What makes comets bright?

First, understand a comet's composition. Ranging from a just a few hundred metres across to kilometres, a comet's nucleus is a loosely held collection of ices, frozen gases, dust, and some rocky particles. read more

Comets are brighter nearer the Sun for two reasons: there is more light to reflect, and as the comet heats up it evaporates dust and gasses into space snd these reflect additional light. The comets are not intrinsic sources of light - just reflectors. read more

We don’t have an exact handle on exactly what makes a comet bright, and there is substantial discussion on the comet lists about the best way to estimate the development of cometary brightness. Light curve of comet Elenin, from Aerith, click to embiggen. read more

Comets don't make any light of their own. We see them due to reflected sunlight. Comets appear bright because they contain large amounts of water ice, which is a very refective material. read more