Star light, star brightest:
the supernova of A.D. 1006
Blast from the past: The brightest supernova on record was visible for months and outshone everything except the sun and moon. This scene shows how the southern sky looked at midnight to observers in Cairo, Egypt, on May 1, 1006. Mars (top center) was at its brightest in Scorpius when the star first appeared. |
Nearly one millennium ago, the sudden appearance of the brightest star in recorded history puzzled and frightened sky watchers around the world. The new star of 1006 was actually a nearby supernova, the visible manifestation of a star's complete destruction. At peak brightness it outshone Venus and was even compared with the moon. Just how bright it was, though, has been something of a mystery estimates in the scientific literature have ranged over a factor of fifteen. Recently, a team led by Frank Winkler of Vermont's Middlebury College reported that it has found the answer in faint strands of gas at the site of the explosion.
Supernovas are rare. In a spiral galaxy such as our Milky Way, containing over a hundred billion stars, astronomers expect only a few such explosions each century. While they have watched thousands of supernovas firing off in distant galaxies, for centuries they've been denied a look at one in our own. Clouds of dust and gas obscure the view of much of our galaxy, so we see only those supernovas that explode within a few thousand light-years of Earth. Johannes Kepler was among those who saw the 1604 supernova, the last stellar blast to occur in our vicinity.
A supernova radiates so much energy — over five billion times the annual energy output of our sun — that it briefly rivals the entire galaxy of stars in which it occurs. Each of these stellar explosions transforms a star into a shell of hot gas that initially expands at speeds of 10,000 kilometers per second or more. The kinetic energy contained in this expanding shell is ten times that of the energy radiated in the blast. By the time the supernova reaches maximum brightness, about two weeks after the explosion, our planetary system would easily fit inside it. Stellar debris races into the interstellar environment, creating a shock wave that compresses and heats the gas it plows into. The shock will continue to expand for millennia, ultimately affecting a bubble of space over a thousand light-years across before losing its identity in the thin matter between the stars.
Dazzling the eyes
On May 1, 1006, observers around the world noticed a brilliant new star in what is now the constellation of Lupus, near Scorpius. Clear historical accounts of the event have been found in Egypt, Iraq, Italy, Switzerland, China, and Japan, with additional possible references from France and Syria. The most detailed account comes from an Egyptian physician and astrologer named Ali ibn Ridwan:
He recorded the positions of the sun, moon and planets when the star first appeared, which helped modern astronomers fix the date. He noted that the " spectacle" did not move independently of the stars and that it disappeared " at once" when the sun came within sixty degrees of longitude of its position. That corresponds to a visibility period of about three months. From Ali bin Ridwan's observing site of Al-Fustat, which now lies within modern Cairo, the star was above the horizon only during the day when it disappeared. Since Venus at its brightest can be seen during the day, his statement suggests that the star had become fainter than this.
Astrologers in Europe and Asia also noticed the new star. One of the most significant observations comes from the Benedictine monastery at St. Gallen in Switzerland:
It's the only event in 1006 that the monastery's chroniclers found important enough to record, placing it on a par with a famine listed in the previous year and a plague outbreak in the following year. To modern astronomers, however, the implication that the star was just barely above St. Gallen's mountainous southern horizon placed important limits on the position of the star and helped pinpoint its location.
In China, too, the phenomenon raised fears. No one could agree on its astrological significance, but there was growing concern that it portended " warfare and ill-fortune." An astute astrologer named Chou K'o-ming came forward with an alternative interpretation:
The emperor not only approved K'o-ming's suggestion — he gave him a promotion as well.
Chinese astrologers watched the star for three months before it became lost in the sun's glare, in agreement with other reports, but they went even further. According to the astronomical treatise in the Annals of the Sung Dynasty, imperial sky watchers recovered the star in December and continued to monitor it for another year and a half.
Only a supernova explains the brilliance and duration indicated by eyewitnesses. The very fact that it captured such attention suggests its extraordinary brightness. The star appeared low on the southern horizon at virtually all of the locations from which it was observed. Yet it was more widely reported than the supernova that appeared forty-eight years later in the constellation Taurus, one more favorably positioned for northern observers. That blast created the Crab nebula.
