December 2025 - Picture of the Month

Digel Cloud 2S in Cassiopeia

Digel Cloud 2 is a molecular cloud about 40,000 light-years from us, and more than 60,000 light-years from the galactic centre where molecular clouds hosting star formation weren't supposed to be found.

In a 1994 paper, Digel et al.¹ presented 11 molecular clouds in the extreme outer galaxy, about 58–91,000 light-years (18–28 kpc) from the centre of our galaxy, that were identified with the 1.2 Meter Millimeter-Wave Telescope (MWT) at the Center for Astrophysics. They were estimated to be 65–130 light-years (20–40 pc) in size and to have lower metalicity than comparable molecular clouds in the inner galaxy. In the extreme outer galaxy gas density is very low and perturbations that initiate star formation are rare, yet they were suspected of hosting forming star clusters.

Skipping forward a few years, a study in 2008 ² captured Near Infra-Red (NIR) data of two areas within the second of Digel's clouds where the radio data of the previous study gave the highest carbon monoxide concentrations. They found that there were indeed forming star clusters, that they formed stars at similar rates to molecular clouds nearer to the Sun and the trigger mechanism was probably a supernova. The images that they obtained with the Subaru 8.2 m telescope and the IRCS infrared imager show that the southern location in the cloud, Digel Cloud 2S, held a more compact cluster.

And it's that cluster you can see in this NIR and Mid-Infra-Red (MIRI) composite image by NASA/ESA/CSA James Webb Space Telescope (JWST), complete with the nebulosity of the molecular cloud in red, vigorous formation of bright and colourful stars and five jets of outflow material from protostars. Features for future investigation in this cluster include the presence of circumstellar disks and the structure of those jets.

I think that it's a very seasonal image, enhanced as it is by those JWST diffraction spikes and many background galaxies. The field of this image is only 1.77 x 1.30 arc-minutes and, whilst there's a lot to see in this region of the night sky, you're not likely to find much just here with an amateur telescope, and certainly not visually.

James Whinfrey - Website Administrator.

References

1. Digel, S., de Geus, E., and Thaddeus, P., “Molecular Clouds in the Extreme Outer Galaxy”, The Astrophysical Journal, vol. 422, IOP, p. 92, 1994. doi:10.1086/173706.
2. Yasui, C., Kobayashi, N., Tokunaga, A. T., Terada, H., and Saito, M., “Star Formation in the Extreme Outer Galaxy: Digel Cloud 2 Clusters”, The Astrophysical Journal, vol. 675, no. 1, IOP, pp. 443–453, 2008. doi:10.1086/524356.
3. Izumi, N., “Overview Results of JWST Observations of Star-forming Clusters in the Extreme Outer Galaxy”, The Astronomical Journal, vol. 168, no. 2, Art. no. 68, IOP, 2024. doi:10.3847/1538-3881/ad4e2e.