CG4 is a cometary globule. Despite the name, they are not related to comets at all.
The name comes from a discovery made in 1976 by the UK Schmidt Telescope. The objects appeared to be elongated comet-like objects located in the Gum Nebula. They have dark, dusty heads with long, wispy tails pointing away from the Vela supernova in the center of the Gum Nebula.
While CG4 makes for a good image above, it’s actually quite faint. Amateur astronomers struggle to spot the nebula in even the darkest skies. The only reason it appears so bright is because of the stars nearby lighting it up.
The most prominent feature of CG4 is its head, which looks like a giant beasts’ mouth opening. This main feature has a diameter of 1.5 light-years. The wispy tail of the cometary globule isn’t seen in the picture above, but it stretches about 8 light-years long below it. That’s pretty small for a gas cloud and is one of the defining features of cometary globules. Plus, all of the cometary globules found so far are isolated small clouds of neutral gas and dust.
The debate rages on as to why CG4 and fellow cometary globules have their distinct form. Astronomers have come up with two theories. One states that cometary globules may have originally been spherical nebulae. The effects of a nearby supernova explosion changed the spherical nebula into the cometary globule we see today.
The second theory points to stellar winds and ionizing radiation from OB stars. These stars are relatively short-lived, but pump out lots of UV radiation.
If astronomers want to learn more about cometary globules, they are going to need a lot more information. Measurements of mass, density, velocity and temperatures would go a long way into shedding light on the mysteries of cometary globules. Another large telescope can take the measurements needed – the Atacama Large Millimeter/submillimeter Array.