The Gum Nebula (Gum 12) is an emission nebula that extends across 36° in the southern constellations Vela and Puppis. It lies approximately 450 parsecs from the Earth. Hard to distinguish, it was widely believed to be the greatly expanded (and still expanding) remains of a supernova that took place about a million years ago. More recent research suggests it may be an evolved H II region. It contains the 11,000-year-old Vela Supernova Remnant, along with the Vela Pulsar. It is located between the two bright and massive stars system ζ Puppis (Naos) and γ2 Velorum (Regor), the former is closer to the edge of the cloud facing the Sun, at a distance of about 330±10 parsecs, while the latter, at about 336+8−7 parsecs, is located closer to the center of the cloud. To its southeastern part lies the Vela Nebula, a supernova remnant that lies between the Gum Nebula and the Solar System, along the same line of sight. The Gum Nebula is one of the largest known nebulae within the Milky Way and constitutes an important scenario for the study of the expansion of bubbles generated by supernova explosions within the environment of the galactic arms, as well as on the interaction between these and the surrounding molecular clouds; in fact, on the edges of the cloud some limited star formation phenomena are active, localized in small clouds (known as cometary globules due to their appearance). Near some of these globules some HH objects are also found, including HH 47.
Observation
Its innermost part was first identified in 1939 by Otto Struve, who did not perceive the true nature of the object, which is why the Gum Nebula is named after its discoverer, the Australian astronomer Colin Stanley Gum (1924–1960). In the early 1950s, the then young Gum reimaged this region of the southern Milky Way in the infrared, thus discovering its true extent: it in fact covers the entire western part of the constellation of Vela, brushes the constellation of Carina to the south, and continues into Puppis, covering its entire southeastern part and ending near the star π Puppis. He published the discovery of the Gum Nebula in 1952 in the journal The Observatory. The observations were made with the Commonwealth Observatory. The report of his discovery was published in 1955, in a work entitled A study of diffuse southern H-alpha nebulae ("A study of southern diffuse H-alpha nebulae") (see Gum catalog). The Gum Nebula appears as a vast and faint nebula covering the region of sky at the boundary between the constellations of Puppis and Vela; its size is such that it appears to be the largest nebula in the sky. It is not visible optically, nor with an amateur telescope, because its filaments are very faint; much of its emission is in the infrared and Hα bands. Its position is strongly southern, therefore visibility from the Northern Hemisphere is greatly reduced: in fact the cloud is fully visible only from lower northern latitudes, south of the 35th–36th parallel north, while beyond 50° N it becomes completely invisible. From the Southern Hemisphere, on the contrary, it is well observable during almost all nights of the year (particularly in the months between December and April). The Gum nebula was photographed during Apollo 16 while the command module was in the double umbra of the Sun and Earth, using high-speed Kodak film.
Description The Gum Nebula contains about 32 cometary globules. These dense cloud cores are subject to such strong radiation from O-type stars γ2 Vel and ζ Pup and formerly the progenitor of the Vela Supernova Remnant that the cloud cores evaporate away from the hot stars into comet-like shapes. Like ordinary Bok globules, cometary globules are believed to be associated with star formation. A notable object inside one of these cometary globules is the Herbig-Haro object HH 46/47. The main ionizing sources are the Vela OB2 association (including Gamma Velorum cluster and the P Puppis cluster), Zeta Puppis and the OB association Trumpler 10. The star cluster NGC 2547 formed 100 pc more distant than Vela OB2 and is an interloper. The Vela OB2 association is surrounded by the so-called "IRAS Vela Shell", located in the south of the Gum Nebula. The star clusters NGC 2547, Trumpler 10, NGC 2451B, Collinder 135 and Collinder 140 all have similar ages of about 30 Myrs. It is suggested that a few stars exploded in supernovae about 30 Myrs ago in the center of the Gum Nebula and triggered the formation of stars in the region.
Associated star formation phenomena The shock wave caused by the expansion of the Gum Nebula and the intense ultraviolet radiation from the region's massive stars have eroded and compressed gas in surrounding clouds, in many cases promoting star formation; these phenomena particularly involve the birth of low- and intermediate-mass stars.
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