Dolphin Head Nebula

In the constellation of Canis Major, about 4,500 light-years away, a giant bubble swells silently through space. It has been nicknamed the Dolphin Head (Sh2-308), and it is the work of a single star, among the rarest and most violent in our galaxy.

At its centre watches EZ Canis Majoris, a Wolf-Rayet star: a colossal star very near its end. About 70,000 years ago, it began to cast off its outer layers, and it now blows a wind so fast, nearly 1,700 kilometres per second, that it sweeps up the material released earlier and inflates it into a bubble 60 light-years wide, slightly larger than the full Moon in the sky. This shell glows mostly in the light of oxygen, which gives it its blue-green hue and its dolphin shape.

It also has a rarity to its name: it is one of only two bubbles of this kind known to emit X-rays, along with the Crescent Nebula. As for the star, its fate is sealed: it will end as a supernova, on the scale of astronomical time.

The object is so faint that special filters were needed to reveal it from Texas: they capture only the light of two gases, oxygen (in blue-green, the dolphin's body) and hydrogen (in red). I assembled the image from two panels to bring into the frame, on the left, a small cloud that reminds me of a miniature coral reef, a perfect companion for the dolphin. The field also hides two small planetary nebulae, those bubbles blown by far more modest dying stars: one, round, nestles at the lower right of the dolphin's head; the other, so compact it passes for an ordinary star, hides further out in the field. The AstroBin button, beneath the image, lets you track them down at full resolution.