Семейство комет Крейца: Солнечные скользящие

Family of comets
The Kreutz sungrazers (/audio=Mgm pronunciation Kreutz Sungrazers. ogg/'/k//r//ɔɪ//t//s/ KROYTS) are a family of sungrazing comets, characterized by orbits taking them extremely close to the Sun at perihelion. At the far extreme of their orbits, aphelion, Kreutz sungrazers can be a hundred times farther from the Sun than the Earth is, while their distance of closest approach can be less than twice the Sun's radius. They are believed to be fragments of one large comet that broke up several centuries ago and are named for German astronomer Heinrich Kreutz, who first demonstrated that they were related. Astronomers at the time, including Edmond Halley, speculated that this comet was a return of a bright comet seen close to the Sun in the sky in 1106. In 1888, Heinrich Kreutz published a paper showing that the comets of 1843 (C/1843 D1, the Great March Comet), 1880 (C/1880 C1, the Great Southern Comet), and 1882 (C/1882 R1, Great September Comet) were probably fragments of a giant comet that had broken up several orbits before. Initially, the name "sungrazer" was applied exclusively to the Kreutz group.

Most sungrazing comets are part of the Kreutz family. The group generally has an eccentricity approaching 1, orbital inclination of 139 144° (precluding close encounters with planets), a perihelion distance of less than 0.01 AU (less than the diameter of the Sun), an aphelion distance of about 100 AU and an orbital period of about 500 1,000 years. Most Kreutz sungrazers have radii of less than , but the brightest ones reach radii of The bodies themselves have irregular shapes and appearances which have been described as diffuse, star like or tailed. The material that makes up their cometary nuclei has a low tensile strength. They have only low concentrations of volatiles and thus become active only close to the Sun, since they have lost most of their volatiles during earlier transits. Their brightness may peak shortly before perihelion at 10 15 solar radii, after which they become dimmer. This may be due to the evaporation of minerals like olivine and pyroxene. Other studies find a more chaotic pattern of brightening and darkening. The water and organic materials of a comet evaporate first, exposing fluffy aggregations of olivines that form dust tails. Dust from these comets remains in the solar corona, where it interacts with the Sun's magnetic field.

The brightest members of the Kreutz sungrazers have been spectacular, easily visible in the daytime sky. The three most impressive have been the Great Comet of 1843, the Great Comet of 1882 and X/1106 C1. The progenitor of all Kreutz sungrazers observed to date may be the Great Comet of 371 BC, or comets seen in 214 BC, 423 AD or 467 AD. X/1381 V1 which was seen from Japan, Korea, Russia and Egypt, two comets seen in 1668 and 1695, C/1880 C1, the Great Southern Comet of 1887, C/1945 X1 (du Toit), C/1970 K1 and C/2005 S1, one of the best observed Kreutz sungrazers.

The Great Comet seen in the winter of 372–371 BC was an extremely bright comet thought to be the progenitor of the entire Kreutz sungrazer family. It was observed by Aristotle and Ephorus during the period in which it was visible to the naked eye. It was reported to have had an extremely long, extremely bright, prominent tail with a reddish colour, as well as a nucleus brighter than any star in the night sky.

The Great Comet of 1843 was first noticed in early February of that year, just over three weeks before its perihelion passage when it passed about from the surface of the Sun. By February 27 it was easily visible in the daytime sky, the tail was calculated to be more than 300 million kilometers (2 AU) long. This held the record for the longest measured cometary tail until 2000, when Comet Hyakutake's tail was found to stretch to some 550 million kilometers in length. The maximum apparent magnitude attained by this comet was −10. (The Earth–Sun distance—1 AU—is only 150 million kilometers.)

