About 1,300 light-years away, in the constellation Puppis, a ghostly hand emerges from the interstellar medium and reaches into the cosmos. This is the cometary cluster known by the acronym CG 4, a cloudy and ominous structure that is also known by the nickname “the hand of God”, which is one of the many cometary clusters present in the Milky Way. The distinctive shape of these objects and their formation remains a topic of debate among astronomers.
Cometary globules are a subclass of dark nebulae known as Bok globules, isolated clouds of dense cosmic gas and dust, surrounded by very hot, ionized material. When a detachment of material occurs in these clouds, resulting in a tail that extends through space, they are known as cometary globules due to their vague resemblance to a comet, although they have nothing in common. The characteristics that classify CG 4 as a cometary globule do not go unnoticed in this image captured with the Dark Energy Camera (DECam), built by DOE and installed on the Víctor M. Blanco 4-meter Telescope of the Cerro Tololo Observatory (CTIO) , an NSF NOIRLab Program. Its dusty head, with a diameter of 1.5 light years, and its long, wispy tail, about eight light years long, make CG 4 a small Bok globule, a general characteristic of cometary globules.
Although these objects were first observed in 1976 from images obtained with the UK’s Schmidt Telescope in Australia, for a long time, cometary globules were not detected by astronomers because they are very faint objects. Their tails, shrouded in dark stardust, block most of the light from passing through. However, currently DECam, with its Hydrogen-alpha filter [1], can capture the faint red glow of ionized hydrogen present in the head of CG 4 and around its outer edge. This light is produced when hydrogen is stimulated after being bombarded by radiation from hot, nearby massive stars.
However, the intense radiation generated by these nearby massive stars is gradually destroying the head of the globule and sweeping away the tiny particles that scatter starlight. Still, CG 4’s dusty cloud contains enough gas to fuel the active formation of several new Sun-sized stars.
While astronomers have observed these structures throughout the Milky Way, most of them, including CG 4, lie inside a huge blob of incandescent gas called the Gum Nebula, which contains at least 31 cometary globules in addition to CG. 4. This nebula is believed to be nothing more than the expanding remains of a supernova that occurred about a million years ago.
The mechanism by which these comet-like objects acquire their characteristic shape is not well understood, but astronomers have at least two main notions about their origins. The first is that, initially, they could have corresponded to spherical nebulae, such as the Ring Nebula, which were subsequently affected by a nearby supernova explosion, possibly the original explosion that created the Gum Nebula.
The second idea suggests that cometary globules are formed by a combination of stellar winds and radiation pressure from hot, nearby massive stars. In fact, all of the cometary globules found within the Gum Nebula appear to have tails that extend away from the center of the nebula, which is where the Vela Supernova Remnant and the Vela Pulsar are located. The latter consists of a fast-spinning neutron star that formed after the collapse of a massive star, and its stellar winds and radiation pressure may be shaping nearby globules.
This image also gives the impression that CG 4 is about to devour the edge-on spiral galaxy ESO 257-19 (PGC 21338), which is visible just in front of it. But in reality, the galaxy lies more than a hundred million light years beyond CG 4 and only appears to be close by a chance alignment.
