Scorpio belongs to a massive belt of stars forming a halo 2000 light years across, known as Gould’s belt, the most prominent feature in our region of the Milky Way. On one side lies Scorpio, on the other Orion, with the most brilliant constellations of the Southern hemisphere, Carina, the Southern Cross and Centaur, arching between. The sun lies off centre from the ring and is not part of it.
The halo lies at a 20 degree angle to the Milky Way and is not part of its spiral structure, suggesting an origin outside of it, possibly a glancing collision between our arm of the Milky Way and a cloud of hydrogen and dust drifting through the immensities of intergalactic space.
This cataclysmic event would have compressed volumes of the gas and triggered a tremendous spate of star birth. Dating back to some 50 million years ago, the brightest stars would have long since burnt themselves out and exploded as supernovae, sending further shock waves storming out through the clouds to trigger more regions of star birth.
About 15 million years ago, a density wave triggered star birth in Centaurus and Lupus leaving us stars of this age. The brightest however have already gone supernova and the resulting density wave passed through the Upper Scorpio cloud about 5 million years ago and triggered new star birth, giving us Shaula, Antares and most of the other bright stars of the constellation along with the zeta star in Ophiucus, all about 450 light years away.
Antares will be the next star to explode, expected within the next few million years, but at a distance of 500 light years, too far away to affect the Earth.
About 2 million years ago, however, the Scorpio cluster was drifting past us, a mere 150 ly away, when a supernova explosion impacted on the Earth disturbing the ozone layer and causing the extinction of plankton, mollusks, and other marine life at the Pliocene-Pleistocene boundary. Evidence in support of this has been the identification of the radioactive isotope, iron-60, in drillings of oceanic crust from this period, a typical production of supernova explosions.
This blog first appeared as a story in the Cairo BCA magazine of 2009, June, to promote astronomy classes in Sinai with an 8″ Meade LX90 Telescope.