Comets Novae and Meteors

The majority of objects we see in the night sky move in regular and readily recognizable cycles, but a few phenomena are much more sporadic. These include meteors, or "shooting stars"; comets, which at their most spectacular are seen as bright, blurry patches with a long tail; and novae, new stars that appear where nothing was visible before.

Meteors are by far the most common of the three. On almost any night a patient observer can see a random shooting star fleetingly passing from one point in the sky to another. In fact, shooting stars are not stars at all but small particles of dust that fly into the earth's atmosphere (or, to put it another way, the earth flies into them), whereupon they burn up. When they are unusually large and close to the observer, they can appear as bright and colored fireballs. Occasionally a small fragment of rock, or a meteorite, will fall to earth; large meteorites are rarer still but can cause local damage or even environmental catastrophe. Meteors often occur in systematic bursts known as meteor showers, with scores or even hundreds visible in the space of a few hours. When watching these closely, an observer soon notes that all the shooting stars appear to radiate from the same point in the sky. It is as though the earth is heading at breakneck speed toward this spot, rushing past stationary points of light. This is, in fact, pretty much what is happening, since the earth's orbit is passing through a stream of material (although the material in the stream is itself in orbit). This means that many meteor showers occur regularly, around the same date each year, when the earth's orbit passes through the same region in space. However, varying conditions can mean that while in some years a given shower will be strikingly visible, in other years it will pass almost unnoticed.

Comets can be truly spectacular, their tails stretching far across the sky, and a few are so bright that they can be seen in daytime. Such occurrences are relatively rare, though. The best examples in the last two centuries were seen in 1811, 1843, 1861, 1882, and 1910. The best example in the later twentieth century (which, though impressive, did not compare with any of these) was Comet Hale-Bopp in 1997. The appearance of any particular comet, and how this varies from night to night, depends on many factors and differs considerably from one comet to another. A comet may take several weeks to reach its maximum brightness or do so extremely rapidly, and it may only be at its most striking for a day or two. Some comets recur regu-larly—the most famous being Comet Halley, which reappears roughly every seventy-six years—but these comets cannot always be relied upon to be conspicuous on every occasion (indeed, Halley passed almost unnoticed in 1986). In practice, then, the appearance of a bright comet to peoples in the past—even those who kept accurate records, such as the Babylonians and Chinese—was unpredictable.

While the majority of stars shine constantly with little perceptible variation for literally billions of years, some are variable and a few—known as novae—are liable to sudden outbursts that cause their brightness to increase considerably. However, the most incredible phenomena of this type are the so-called supernovae—stars that literally explode or implode and whose brightness, as a result, suddenly increases by a huge factor. When supernovae occur relatively close to us (in astronomical terms), then what before was invisible to the naked eye suddenly makes its appearance as a guest star, which, in some cases, can outshine everything in the sky apart from the moon and sun. There have been no great supernovae in the last four centuries and only around eight have been reliably recorded in historical times. The earliest, in the constellation of Centaurus, was recorded by Chinese astronomers in c.e. 185, although there is some dispute about whether this was actually a supernova. The most recent was Kepler's Supernova of 1604. It has been suggested that a guest star that appeared in 1054, and was certainly recorded by the Chinese and possibly by others in Asia and Europe, was also recorded in North America. However, the idea that a famous pic-

tograph at Chaco Canyon was a depiction of this supernova does not withstand close scrutiny.

Any unexpected and conspicuous occurrence in the sky—including solar and lunar eclipses as well as rare meteorological phenomena—has the potential to cause panic and even social upheaval. The political control wielded by ruling elites often rests upon a spiritual power derived from their apparent links with the heavens. Unanticipated and unnerving celestial events can serve to undermine this power, sometimes catastrophically.

See also:

Catastrophic Events; Power.

Chaco Supernova Pictograph; Chinese Astronomy.

References and further reading

Brandt, John C., and Robert D. Chapman. Introduction to Comets, 1-54. Cambridge: Cambridge University Press, 2004.

Frommert, Hartmut, and Chrisine Kronberg. Supernovae Observed in the Milky Way: Historical Supernovae. http://seds.lpl.arizona.edu/messier/more/mw_sn.html.

Ridpath, Ian, ed. Norton's Star Atlas and Reference Handbook (20th ed.), 88-94, 126-129. New York: Pi Press, 2004.

Stephenson, F. Richard, and David A. Green. Historical Supernovae and Their Remnants. Oxford: Oxford University Press, 2002.

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