Drop a stone in a pond and ripples spread into every widening circles. Blow up a star and its blast wave sweeps up and excites thinly strewn atoms to form an ever-expanding glowing circle in space. A team of astronomers stumbled on just such a ring about 600 light years away that spills into four different constellations including the familiar Big and Little Dippers.
The 30° arc of compressed hydrogen gas is extremely faint so you’ll have to use your imagination to picture it curving between the two dippers. Andrea Bracco of the University of Paris and her team discovered it by chance while examining images taken by NASA’s Galaxy Evolution Explorer (GALEX), an orbiting telescope that photographed the sky in ultraviolet light from 2003 until early 2012. While examining what appeared to be a linear filament of gas they noticed it was actually slightly curved. The “line” proved to be only a short segment of a much larger arc.
Dubbed the Ursa Major Arc it appears to be an expanding shock wave originating in a supernova (or possibly nova) explosion that occurred about 100,000 years ago. Astronomers have photographed similar arcs and filaments that form in the wake of supernovae explosions called supernova remnants. One of the most famous is the Crab Nebula in Taurus. While the new discovery is much too faint to see visually in a telescope it takes the cake for size — if you complete the arc it extends across 48° of sky!
100,000 years ago early humans (us) emerged in Africa and soon migrated into Europe and Asia. One wonders whether our ancestors may have seen the original supernova blast and stopped to look, wonderstruck. Would it have resembled the explosion in 1054 that birthed the Crab Nebula? Chinese astronomers recorded a brand new “star” in Taurus in July that year brighter than Venus. Anyone with a modest telescope can see the explosion’s expanding remains today. Would that the Ursa Major remnant were as bright, but it occurred so long ago it has since faded.
You don’t need a satellite to photograph it however. Amateur astronomers David Mittelman, Dennis di Cicco, and Sean Walker have been working on a project called the MDW Survey to photograph the entire sky in the light of glowing hydrogen gas called “hydrogen alpha (H-alpha)” light using a special “narrow band” filter. Stellar birth clouds called nebulas are rich in hydrogen which fluoresces a beautiful pink when ultraviolet light from massive, newborn stars excites the gas. Their photos show an extremely narrow circular arc that lines up neatly with the same arc seen in the GALEX ultraviolet images.
Diagrams and photos show the arc — part of a circle — in two dimensions. To really picture what hovers near the Big Dipper we must imagine the blast wave as an expanding sphere in three dimensions. A gigantic, continuously-inflating bubble. The arc we see defines the edge of that bubble while the center — invisible thus far in any photos — bulges toward us and the even more distant while backside bows away.
Supernova blast waves have a side benefit. They sweep away the dust and gas scattered across space like a brisk wind, clearing the view into the great beyond where we might discover the faintest and most remote galaxies.
Visible or not, the enormity of this supernova fossil is a feast for the imagination.