In spite of the name, cometary globules don't actually have anything to do with comets. They are a type of nebula, a sub-type of what is known as a Bok globule -- a very compact, very dense, very cold nebula -- the smallest type of dark nebula, only a light-year or two across. Inside that region is a mass that can roughly vary between 2 and 100 times the mass of our sun.
These globules often appear as dark patches in the sky, emanating no light (which makes them very difficult to detect and study). And, although they are some of the coldest objects we have seen in the universe, inside often burns a very warm core -- a forming star, or multiple stars.
CG4, also known as God's Hand, is a cometary globule -- a Bok globule on which one side has been blown outwards into a long tail, resembling a comet. It is just one of several such cometary globules located in the Gum Nebula, all with their tails trailing away from the Vela supernova remnant -- which may indicate that the exploding star is what caused the tails.
Another hypothesis is that the shape is caused by stellar winds and ionising radiation from very hot, very large OB stars nearby. These, astronomers believe, could initially form the structures known as elephant trunks -- structures like those seen in the Pillars of Creation in the Eagle Nebule -- and eventually cometary globules.
Normally, the globule is very faint. It's located 1,300 light-years from Earth, in the constellation of Puppis, and, true to type, is made of very dense, very dark material. Its head, which resembles a gaping mouth in a new photo by the European Southern Observatory's Very Large Telescope, is only 1.5 light-years across, with the tail extending eight light years behind.
In the image, the head of CG4 is not glowing under its own light; rather, it is illuminated by the light of nearby stars. Although the radiation from these stars is gradually eroding CG4, it is still working hard as a stellar nursery; in its dusty depths, new stars are forming.