A party of observers gathered in Egypt to watch a solar eclipse in May 1882 also observed a bright streak near the Sun once totality began. The streak was the perihelion passage of a Kreutz comet, and its sighting during the eclipse was the only observation of it. Photographs of the eclipse revealed that the comet had moved noticeably during the 1m50s eclipse, as would be expected for a comet racing past the Sun at almost 500 km/s. The comet is sometimes referred to as Tewfik, after Tewfik Pasha, the Khedive of Egypt at the time. It grew rapidly brighter and was eventually so bright it was visible in the daytime for two days (16–17 September), even through light cloud. After its perihelion passage, the comet remained bright for several weeks. During October, its nucleus was seen to fragment into first two and then four pieces. Some observers also reported seeing diffuse patches of light several degrees away from the nucleus. The rate of separation of the fragments of the nucleus was such that they will return about a century apart, between 670 and 960 years after the break up. Japanese astronomers used a coronagraph to observe how the comet broke into three pieces 30 minutes before perihelion. When the comet reappeared in the morning sky in early November, two of these nuclei were definitely detected with the third suspected. The comet developed a very prominent tail, about 25° in length, before fading throughout November. It was last detected in January 1966.

A study by Brian G. Marsden in 1967 was the first attempt to trace back the orbital history of the group to identify the progenitor comet. This meant it was realistic to presume that they were two parts of the same comet which had broken up one orbit ago. The best candidate for the progenitor comet was the Great Comet of 1106: Ikeya–Seki's derived orbital period indicated that its previous perihelion matched that of 1106, and while the Great Comet of 1882's derived orbit implied a previous perihelion a few decades later, it would only require a small change in the orbital elements to bring it into agreement. These form the 'non Kreutz' or 'sporadic' sungrazers. The ultimate origin of the Kreutz sungrazers is probably the Oort cloud, with unknown physical processes reducing the semi major axis until a sungrazing comet resulted. This process may occur a few times every million years, which may either be an underestimate or may indicate that humanity is lucky that such a Kreutz sungrazer family exists just now. Studies have shown that for comets with high orbital inclinations and perihelion distances of less than about 2 AU, the cumulative effect of gravitational perturbations tends to result in sungrazing orbits. One study has estimated that Comet Hale–Bopp has about a 15% chance of eventually becoming a Sun grazing comet. Comet families resembling the Kreutz group have been detected around the star Beta Pictoris.

Until recently, a very bright member of the Kreutz sungrazers could pass through the inner Solar System unnoticed if its perihelion had occurred between about May and August. During the 1980s, two Sun observing satellites serendipitously discovered several new members of the Kreutz family. Since the launch of the SOHO Sun observing satellite in 1995, it has been possible to observe comets very close to the Sun at any time of year. New Kreutz sungrazers are discovered roughly once every three days, while many are likely going unobserved. Their frequency increased from 1997–2002 to 2003–2008. They probably have radii of only a few dozens of metres. Apart from Comet Lovejoy, none of the sungrazers seen by SOHO has survived its perihelion passage; some may have plunged into the Sun itself, but most are likely to have simply evaporated away completely. Centrifugal breakup is another important process that destroys smaller Kreutz sungrazers, and may explain the delayed breakup of some Kreutz comets long after they passed through perihelion and are moving away from the Sun. The 1,000th known Kreutz sungrazer was observed by SOHO on 10 August 2006, and is named C/2006 P7 (SOHO). as of 2020, 85% or about 3,400 of the 4,000 comets that have been identified using SOHO data, mostly by amateur astronomers analysing SOHO's observations via the Internet, were Kreutz sungrazers. as of 2024, NASA's JPL Small Body Database lists about 1,300 Kreutz sungrazers. Sungrazers frequently arrive in pairs or triplets to four times the number of Subgroup II members. This suggests that the 'grandparent' comet split into parent comets of unequal size. Comet White–Ortiz–Bolelli in 1970 reached an apparent magnitude of 1. In December 2011, Kreutz sungrazer C/2011 W3 (Lovejoy) survived its perihelion passage for some time and had an apparent magnitude of −3. This comet is probably not the herald of another arrival of bright Kreutz sungrazers.