A Ghostly Bubble in the Big Dog
Sh2-308 is a massive shell of ionized gas blown by the powerful stellar winds of the Wolf-Rayet star EZ Canis Majoris. This star is in a brief but violent phase of its life, shedding its outer layers at incredible speeds. The nebula is roughly 60 light-years across and is approximately 70,000 years old. Its distinct 'Dolphin' shape comes from the way the gas expands into the uneven density of the surrounding interstellar medium. Unlike most nebulae that glow red from Hydrogen, the Dolphin is dominated by Oxygen-III, giving it its signature ghostly blue-teal appearance.
Imaging Challenges
- • Faintness: Despite its fame, the OIII shell is quite faint. You need significant integration time (10+ hours) to get a clean, noise-free 'skin' on the dolphin.
- • Low Altitude: For many Northern Hemisphere observers, it sits very low on the horizon, meaning you have a narrow window each night to capture it through thick atmosphere.
- • Processing the 'Void': The nebula sits in a relatively empty area of the sky; maintaining a natural-looking background while stretching the faint shell is a delicate balance.
The 'eye' of the Dolphin is actually the Wolf-Rayet star EZ Canis Majoris itself, which is one of the brightest stars of its type.