The Cometary Globule CG4 appear to be gobbling up the galaxy on the left of the image but these objects are actually unrelated and projected adjacent to each other in the image. The Galaxy PGC 21338 (ESO 257-19) is actually over 100 million light years distant, while CG4 is only 1300 light years away. CG4 is an example of a Cometary Globule. The cometary head is about 1.5 light years in diamter while the tail stretches over 8 light years long. CG4 has enough material to make several sunlike stars.

Bright rimmed globules and their more evolved cousin the cometary globule represent fascinating dynamic structures formed by the interplay of cold molecular clouds and hot ionizing stars. Typically the head of the globule faces a hot O-type star. Intense radiation from the star boils away lower density gas from the head. The evaporated rim of gas becomes ionized by the stars ultraviolet flux forming a bright glowing rim we associate with many of these globules including CG4. Intense stellar winds from the ionizing star evaporate gas and dust away from the head forming the "tail" and completing the cometary shape. The globules are known to be the birthplace of low mass stars. Stars form within the globules by the mechanism known as "radiation driven implosion". This process occurs when ultraviolet flux from a hot star compresses surviving clumps of cold molecular gas eventually causing collapse and core formation within the dense compact clouds. Lower and intermediate mass stars ultimately form from the compact gas and dust within the globules.

The evolution of a cometary globule occurs in two phases. The first phase involves collapse, compression, and subsequent equilibrium of the cloud driven by photoionization of its surface by the ultraviolet flux of a hot B-type star. The collapse occurs relatively quickly within about 30,000 years followed by a longer transient phase lasting some 100,000 years where the cloud undergoes several radial reexpansions and recompressions. The second phase involves photoevaporation and acceleration of globular material away from the ionizing source forming the "tail" of the globule. The evaporation phase can last a million years or so and ends with the dissolution of the globule.