This ring nebula surrounds the Wolf-Rayet star WR 6
(EZ CMa), which is one of the closest Wolf-Rayet stars to our solar system and may also partially ionise Sh 2-303 and Sh 2-304.
This distance to WR 6 is a matter of some controversy. The Hipparcos astrometric satellite results were used in 1997 to determine a parallax suggesting a distance of about 575 parsecs. However, other researchers strongly disagreed with this estimate and proposed distances of up to three times this amount.
A revised Hipparcos analysis published in 2007 using the same data determined a very different parallax and a distance of about 1400 pc, much closer to other estimates.
A striking narrow-band image here
shows the strong ionised oxygen emission from Sh 2-308, which the photographer calls the Gourd nebula.
This ring nebula, also called RCW 11, is associated with WR 6. [1
WR 6 may also ionise Sh 2-303 and Sh 2-304. [2
1. ^ Arnal, E. Marcelo & Cappa, Cristina E. (1996). "The HI distribution in the
environment of the WR star HD 50896", Monthly Notices of the Royal
Astronomical Society, Vol. 279, 788-798. [1996MNRAS.279..788A]
2. ^ Chu, Y.-H., Troland, T. H., Gull, T. R., et al. (1982). "Galactic ring
nebulae associated with Wolf-Rayet stars. IV - The ring nebula S308 and its
interstellar environment", The Astrophysical Journal, Vol. 254, 562-568.