Trumpler 16

Trumpler 16 is a massive open cluster that is home to some of the most luminous stars in the Milky Way Galaxy. It is situated within the Carina Nebula (Caldwell 92) complex in the Carina-Sagittarius Arm, located approximately 7,500 light-years from Earth. The cluster has one naked eye member star, Eta Carinae.

Its most luminous members are Eta Carinae and WR 25, with both having luminosities several million times that of the Sun, and there are six other extreme stars with O3 spectral classes. Both η Carinae and WR 25 are binaries, with the primary stars contributing most of the luminosity, but with companions which are themselves more massive and luminous than most stars. Across all wavelengths, WR 25 is estimated to be the more luminous of the two, 6,300,000 times the Sun's luminosity (absolute bolometric magnitude -12.25) compared to Eta Carinae at 5,000,000 times the Sun's luminosity (absolute bolometric magnitude -12.0). However in the image on the right Eta Carinae appears by far the brightest object, both because it is brighter in visual wavelengths and because it is embedded in nebulosity which is exaggerated in this type of image. WR 25 is very hot and emits most of its radiation as ultraviolet. It can be seen in the image below and to the right of Eta Carinae, just beyond the edge of the brightest nebulosity and to the right of an orange foreground star.