Great comet

A great comet is a comet that becomes exceptionally bright and easily observable to the naked eye. There is no official definition; the term is often attached to comets such as Halley's Comet, which, during certain appearances, are bright enough to be noticed by casual observers not looking for them and become well known outside the astronomical community. Typically, they are as bright or brighter than a second-magnitude star and have tails that are 10 degrees or longer under dark skies.

Great comets appear at irregular, unpredictable intervals, on average about once per decade. Although comets are officially named after their discoverers, great comets are sometimes also referred to by the year in which they appeared great, by using the formulation "The Great Comet of ...", followed by the year. The term can also be used as a generic name when a very bright comet is discovered by many observers simultaneously.

Causes

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 0.5 AU (75 million km) 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 0.1 AU (15 million km; 39 LD).

List of great comets

Great comets of the past two millennia include the following below. This list includes multiple bright apparitions of Halley's Comet since 86 BC:

Notes

External links

• John E. Bortle (W. R. Brooks Observatory)
• Gary W. Kronk.