SH2-69 (Sharpless 69)

SH2-69 is an H II region in the constellation of Aquila that glows mainly at the wavelength of the Hydrogen-alpha spectral line at 656.3 nm. H II regions are ionized hydrogen atoms – in other words protons stripped of their electron – and when an electron is recaptured by these ions, a photon of light is emitted with an energy dependent on the atomic energy level the electron settles in. The H-α spectral line correspondes to the energy of the second electron shell (known as the 2S shell).

With hydrogen being the most abundant element in the universe it is not surprising that there are a great many H II regions, and Stewart Sharpless, while working at the US Naval Observatory at Flagstaff Arizona in the 1950s,  catalogued them by studying images from the Palomar Sky Survey. The second and final version of his catalogue was published in 1959.

SH2-69 is very faint. The brightest star in this image is mag 6.9 and the stars were imaged for a total of 16 hours through standard broadband filters. Yet the nebula is barely discernable after 4 hours’ exposure through the red filter. To bring out the nebula required 37.5 hours’ exposure through the H-α afilter to produce this image.

The field is 30 minutes of arc wide. This object was first imaged in October 2020; the image was reprocessed in May 2021 from the same base data.

Technical details:

Planewave CDK-14 corrected Dall-Kirkham reflector, FLI Proline P9000 cooled CCD camera and filter wheel with Astrodon LRGB and 3nm narrowband filters. Total exposure times were 4 hours through each of the LRGB filters and 44 hours H-α. Processed in PixInsight.