The solar system is home to many comets, small icy bodies that orbit the Sun in large, looping elliptical paths. As comets approach the Sun, they warm up and the various water, carbon dioxide and ammonia ices sublimate (go from solid to gas phase - there's not enough pressure in space for the liquid phase to be stable). During the sublimation stage, any solid particles embedded in the ices ('dust') is also released. We see evidence for these two types of materials in many comets' tails. In Comet Hyakutake, pictured below, the dust forms the yellowish tail (the particles are large enough to reflect sunlight) and the gases scatter only the shorter wavelengths and thus appear blue.
The relative intensities of these two tails, which sometimes appear to lie directly on top each other, is a function of the composition of the comet and its 'freshness' (how many close approaches to the Sun has the comet already made?). Also noticeable is the blue-green fluorescence of cyanogen compounds in the 'coma', the hazy cloud at the head of the comet.
Astronomers have grouped cometary orbits into several broad categories. Short-period comets originate in the Kuiper belt, which lies beyond the orbit of Neptune, though there are several comets that ply paths closer to the Sun. Short-period comets commonly also orbit the Sun in the same direction as do the planets and have orbital periods of less than 200 years. In contrast, longer-period comets are thought to originate in the Oort cloud, a spherical cloud of icy bodies in the outer Solar System, and their elliptical paths through the inner solar system are randomly oriented. Long-period comets are thought to initiate their plunge towards the Sun from the Oort cloud because of gravitational perturbations caused by either the massive outer planets of the Solar System (Jupiter, Saturn, Uranus, and Neptune), or passing stars.
A special class of comets are the Sungrazers, whose minimum distance to the Sun is extremely small, sometimes less than the radius of the Sun itself, meaning that the comet actually plunges into the Sun. Others simply dissipate in the extreme environment of the Sun's inner corona: high intensity sunlight from the 6000 degree Sun and the million degree corona that lies above the solar surface. The most famous sungrazers are the Kreutz Sungrazers, which are believed to originate from one giant comet that broke up into many smaller comets during one of its intial visits through the inner solar system. An extremely bright comet seen by Aristotle and Ephorus in 371 BC is a possible candidate for this parent comet. The Great Comets of 1843 and 1882, and Comet Ikeya-Seki in 1965 were all fragments of the original comet. Each of these three was briefly bright enough to be visible in the daytime sky, next to the Sun, outshining even the full moon.
Comet Lovejoy was discovered on 27 November 2011, by amateur astronomer Terry Lovejoy. He had previously been one of hundreds of people who had done successful on-line comet searches using images gathered by the SOHO (Solar Heliospheric Observatory) The comet's closest approach, or perihelion, took it through the Sun's corona on 16 December 2011 and it passed approximately 140,000 kilometres above the Sun's surface. It was not expected to survive the encounter, but the Solar Dynamics Observatory (SDO), as well as other Sun-monitoring spacecraft, observed the comet emerge from the corona intact. In the picture above, the comet's head is so bright that the image has started to 'bleed' to the right and left, creating the horizontal lines. The comet was estimated to have reached brightnesses similar to those of Venus in our skies; unfortunately, Comet Lovejoy was very close to the Sun and thus essentially invisible.
See the amazing Comet Lovejoy video:
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