According to a 1982 study, this nebula is ionised by the Wolf-Rayet star WR 6
(EZ CMa) along with Sh 2-304 and Sh 2-308. However, a look at this region in hydrogen-alpha
suggests that Sh 2-303 is the brightest part of a ring that appears to surround several B class giants
, including LS 92
(B2/3 III) and HD 51200
(B2 III/IV). So it seems as though WR 6 may not be the only ionising star for this nebula.
Avedisova includes Sh 2-303 = Bran 3 in the star formation region SFR 233.40-9.42
along with the smaller nebula Bran 1
Along with Sh 2-304 and Sh 2-308, this HII region is ionised by WR 6 (EZ CMa). [1
The distance estimate given below for Sh 2-304 is actually the distance to WR 6. A 2007 paper gives a distance estimate for HD 51200 as 681 parsecs, suggesting that Sh 2-304 may actually lie a bit further away than the ring nebula Sh 2-308 surrounding WR 6. [2
1. ^ 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.
2. ^ Kaltcheva, Nadia & Makarov, Valeri (2007). "The Structure and the Distance
of Collinder 121 from Hipparcos and Photometry: Resolving the Discrepancy",
The Astrophysical Journal, Vol. 667, L155-L157. [2007ApJ...667L.155K]