Dolphin Head Nebula Sh 2-308 + Collinder 121
History
In 1931 Swedish astronomer Per Collinder published his «Catalogue of Open Galactic Clusters» based on a survey of Lundmark using the Franklin-Adams plates and Collinder using the Franklin-Adams charts. Cluster nr. 121 (Collinder 121, Cr 121) is listed there with dimensions of circa 60×40 arcminutes and containing 18-20 stars. [455]
In the 1950-ies the American astronomer Stewart Sharpless did a survey for H II regions on the photo plates of «Palomar Observatory Sky Survey» made with the 48-inch Schmidt telescope. In 1959 he published his revised catalogue of H II regions where the nebula is listed as number 308 (Sh 2-308, S 308). He described it as a medium brightness nebula of irregular form with filamentary structure and a diameter of 35 arcminutes. [310]
Due to its shape revealed on long exposure photography the nebula got the nickname «Dolphin Head Nebula» or just «Dolphin Nebula».
Physical Properties
Collinder 121 is a small open cluster containing ~20 main-sequence B-stars spanning an area of 50×35 arcminutes and the red supergiant star ο1 Canis Majoris which according to Gaia DR3 is at a distance of ~777 parsec. [145]
Sh 2-308 is a Wolf-Rayet bubble surrounding the 6.9 mag and WN4b spectral type star EZ Canis Majoris (HD 50896, WR 6). Wolf-Rayet stars are a class of massive stars descended from stars greater than 20 solar masses. They are characterized by the presence of strong broad emission lines in their spectra, which indicates a strong outflow of stellar wind, forming a bubble in the excited interstellar medium surrounding the star. Sh 2-308 is one of two known Wolf-Rayet bubbles emmitting X-ray. The other one is NGC 6888. [39] According to Gaia DR 3 the distance to the star is ~1537 parsec and therefore possibly unrelated to Collinder 121, which center lies ~50 arcminutes further south. [145]
Finder Chart
The Dolphin-Head Nebula Sh 2-308 is located in the constellation Canis Maior. The best time for observation is in the months from November until February.
