Fly Nebula
| Polish version is here |
NGC 1931 is a small reflection and emission nebula associated with a young open star cluster in the constellation Auriga. It bears a striking resemblance to the famous Orion Nebula (Messier 42), often described as a miniature version of it. With an apparent magnitude of around 10m, NGC 1931 is far too faint to be seen with the naked eye. In the sky, it spans only a few arcminutes, appearing as a small, misty patch best observed through a telescope.
The nebula was discovered on February 4, 1793, by William Herschel, who described it as a faint, diffuse glow containing several embedded stars. In 1931, Swedish astronomer Per Collinder included the cluster in his catalog of open star clusters as Collinder 68. Later, in 1959, American astronomer Stewart Sharpless listed its bright emission region as the H II region Sharpless 2-237 (Sh2-237). NGC 1931 is also known as the Fly Nebula, particularly when mentioned alongside its nearby companion, IC 417. Together, the two form a striking pair often referred to as “The Spider and the Fly.”
Observations
October 16, 2025, about 11:30 p.m. - Katowice, Poland
urban conditions, very high level of light pollution
NGC 1931 lies in a relatively distant part of our Galaxy, within the outer Perseus Arm, about 7,500 light-years from the Sun. Together with the nearby IC 417 nebula, it forms part of a larger complex of H II regions. On the sky, it covers only a few arcminutes, which corresponds to an actual diameter of roughly a dozen light-years.
NGC 1931 is composed of two main parts: an emission region and a reflection region. The emission nebula is a cloud of ionized gas, mostly hydrogen, that glows with its own light as it is energized by radiation from nearby hot, young stars. The optical spectrum of this region shows strong emission lines of hydrogen and ionized oxygen, giving the nebula its characteristic reddish and greenish glow. The reflection nebula, in contrast, shines with starlight scattered by tiny grains of interstellar dust, usually appearing bluish in color. The presence of dense dust also absorbs some of the light, making the deeper, central parts of the nebula appear heavily darkened in visible light.
At the core of NGC 1931 lies a young open star cluster, estimated to be around two million years old. Its brightest members are hot, blue stars of spectral type B that emit intense ultraviolet radiation, ionizing the surrounding gas and sustaining the nebula’s glow. Near the cluster’s core, astronomers have identified a group of massive, newly formed stars. Their powerful stellar winds and energetic radiation sculpt the nearby gas, shaping the nebula’s structure and influencing further star formation. Star formation within NGC 1931 is believed to be ongoing, as very young stellar objects (protostars) have been detected inside the cloud, still in the earliest stages of their evolution.
Photo 1 parameters:
- Total exposure time: 50 minutes (stack of 200 RAW frames at 15s each)
- DWARF3
- Lens: f=150mm (aperture: 35mm)
- Mount: photographic tripod
Further readings:
- Ambartsumian V. A., Multiple Systems of Trapezium type, Communications of the Byurakan Astrophysical Observatory, 1954, 15, pp. 3-40
- Dewangan L. K., Ojha D. K., Zinchenko I., Janardhan P., Luna A., Multiwavelength Study of the Star Formation in the S237 H II Region, Astrophysical Journal, 2017, 834(1), p. 22
- Herschel W., Catalogue of 500 New Nebulae, Nebulous Stars, Planetary Nebulae, and Clusters of Stars; With Remarks on the Construction of the Heavens, Philosophical Transactions of the Royal Society of London, 1802, 92, pp. 477-528
- Lim B., Sung H., Bessell M. S., Kim J. S., Hur H., Park B.-G., The Sejong Open Cluster Survey (SOS). IV. the Young Open Clusters NGC 1624 and NGC 1931, Astronomical Journal, 2015, 149(4), p. 127
Marek Ples