Nick James, Director of the BAA Comet Section: Talk to Newbury Astronomical Society, 2nd November 2018

Comets have a small solid nucleus usually just a few kilometres across, Sublimation from the nucleus creates a thin cloud of gas hundreds to thousands of kilometres across. This gas is ionised by solar radiation so it is affected by the solar wind and pushed away to form a tail which fluoresces with a weak blue-green colour. The gas leaving the surface takes with it some dust, finer than talcum powder. This follows the comet but light pressure from the sun gently pushes it away to form a dust tail reflecting the sunlight.

Nick’s best comet was Hyakutake in March 1996 which he photographed and observed from Tenerife. Hyakutake had a small nucleus and not much dust but its close approach meant the ion tail appeared very long at 30 to 40 degrees, but only for a few nights. Next of course was comet Hale-Bopp in the spring of 1997. This came from an unusually large nucleus 60-80km across so it showed prominent dust and gas tails and was visible for several months.

The media like to big up comets, so we’ve had multiple Comets of the Century. There have been several bright comets over the last 50 years but usually are only bright for a few days. Examples were Comet West in spring 1976, Kouhotek in early 1974 didn’t live up to early expectations. Comet Halley in 1986 was a disappointment for many, being much further away than in 1910, but still a better show than we’ll get in 2062.

Comet McNaught in January 2007 was the most spectacular in the last 50 years, from the southern hemisphere. It developed a broad dust tail with striations which resembled De Chesaux’s comet drawings from 1744. Comet Lovejoy in 2011 was a sungrazer which broke apart and was prominent in the southern skies for a few days. Comet ISON in November 2013 was expected to do the same but broke up just before perihelion and completely disappeared.

Comet discoveries used to be mostly visual, often by amateurs such as Bill Bradfield, but that is now a thing of the past. With so many surveys reaching very faint objects the comets are discovered long before they brighten to visual range, although a few are still being discovered electronically by amateurs. Amateurs can still play a role by estimating the magnitude of the brighter comets and imaging the changing tail structure, which helps in studies of the solar wind.

Comets are named after the discoverers but there are now about 150 comets with the name PANSTARRS, found by a telescope looking for hazardous asteroids. Another prolific source has been the SOHO observatory which can pick up small comets as they get too close to the Sun.

The next comet which may become bright enough to see in binoculars is 46P Wirtanen. Closest approach is in mid-December 2018 when it will move from Eridanus into Taurus. It will be best seen in binoculars or imaged with a DSLR and telephoto lens. It is a gassy comet so should show up as a green blob with a DSLR but probably with little tail.

Notes and summary by Richard Fleet.

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