SH2-290 is a planetary nebula located in the constellation Cancer.  It was discovered in 1955 by George Abell and is also catalogued as Abell 31, PK 219.1+31.2, and ARO 135.

The designation planetary nebula is an unfortunate misnomer, as these objects have nothing to do with planets.  The name was chosen long ago because many of these resembled planets when seen through small telescopes, but in reality planetary nebulae are the result of the physics of the end stage of the life cycle of intermediate mass stars.  Planetary nebulae are comprised of stellar material ejected from the progenitor star, and this material becomes visible to us for a brief period (thousands of years) during which the projenitor star emits ultraviolet radiation that in turn ionizes the ejected gas.  The projenitor star becomes a white dwarf and eventually will expire, while the ejected material will slowly dissipate into the interstellar medium.  There are roughly 3000 known planetary nebulae in our galaxy, the most famous bearing familiar names such as the Ring Nebula, Dumbbell Nebula, and Helix Nebula.  It is expected that our Sun will eventually form a planetary nebula.  

SH2-290 is one of the largest planetary nebulae in the sky, with an angular size about half that of our Moon.  It lies at a distance of approximately 2000 light years from Earth.  Its exact age is unknown, but based on the properties of the central star as well as the nebula’s size, expansion rate, and chemical composition it is considered to be very old compared to most planetary nebulae.  The nebula is moving through space in the southerly direction, which is approximately the 5 PM position on my image.  The edge of the nebula on the south side is compressed by the surrounding interstellar medium, a physical interaction known as a bow shock.  Conversely, the opposite side of the nebula is more diffuse.

SH2-290 is very faint, but ionized oxygen and hydrogen emissions are visible, especially with the aid of narrowband filters.  For this image I used 3nm OIII and 3nm Ha filters, which narrows the view to the relevant wavelengths and improves contrast.  There are various options for assigning colors to produce the final image.  One common approach is the HOO palette, which assigns Ha to red, OIII to blue, and OIII to green.  For my image I decided to use the Foraxx palette, where Ha is assigned to red, OIII to blue, and where the green assignment is a dynamic mix of OIII and Ha that depends on their relative strengths.  I chose this palette because it brought forth a yellow-orange hue at the boundary between the inner portion of the nebula (dominated by OIII emissions) and the outer shell (dominated by Ha), which I felt gave a more nuanced appearance.  For reference, see this article:  https://thecoldestnights.com/2020/06/pixinsight-dynamic-narrowband-combinations-with-pixelmath/   I collected a minimal amount of RGB data to produce stars with normal colors.