Kilometer-sized "dirty snowball" composed of dust (refractory material) and primarily water-ice which gives rise to all of the features observers associate with comets. As the nucleus approaches the Sun, the temperatures rise sufficiently to cause the sublimation of the ices. As the gas leaves the nucleus, it is able to drag some of the dust with it in the low gravity. Sunlight reflected off the dust is what we see as the yellowish coma and tail of the comet, and interaction of the solar radiation with the gases gives us the characteristic blue appearance of the plasma tail.