Names of stars usually start with letters NC and are followed by numbers. What does NC stand for and why are they named as such?
I believe the questioner is referring to the 'NGC' designations found in the 'New General Catalogue' compiled by Henry Dreyer and Annie Jump Cannon in the late 19th and early 20th century (subsequently revised, enlarged and republished a number of times). This catalogue did not list stars, as the questioner suggests, but non-stellar objects found in astro-photographic plates (and for many of which the nature was not known until many years later). It was an update to Sir John Herschel's 'General Catalogue', hence the name 'NGC'. See http://www.ngcic.com/history.htm
Each catalogue entry is of the form NGC, and the types of objects catalogued include galaxies, nebulae, open and closed clusters and globular clusters. On-line versions of the catalogue are available at a number of sites, including http://www.seds.org/~spider/ngc/ngc.html
The NGC was the most extensive catalogue of its time, covering the entire sky down to -17.5 declination, a limiting magnitude of about 16, professional-class photographic plates and accurate photomicrometry. It became a standard reference and, despite being complemented and supplanted by later specialized catalogues, the NGC remains perhaps the most widely used astronomical catalogue after Messier's much-more-restricted version.
Mick Wilson, B.S., Environmental information systems manager, Nairobi
The very brightest stars in the sky were named many, many years ago and these names have stuck. Astronomers today still refer to 'Betelguese' and 'Polaris' and 'Dubhe'. This technique couldn't continue forever, however, for there are many millions of stars that the unaided eye cannot see and an astronomer would need to spend a lifetime just to learn their names.
As telescopes became more powerful and more objects were identified, various classifications were developed to keep track of all these new sightings. Compilations of faint stars that are used by astronomers today include the 'Bonner Durchmusterung', the 'Henry Draper Catalogue' and the 'Smithsonian Astrophysical Observatory Catalogue'. Stars in these catalogues are prefixed with the letters BD, HD and SAO respectively. So the usual reason that a star's 'name' starts with a particular set of letters is because it is actually being referred to by its listing in a particular catalogue.
These catalogue systems are the most common system to reference faint stars. However, other astronomical objects are often refereed to by a sequence of letters and numbers where the letters stand for what the object is. For example, 'SN 1983N' refers to a supernova and 'WD 2034-53' refers to a white dwarf. So, finally, we come to your question. The letters NC actually indicate a particular type of nova. Novae are binary systems - two stars circling around their common centre of mass. One of the stars is a hot, dwarf star which varies in brightness, suddenly becoming brighter and then, more slowly, returning to its original state. The companion star is usually a much larger, cooler star. The letter C indicates a slow nova, one that takes over 10 years for the dwarf star return to its original luminosity.
Sally Riordan, M.A., Management Consultant, London
'Where the telescope ends, the microscope begins. Which of the two has the grander view?'