The Hubble Space Telescope has captured a delicate ribbon of gas floating eerily in our Galaxy, a ghostly reminder of a supernova explosion that occurred over 1,000 years ago, and which is still expanding at almost 10 million kilometres per hour today.
SN 1006 resides within our Milky Way Galaxy, located more than 14 degrees off the plane of the Galaxy's disc. The supernova would have occurred far off the lower right corner of the image, and the motion is toward the upper left. The image is a composite of hydrogen-light observations taken with Hubble's Advanced Camera for Surveys, and Wide Field Planetary Camera 2 observations in blue, yellow-green, and near-infrared light. The supernova remnant is visible only in the hydrogen-light filter and is assigned a red hue in this image. Image: NASA, ESA, and the Hubble Heritage Team (STScI/AURA); W. Blair (Johns Hopkins University).
Unbeknown at the time to ancient observers of the year
1,006 A.D, the arrival of a bright light in the sky marked the explosion of a white dwarf star located 7,000 light years away. The supernova became the most prominent feature in the night sky for several weeks, after the Moon, and records suggest that it remained visible to the naked eye for at least two and a half years before fading away.
It wasn’t until the mid 1960s, however, that radio astronomers first detected a nearly circular ring of material at the recorded position of the supernova almost 30 arcminutes across, the same angular diameter as the full moon. Some quick calculations revealed that the supernova remnant had expanded at around 32 million kilometres per hour since the explosion occurred.
In 1976, the first detection of exceedingly faint optical emission of the supernova remnant was reported, but only for a filament on the northwest edge of the radio ring, a tiny portion of which is revealed in detail in the new Hubble image. The twisting ribbon of light seen by Hubble corresponds to locations where the expanding blast wave from the supernova is now sweeping into very tenuous surrounding gas.
The hydrogen gas heated by this fast shock wave emits radiation in visible light, providing astronomers with a detailed snapshot of the actual position and geometry of the shock front at any given time. Bright edges within the ribbon correspond to places where the shock wave is seen exactly edge on to our line of sight.
Today we know that SN 1006 has a diameter of nearly 60 light years, and that it is still expanding at roughly 10 million kilometres per hour. Even at this tremendous speed, however, it takes observations separated by several years to see significant outward motion of the shock wave against the grid of background stars and galaxies, which in the image shown here are represented by white flecks and orange ellipses respectively.
This composite image shows the whole extent of SN 1006 comprising X-ray data from NASA's Chandra X-ray Observatory (blue), optical data from the University of Michigan's 0.9 metre Curtis Schmidt telescope at the NSF's Cerro Tololo Inter-American Observatory (CTIO; yellow) and the Digitized Sky Survey (orange and light blue), plus radio data from the NRAO's Very Large Array and Green Bank Telescope (VLA/GBT; red). The Hubble 'ribbon' is a close up of the region in the upper right of this image.