Great comet
Encyclopedia
Comet
A comet is an icy small Solar System body that, when close enough to the Sun, displays a visible coma and sometimes also a tail. These phenomena are both due to the effects of solar radiation and the solar wind upon the nucleus of the comet...
that becomes exceptionally bright. There is no official definition; often the term will be attached to comets that become 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
Decade
A decade is a period of 10 years. The word is derived from the Ancient Greek dekas which means ten. This etymology is sometime confused with the Latin decas and dies , which is not correct....
. While 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.
Causes
The vast majority of comets are never bright enough to be seen by the naked eye, and generally pass through the inner solar systemSolar System
The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...
unseen by anyone except astronomers. However, occasionally a comet may brighten to naked eye visibility, and even more rarely it may become as bright or brighter than the brightest stars. The requirements for this to occur are: a large and active nucleus, a close approach to the Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
, and a close approach to the Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...
. 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
Comet Hale-Bopp
Comet Hale–Bopp was perhaps the most widely observed comet of the 20th century, and one of the brightest seen for many decades...
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
Comet Hyakutake
Comet Hyakutake is a comet, discovered on January 31, 1996, which passed very close to Earth in March of that year. It was dubbed The Great Comet of 1996; its passage near the Earth was one of the closest cometary approaches of the previous 200 years. Hyakutake appeared very bright in the night...
was a rather small comet, but appeared bright because it passed extremely 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 gasGas
Gas is one of the three classical states of matter . Near absolute zero, a substance exists as a solid. As heat is added to this substance it melts into a liquid at its melting point , boils into a gas at its boiling point, and if heated high enough would enter a plasma state in which the electrons...
and dust
Dust
Dust consists of particles in the atmosphere that arise from various sources such as soil dust lifted up by wind , volcanic eruptions, and pollution...
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
17P/Holmes
Comet Holmes is a periodic comet in our solar system, discovered by the British amateur astronomer Edwin Holmes on November 6, 1892...
in 2007 showed the importance of the activity of the nucleus in the comet's brightness. Between 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 a 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.
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 doubles, its brightness is quartered. However, comets behave differently, due to their ejection of large amounts of volatile gas which then also reflect sunlight and may also fluoresceFluorescence
Fluorescence is the emission of light by a substance that has absorbed light or other electromagnetic radiation of a different wavelength. It is a form of luminescence. In most cases, emitted light has a longer wavelength, and therefore lower energy, than the absorbed radiation...
. Their brightness varies roughly as the inverse cube of their distance from the Sun, meaning that 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
Astronomical unit
An astronomical unit is a unit of length equal to about or approximately the mean Earth–Sun distance....
or less may have a chance of becoming a Great Comet.
Close approach to the Earth
Comet Halley
Halley's Comet or Comet Halley is the best-known of the short-period comets, and is visible from Earth every 75 to 76 years. Halley is the only short-period comet that is clearly visible to the naked eye from Earth, and thus the only naked-eye comet that might appear twice in a human lifetime...
, for example, is usually very bright when it passes through the inner solar system every seventy-six 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 was definitely unspectacular. On the other hand, the intrinsically small and faint Comet Hyakutake
Comet Hyakutake
Comet Hyakutake is a comet, discovered on January 31, 1996, which passed very close to Earth in March of that year. It was dubbed The Great Comet of 1996; its passage near the Earth was one of the closest cometary approaches of the previous 200 years. Hyakutake appeared very bright in the night...
(C/1996 B2) appeared very bright and spectacular due to its very close approach to Earth at its nearest during the March of 1996. Its passage near the Earth was one of the closest cometary approaches on record.
List of Great Comets
Great Comets of the past two millennia include the following:- Caesar's CometCaesar's CometCaesar's Comet – also known as Comet Caesar and the Great Comet of 44 BC – was perhaps the most famous comet of antiquity...
– 44 BC - Halley's Comet – 1066
- Great Comet of 1106X/1106 C1X/1106 C1, also known as the Great Comet of 1106, was a Great Comet that appeared on February 2, 1106, and was observed across the world from the beginning of February through to mid-March. It was recorded by astronomers in Wales, England, Japan, Korea, China and Europe. It was observed to split...
- Great Comet of 1264Great Comet of 1264The great comet of 1264 was one of the brightest comets on record. It appeared in July 1264 and remained visible to the end of September...
