Dolphin Head Nebula

In the constellation Canis Major, roughly 4,530 light-years from Earth, a colossal bubble is silently expanding through the interstellar medium. The Dolphin Head Nebula — Sh2-308 — is the spectacular result of the stellar winds of one of the most violent stars in our Galaxy.

This bubble of ionised hydrogen spans approximately 60 light-years in diameter, appearing slightly larger than the full Moon in the sky. It has been shaped over tens of thousands of years by the fierce stellar winds of EZ Canis Majoris (WR 6), a Wolf-Rayet star that continuously ejects material at tremendous speed. Sh2-308 is one of only two known Wolf-Rayet bubbles that emit X-rays, alongside the Crescent Nebula (NGC 6888).

EZ Canis Majoris is a massive star in the final stage of its evolution, with a spectral type of WN4b and an apparent magnitude ranging from 6.71 to 6.95. Having already shed its outer hydrogen layers, it now burns through heavier elements at extreme temperatures. A supernova is inevitable on astronomical timescales. Sh2-308 was catalogued by American astronomer Stewart Sharpless in his 1959 catalogue of HII regions.

Due to its exceptionally low surface brightness, Sh2-308 is a challenging target for astrophotographers. The IDAS NBZ-II narrowband filter, sensitive to Hα and OIII emission lines, was essential to reveal the filamentary structure of this cosmic bubble from Texas. A 2-panel mosaic was required to capture the full extent of the nebula.

The field also reveals several other notable objects. To the left of the bubble lies Sh2-303, an HII region also ionised by EZ Canis Majoris. On the lower perimeter of the dolphin appears MPA J0656-2356, a small round planetary nebula whose central star is a white dwarf at approximately 80,000 K. Above the left-hand cloud, PK 233-10.1 is another catalogued planetary nebula — so compact that it resembles a star unless zoomed in. These details are much easier to appreciate at full resolution — the AstroBin button at the bottom of the page offers a far greater zoom than the preview here.