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Sh2-261

Ori

• H alpha
• Oxy 3

• 16 x 900 sec - 4 hours
• 14 x 900 sec - 3 hours 30 min

• UNBINNED
• UNBINNED

Sh2-261, sometimes also known as the Lower Nebula, is a visible emission nebula in the Orion constellation. It is found in the northernmost part of the constellation, about 6 ° west of Alhena (γ Geminorum); it is easily visible in long exposure photos taken even with medium power amateur instruments. It lies in the clear wake of the Milky Way in opposition to the galactic center and is surrounded by rich star fields at the bottom. The best time for its observation in the evening sky falls in the months between January and May, and although it is located in the northern celestial hemisphere, it can be observed from all populated areas of the Earth. It is an H II region located on the outermost edge of the Arm of Orion, on the edge of the galactic region between it and the Arm of Perseus; it is located on the line of sight of the southern part of the Gemini OB1 association. One of the stars responsible for the ionization of the gases in the region would be the blue star HD 41997, a fugitive star of spectral class O7.5V; to this are added other stars, among which the blue star BD + 15 ° 1079, of class B0. This star was sometimes referred to as class O7, thus shifting the nebula distance up to 2000 parsecs due to the increase in the discrepancy between absolute magnitude, evidently brighter in a class O star, and apparent magnitude; through the analysis of the other ionizing stars, a distance of about 1000 parsecs (3260 light years) has been calculated, which seems to be more reliable, given the redetermination of the star BD + 15 ° 1079 as belonging to class B0.