January 2026 - Picture of the Month

Sharpless 261 (Sh2-261) in Orion

This HII region in the northern part of Orion was discovered in 1939 by Harold and Charles Lower using an 8-inch f/1 Schmidt camera to make a 20 minute exposure on a photographic film. Brendan's SHO narrowband image above contains a total of 24 hours of exposure and a lot more detail.

As a consequence of the discovery this region is sometimes known as Lower's nebula. It was later catalogued as Cederblad 64 (Ced 64), then Siméis 34 (Sim 34) before reaching Stewart Sharpless' list as number 261 (Sh2-261). The same region later produced three bright nebulae designations in the Lynds' Catalogue of Bright Nebulae: LBN 862, LBN 863 and LBN 864.

Based on photographic plates, Stewart Sharpless estimated Sh2-261 to be 45 arc-minutes in diameter with an 'irregular' form in his 1959 paper ¹. In this paper Sharpless considered this HII region to be associated with star HD 41997 which he estimated to be B8.5 class.

Radio measurements of CO emission appear to show that Sh2-261 lies beyond the western edge of the Gemini OB1 Molecular Cloud Complex ² at a distance of about 1.89 kpc (6200 light-years) ³, although other estimates of its distance vary widely. As a result it's uncertain whether it lies in the Perseus arm of the galaxy or our own Orion spur.

HD 41997 (Gaia DR3 3345950879898371712) is now classified as an O7.5III star with a visual magnitude of 8.45 at a distance of around 1.75 kpc (5705 light-years) and it's a runaway ⁴ with significant peculiar velocity with respect to [its] environment having been ejected for wherever it formed.

Brendan's wonderful image captures the runaway right where you might expect it, below the high energy blue structure in the central hollow, just to the right of centre, from where it illuminates this whole region of dust and gas. The brightest region surrounding the star corresponds to LBN 864.

Unsurprisingly perhaps, according to The Night Sky Observer's Guide Volume 4, Sh2-261 is barely visible as a half-degree patch of very faint haze with a 16–18 inch telescope and, I assume, a good night sky. So, probably one for the imagers or an ambitious visual observer.

James Whinfrey - Website Administrator.

References

1. Sharpless, S., “A Catalogue of H II Regions.”, The Astrophysical Journal Supplement Series, vol. 4, IOP, p. 257, 1959. doi:10.1086/190049.
2. Wang, C., Yang, J., Xu, Y., Li, F., Su, Y., and Zhang, S., “Molecular Gas toward the Gemini OB1 Molecular Cloud Complex. I. Observation Data”, The Astrophysical Journal Supplement Series, vol. 230, no. 1, Art. no. 5, IOP, 2017. doi:10.3847/1538-4365/aa6c6b.
3. Foster, T. and Brunt, C. M., “A CGPS Look at the Spiral Structure of the Outer Milky Way. I. Distances and Velocities to Star-forming Regions”, The Astronomical Journal, vol. 150, no. 5, Art. no. 147, IOP, 2015. doi:10.1088/0004-6256/150/5/147.
4. Carretero-Castrillo, M., Ribó, M., and Paredes, J. M., “Galactic runaway O and Be stars found using Gaia DR3”, Astronomy and Astrophysics, vol. 679, Art. no. A109, EDP, 2023. doi:10.1051/0004-6361/202346613.