Thor's Helmet

A warrior's helmet floats in the dark, complete with horns and a crest: astronomers saw in it the helm of Thor, the Norse god of thunder. It is a bubble of gas nearly 30 light-years across, lost far away, some 15,000 light-years off, in the constellation of Canis Major.

At its centre burns an extraordinary star: a Wolf-Rayet, sixteen times heavier than the Sun and nearly 300,000 times more luminous. It is this star that blew the helmet: its wind, of unimaginable violence, sweeps up and compresses the gas around it, and the horns are born where that blast meets a neighbouring cloud. Such a star lives fast and dies young: this one should explode as a supernova within a few thousand years.

William Herschel had already spotted it in 1785, without guessing at the furnace that drives it. Today, from Texas, it takes special filters and long exposures to wrest its faintest filaments from the darkness.