Page:Astronomy for Everybody.djvu/279

Comets differ from the heavenly bodies which we have hitherto studied in their peculiar aspects, their eccentric orbits, and the rarity of their appearance. Some mystery still surrounds the question of their constitution, but this does not detract from the interest of the phenomena which they present. When one of these objects is carefully examined we find it to embody three features which, however, are not separate and distinct, but merge into each other.

First we have what, to the naked eye, appears to be a star of greater or less brilliancy. This is called the nucleus of the comet.

Surrounding the nucleus is a cloudy nebulous mass, like a little bunch of fog, shading off very gradually toward the edge, so that we cannot exactly define its boundary. This is called the coma (Latin for hair). Nucleus and coma together are called the head of the comet, which looks like a star shining through a patch of mist or fog.

Stretching away from the comet is the tail, which may be of almost any length. In small comets the tail may be ever so short, while in the greatest it stretches over a long arc of the heavens. It is narrow and bright near the head of the comet and grows wider and more diffuse as it