR 134, the Soap Bubble. The oxygen shows where the energy is.

The energy sources are three Wolf-Rayet stars, bare helium cores of former O stars. WR 136 (HD 192163, 7.5 mag, WN6) in the Crescent and WR 134 (HD 191765, 8.1 mag, WN6) at the upper right are of the same type; both have caught up with the slow wind of their earlier giant phase and swept it into a shell. With WR 136 that became the closed Crescent, with WR 134 a wide-open [O III] arc. WR 134 is also variable (V1769 Cyg) with a periodicity of about two days whose cause — a companion or rotating wind structures — is still debated. WR 135 (HD 192103, 8.1 mag), just below the ring, belongs to the carbon sequence WC8: in its case the ashes of helium burning already lie at the surface. It no longer has a ring nebula of its own. All three stand at roughly 1.7 to 2 kpc and belong to the OB associations of Cygnus.

The fourth special star is P Cygni (34 Cyg, 4.8 mag) at the lower left: a luminous blue variable of several hundred thousand solar luminosities, which flared up to naked-eye visibility in an outburst in 1600 and whose spectral profile — emission beside blueshifted absorption — is to this day the textbook signature of dense stellar winds. It is on the road the three WR stars have already travelled: an LBV is the phase in which the envelope is lost. The cluster Feibelman 1, which appears to surround it, lies according to Gaia more than twice as far away — P Cygni is not a member.

Two planetary nebulae lie in the field: the Soap Bubble (PN Ju 1), discovered in 2008 by the amateur Dave Jurasevich and glowing almost exclusively in [O III], and NGC 6881, a compact nebula of only four arcseconds, a starlike point in this image. At the lower edge stands Sh2-104, a ring-shaped H II region around an O6 star at about 4 kpc — far behind everything else. It is the textbook case of star formation triggered by an expanding bubble itself: regularly spaced molecular clumps with young star clusters inside them sit along the rim of the ring. Add the open star clusters IC 4996, Dolidze 1 and Gulliver 23, and a dark cloud of the Cygnus Rift running across the middle — dust that hides the glow behind it.

The image is a map of two emission lines in photographic color rendering: Hα in the red channel, [O III] in green and blue. Star colors survive the filter only as a remnant. Morphology may be read freely — shells, fronts, filaments, dark clouds — but the absolute line intensities are processed.