Great comet

A great comet is a comet that becomes exceptionally bright. There is no official definition; often the term is attached to comets such as Halley's Comet, which are bright enough to be noticed by casual observers who are not actively looking for them, and become well known outside the astronomical community. Great comets are rare; on average, only one will appear in a decade. Although comets are officially named after their discoverers, great comets are sometimes also referred to by the year in which they appeared great, using the formulation "The Great Comet of ...", followed by the year.

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 these criteria will certainly be spectacular. Sometimes, a comet failing on one criterion will still be extremely impressive. For example, Comet Hale–Bopp had an exceptionally large and active nucleus, but did not approach the Sun very closely at all, yet it still became an extremely famous and well observed comet. Equally, Comet Hyakutake was a rather small comet, but appeared bright because it passed extremely close to the Earth.

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, due to 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 which are making their first passage through the Solar System.

The sudden brightening of comet 17P/Holmes in 2007 showed the importance of the activity of the nucleus in the comet's brightness. On October 23–24, 2007, the comet suffered a sudden outburst which caused it to brighten by factor of about half a million. 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 these 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.

...
Wikipedia