This delicate Hubble Space Telescope image shows a tiny portion of the Cygnus loop, a supernova remnant in the constellation of Cygnus, the Swan. Measurements on this super-detailed image of a cosmic veil shows that the original supernova explosion took place only 5,000 years ago. © ESA & Digitized Sky Survey (Caltech) The Hubble Space Telescope established that the Cygnus Loop supernova may have exploded just 5000 years ago. This means that the initial explosion could have been seen and recorded by ancient civilizations. For a short time it would have been 40 times brighter than Venus at its brightest and visible during the day.

This discovery was made by comparing photographs of the Cygnus Loop, taken from the ground in 1953, with recent images obtained with the Hubble Space telescope.

The Cygnus Loop is a well known, roughly spherical, mass of gas that is expanding at at a speed of 600,000 km per hour in the aftermath of a stellar explosion or supernova that took place at its centre. The edge of the expanding shell glows where the fast moving gas collides with stationary interstellar gas. Comparing the photograph taken in 1953 with the recent Hubble picture shows that the sphere of gas has expanded. This, and the known velocity of expansion measured from the Doppler shift with a spectrograph, enable the ESO team to estimate when the nebula first started to expand. They concluded that the explosion was only 1500 light years away and happened far more recently than we previously thought.