RCW 42-GUM 26

Gum 26, also known as RCW 42, is a small emission nebula visible in the constellation of Vela.

It is observed in the central-southern part of the constellation, about 2° north of the star κ Velorum, which is part of the asterism of the False Cross; it can be photographed without excessive difficulty with the help of filters through a medium-power telescope. Its strongly southern declination means that from the boreal regions its observation is particularly difficult already north of the tropical belt (north of 38°N it is always invisible). From the southern hemisphere, on the other hand, it can be observed on average for almost all nights of the year. The best time for its observation in the evening sky is from December to May. The nebula lies beyond the Perseus Arm or perhaps on the extreme edge of the Sagittarius-Carina Arm, in an outermost region of the Milky Way; it is a large H II region, one of the most massive known, whose distance is estimated at 6400 parsecs. The cloud hosts a cluster of infrared sources deeply immersed in gases, indicated by number 38 of a catalog of infrared clusters compiled in 2003; however, this study indicates a greater distance for the nebula, up to 7100 parsecs.

According to some studies, this nebula is associated with a giant galactic chimney, indicated by the abbreviation GSH 277+00+36; This superbubble, with a diameter of about 600 parsecs, crosses the entire galactic plane, having a length of 1000 parsecs. The structure appears extremely massive, with a mass of no less than 870,000 M⊙, while its expansion rate would be about 45 km s−1. Through these estimates, an age of the superbubble of about 20-25 million years has been indicated, which for a structure of this size is still very low. A large amount of neutral hydrogen is associated with it, while the column structure that makes up the body of the chimney appears hollow inside. Its origin is not well understood, since at the distance at which it lies it has not been possible to detect stellar objects or galaxy clusters in dissipation; Among the various possibilities was included that of a possible impact of a high-speed cloud with the galactic plane.