A large, almost circular emission nebula in the constellations Vela and Puppis, discovered by the Australian astronomer Colin Stanley Gum (1924–60). The Gum Nebula is 36° across and is estimated to be centred 1500 l.y. away, which gives it a true diameter of about 800 l.y. It is expanding at about 10 km/s, although the near side is expanding faster than the far side. The nebula is thought to be an HII region ionized by massive star formation. It contains the Vela OB2 and Trumpler 10 (Tr 10) associations, the hot stars of which contribute to the photoionization along with Zeta Puppis and Gamma-2 Velorum. The Vela Supernova Remnant and the Vela Pulsar are embedded within the Gum Nebula but are much younger.