over
Rotterdam as painted by
Lieve Verschuier The vast majority of comets are never bright enough to be seen by the naked eye and generally pass through the inner
Solar System unseen by anyone except
astronomers. However, occasionally, a comet may brighten to naked eye visibility, and even more rarely, it may become as bright as or brighter than the brightest stars. The requirements for this to occur are: a large and active
nucleus, a close approach to the
Sun, and a close approach to the
Earth. A comet fulfilling all three of those criteria will certainly be very bright. Sometimes, a comet failing on one criterion will still be bright. For example,
Comet Hale–Bopp did not approach the Sun very closely but had an exceptionally large and active nucleus. It was visible to the naked eye for several months and was very widely observed. Similarly,
Comet Hyakutake was a relatively small comet but appeared bright because it passed very close to the Earth.
Size and activity of the nucleus Cometary nuclei vary in size from a few hundreds of metres across or less to many kilometres across. When they approach the Sun, large amounts of
gas and
dust are ejected by cometary nuclei by solar heating. A crucial factor in how bright a comet becomes is how large and how active its nucleus is. After many returns to the inner Solar System, cometary nuclei become depleted in volatile materials and thus are much less bright than comets that are making their first passage through the Solar System. The sudden brightening of
Comet Holmes in 2007 showed the importance of the activity of the nucleus in the comet's brightness. On October 23–24, 2007, the comet underwent a sudden outburst which caused it to brighten by factor of about 480,000 times. It unexpectedly brightened from an
apparent magnitude of about 17 to about 2.8 in a period of only 42 hours, making it visible to the naked eye. All of those factors temporarily made comet 17P the largest (by radius) object in the Solar System although its nucleus is estimated to be only about 3.4 km in diameter.
Close perihelion approach The brightness of a simple reflective body varies with the
inverse square of its distance from the Sun. That is, if an object's distance from the Sun is halved, its brightness is quadrupled. However, comets behave differently because of their ejection of large amounts of volatile gas, which then also reflect sunlight and may also
fluoresce. Their brightness varies roughly as the inverse cube of their distance from the Sun. That is, if a comet's distance from the Sun is halved, it will become eight times as bright. This means that the peak brightness of a comet depends significantly on its distance from the Sun. For most comets, the
perihelion of their orbit lies outside the Earth's orbit. Any comet approaching the Sun to within or less may have a chance of becoming a great comet.
Close approach to the Earth For a comet to become very bright, it also needs to pass close to the Earth.
Halley's Comet, for example, is usually very bright when it passes through the inner Solar System every 76 years, but during its 1986
apparition, its closest approach to Earth was almost the most distant possible. The comet became visible to the naked eye but unspectacular. On the other hand, the intrinsically small and faint
Comet Hyakutake (C/1996 B2) appeared very bright and spectacular due to its very close approach to Earth at its nearest during March 1996. Its passage near the Earth was one of the closest cometary approaches on record, with a distance of . == List of great comets ==