July 2026 - Picture of the Month
Cometary Globule CG4 in Puppis
I was going to bring you an image by an amateur this month, but things (not least a heat wave) have conspired against me, hopefully next month.
All the same, I think that this stunning image of a dark cloud, check out CG 4 on SIMBAD and the Digitized Sky Survey (DSS), which looks anything but dark is a worthy alternative. It was captured by the Department of Energy-fabricated Dark Energy Camera on the U.S. National Science Foundation Víctor M. Blanco 4-meter Telescope at Cerro Tololo Inter-American Observatory.
Apparently CG 4 (G259.43-12.72) in Puppis to left of centre in this image is nicknamed ‘God’s Hand’, however its structure reminds me more of the sandworms in Dune, and I believe that it's the edge-on barred spiral galaxy PGC 21338 that's about to be swallowed. It does make for a fabulous image, and even this larger jpeg version still pales in comparison with the full original which is extremely detailed.
There are plenty of these dark nebulae around the galaxy which are known as Bok globules. The cometary variety, of which CG 4 is an example, are characterised by their dense 'head' which fades into a diaphanous tail. In fact there's another example in the upper right of this image, CG 6 which is is more classically cometary with its pointed tip. CG 4 is about 1 pc wide and 3 pc long containing about 60 solar masses of hydrogen, and that mouth is 0.5 pc in diameter. CG 6 is smaller, only about 0.25 pc wide and 1 pc long, and only contains about 5.5 solar masses. 1
There's some uncertainty about the origin of these structures. For the 32 cometary nebulae found around the edge of the Gum Nebula it's been noted that they 'point' roughly towards a common centre, perhaps in the direction of the Vela Supernova Renmant, suggesting that this event about 11,000 years ago, and the resulting Vela pulsar, may have some part in their development. However the leading theory is that the stellar wind of hot nearby O-type stars, principally γ2 Velorum in the case of the two objects in this image, caused the stripping of gas from the dense nebula to form the tail. 2
CG 4 is relatively close at about 400 pc (1,300 light-years) and is certainly not an object for visual observation. The Very Large Telescope (VLT) captured an excellent image of the gaping maw full of stars, but sadly no sign of PGC 21338 which lay outside the field. In reality PGC 21338 is in no danger from this particular sandworm as it's somewhere around 45 Mpc (147 million light-years) further away.
James Whinfrey - Website Administrator.
References
- Gonzalez-Alfonso, E., Cernicharo, J., and Radford, S. J. E., Molecular gas in cometary globules: CG4 and CG 6 in the GUM Nebula., Astronomy and Astrophysics, vol. 293, EDP, pp. 493–506, 1995.
- Sridharan, T. K., Kinematics of the Cometary Globules in the GUM Nebula, Journal of Astrophysics and Astronomy, vol. 13, no. 3, Springer, pp. 217–239, 1992. doi:10.1007/BF02702292.
