The Cometary Globules CG 30/31/38 appear as finger-like extensions extending out in different directions. Although cometary globules are located in both the northern and southern skies there is a greater number in the south particularly within the Gum Nebula region spanning the constellations of Vela and Puppis. The reasons are uncertain but are likely related to a greater frequency of supernova blasts in that region.
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 globule head of CG 30 contains the Herbig Haro object HH 120, which is the outflow signature of a pre-main sequence star.
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.