- Great Comet of 1402
- Great Comet of 1556
- Great Comet of 1577Great Comet of 1577The Great Comet of 1577 was a comet that passed close to Earth during the year 1577 AD. It was viewed by people all over Europe, including famous Danish astronomer Tycho Brahe. From his observations of the comet, Brahe was able to discover that comets and similar objects travel above the Earth's...
- Great Comet of 1680C/1680 V1C/1680 V1, also called the Great Comet of 1680, Kirch's Comet, and Newton's Comet, has the distinction of being the first comet discovered by telescope. Discovered by Gottfried Kirch on 14 November 1680, New Style, it became one of the brightest comets of the 17th century--reputedly visible even in...
- Great Comet of 1744
- Great Comet of 1811C/1811 F1The Great Comet of 1811, formally designated C/1811 F1, is a comet that was visible to the naked eye for around 260 days, a record it held until the appearance of Comet Hale-Bopp in 1997...
- Great Comet of 1843Great Comet of 1843The Great Comet of 1843 formally designated C/1843 D1 and 1843 I, was a long-period comet which became very bright in March 1843 . It was discovered on February 5, 1843 and rapidly brightened to become a great comet...
- Donati's CometComet DonatiComet Donati, or Donati's Comet, formally designated C/1858 L1 and 1858 VI, is a long-period comet named after the Italian astronomer Giovanni Battista Donati who first observed it on June 2, 1858. After the Great Comet of 1811, it was the most brilliant comet that appeared in the 19th century. It...
– 1858 - Great Comet of 1861C/1861 J1The Great Comet of 1861 formally designated C/1861 J1 and 1861 II, is a long-period comet that was visible to the naked eye for approximately 3 months...
- Great Comet of 1882Great Comet of 1882The Great Comet of 1882 formally designated C/1882 R1, 1882 II, and 1882b, was a comet which became very bright in September 1882...
- Comet HalleyComet HalleyHalley's Comet or Comet Halley is the best-known of the short-period comets, and is visible from Earth every 75 to 76 years. Halley is the only short-period comet that is clearly visible to the naked eye from Earth, and thus the only naked-eye comet that might appear twice in a human lifetime...
– 1910 - Great Daylight Comet of 1910Great Daylight Comet of 1910The Great January Comet of 1910, formally designated C/1910 A1 and often referred to as the Daylight Comet appeared in January 1910. It was already visible to the naked eye when it was first noticed, and many people independently "discovered" the comet...
- Comet Skjellerup-MaristanyComet Skjellerup-MaristanyComet Skjellerup-Maristany, formally designated C/1927 X1, 1927 IX, and 1927k, was a long-period comet which became very bright in 1927. This great comet was observable to the naked eye for about 32 days...
– 1927 - Comet Arend-RolandComet Arend-RolandComet Arend–Roland was discovered on November 8, 1956, by Belgian astronomers Sylvain Arend and Georges Roland on photographic plates. As the eighth comet found in 1956, it was named Arend–Roland 1956h after its discoverers. Because it was the third comet to pass through perihelion during 1957, it...
– 1957 - Comet Seki-Lines – 1962
- Comet Ikeya-SekiComet Ikeya-SekiComet Ikeya–Seki, formally designated C/1965 S1, 1965 VIII, and 1965f, was a long-period comet discovered independently by Kaoru Ikeya and Tsutomu Seki...
– 1965 - Comet BennettComet BennettComet Bennett, formally known as C/1969 Y1 , was one of two brilliant comets to grace the 1970s, along with Comet West...
– 1970 - Comet WestComet WestComet West formally designated C/1975 V1, 1976 VI, and 1975n, was a spectacular comet, sometimes considered to qualify for the status of "great comet".- Discovery :...
– 1976 - Comet HyakutakeComet HyakutakeComet Hyakutake is a comet, discovered on January 31, 1996, which passed very close to Earth in March of that year. It was dubbed The Great Comet of 1996; its passage near the Earth was one of the closest cometary approaches of the previous 200 years. Hyakutake appeared very bright in the night...
– 1996 - Comet Hale-BoppComet Hale-BoppComet Hale–Bopp was perhaps the most widely observed comet of the 20th century, and one of the brightest seen for many decades...
– 1997 - Comet McNaught – 2007
External links
- The bright-comet chronicles. John E. Bortle (W. R. Brooks Observatory)
- Memorable Comets of the Past Gary W. KronkGary W. KronkGary W. Kronk is an American amateur astronomer and writer.-Biography:Kronk was born in Granite City, Illinois, United States, on March 23, 1956...
. - Brightest comets seen since 1935