The legend of a brilliant new star that suddenly blazes forth in the heavens to augur the dawn of a new age is a potent image, intimately linked with the annual celebration of the birth of Jesus.
But the "guest star," as such phenomena have been called, is not an exclusively Christian symbol. The appearance of a mysterious new star has been recorded several times in history. Dazzled observers in many ages have taken such celestial fireworks to portend a major new era in their own cultures.
Astronomers now know that about half a dozen "guest stars" recorded during the last millennium were supernovas -- cataclysmic stellar explosions that are believed to occur about once every 30 years in our galaxy, the Milky Way. A supernova is the spectacular death agony of a star at least four times as massive as the sun. In less than a second, the star's center collapses into a dense ball no bigger than Washington, D.C., then explodes, spewing out enormous quantities of energy in the form of subatomic particles, rapidly expanding gases, and radiation, including visible light.
The timing of a supernova is determined by physical conditions deep in a star's core, not, of course, by any connection with events on Earth. But witnessing such a cosmic explosion can profoundly influence human observers. There is evidence to suggest the nearest known supernova, and perhaps the brightest, may have inspired the ancient Sumerians, people who lived in Mesopotamia on the shores of the Persian Gulf about 5,000 years ago, to develop the beginnings of astronomy, mathematics and writing.
That supernova exploded about 1,500 light years from Earth, in the vicinity of Vela, a constellation in the southern sky. Known as the Vela X supernova, it left behind a cloud of visible, expanding debris and a pulsar -- a source of intense, rhythmic bursts of radio waves -- that radio-astronomers discovered in 1968.
At the time of the pulsar's discovery, astronomers had trouble dating the original explosion, and initially concluded it could have happened any time from 15,000 to about 6,000 years ago. They suggested that archaeology might help pinpoint the date, if a record of human observation of the Vela supernova could be found.Nearest Known Supernova
That challenge fired the imagination of George Michanowsky, a New York independent scholar and historian of ancient astronomy who has been fascinated for most of his life by the Sumerians and by supernovas.
"What was so fantastic about the pulsar in the constellation Vela was that it is by far the nearest supernova that science has any knowledge of," he said. If humans had indeed watched a star explode so close to their own solar system, it seemed to Michanowsky that there ought to be a record somewhere.
Such a "guest star" would probably have shined brighter than the moon. It would have been visible by day. It would have blazed for months, perhaps as long as a year, before flickering out. Except for the sun, "it was the brightest thing we have any historical knowledge of," Michanowsky said.
In the writings of earlier Sumerian scholars, Michanowsky saw frequent references to a star or constellation called "Mul Nunki," the old name for the southern constellation Vela. The region of the sky containing Mul Nunki was sacred to the Sumerians, who associated it with their god Ea, a water deity who befriended humans and was a forerunner of the Prometheus figure in Greek mythology.
Michanowsky found the clue he was looking for in the early 1970s, when he made a new translation of a small Mesopotamian clay tablet now in the British Museum. Thought to date from about 1500 B.C., it contains 40 lines of cuneiform, the form of writing developed more than 4,000 years ago by the Sumerians and passed on to later inhabitants of Mesopotamia.
The tablet was probably "a votive offering put together by a pious temple scribe," Michanowsky said. It contained "a compendium of much earlier astronomical observations."
The second and third lines, as translated by Michanowsky, refer to "the giant star of the god Ea in the constellation Vela of the god Ea. The star or constellation immediately to the north is the star Exalted Lady." Exalted Lady is a constellation now called Puppis, which the Sumerians associated with their mother-goddess, Nin-Mah.
Vela, the constellation where the supernova exploded, contains no especially bright star today. By describing a "giant star" in Vela and near Puppis, Michanowsky said, the Sumerians pinpointed the area of the sky where the Vela X supernova exploded.
To ancient people, a new and unusually bright star bursting into view on the southern horizon -- it would have been visible just above the waters of the Persian Gulf -- would have been an awe-inspiring sight. Michanowsky said the star's association with Ea suggests that the Sumerians revered it, since Ea was the giver of knowledge, including mathematics, astronomy, canal irrigation and writing. He speculates that the Sumerians developed cuneiform, the first known writing system, and were the first to name the constellations because they were inspired by the spectacle of the supernova.
"This gigantic star, which eventually disappeared, was considered the star of the culture hero and the source of wisdom, knowledge and science," said Michanowsky. "It was expected to return again."
Michanowsky's theory, described in his 1977 book "The Once and Future Star," earned praise from Sumerian experts, but many astronomers argued that the Vela X supernova had come and gone long before the Sumerians appeared.
Recently, however, astronomers have refined their estimate of the supernova's age, making it a plausible player in Michanowsky's scenario.An Asymmetrical Explosion
Earlier estimates of the supernova's age depended on measuring the rate of expansion of the cloud of gas and debris blasted out by its explosion. In 1989 Australian astronomers used a different method.
A pulsar travels through space like a spinning soccer ball because of the "kick" provided by the explosion. Australian astronomers, used the pulsar's velocity and direction to trace its path backward through time. They showed that the supernova had exploded asymmetrically, one side of the gas cloud expanding faster than the other.
That asymmetry means that previous estimates, which assumed a symmetrical burst, would have been off by a factor of two, said Richard N. Manchester, chief research scientist at the Australia Telescope National Facility. In an article in The Astrophysical Journal, Manchester and other astronomers calculate that the supernova occurred between 4,500 and 8,000 years ago, a range that overlaps the time of the Sumerians.
"These things are very uncertain," he said. But he said it was "possible" that the Sumerians witnessed the Vela supernova. If so, they appear to have launched the enduring tradition of linking the appearance of a bright new star to the rise of a new era in history.