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tokens: gas: we see the bubble expanding in two directions. Since the [C II ] emission from NGC 1977 is fainter than that of M 42, we have opted to determine the expansion velocity from spectra towards this region ( Fig. 18 ), resulting in v exp ≃ 1.3 km s −1 . This is consistent with the pv diagram shown in Fig. 17 , from which we obtain the bubble radius r ≃ 500′′ ≃ 1.0 pc with the expansion velocity of v exp ≃ 1.5 km s −1 . As for the Veil shell and M 43, the systematic uncertainty of the mass estimate are of the order of 50%, systematic uncertainties of the estimates of the extent of the shell and of the expansion velocity are about 30%. Table 4 Gaussian fit parameters of spectrum in Fig. 18 . 4 Discussion OMC1, including the Trapezium cluster, is the nearest site of active intermediate- and high-mass star formation (e.g., Hillenbrand 1997 ; Megeath et al. 2012 , 2016 ; Da Rio et al. 2014 ; Großschedl et al. 2019 for discussions of the stellar content). Most of the stars formed and forming are low-mass stars, but also some high-mass stars, that have profound impact on the characteristics and evolution of their environment. Outside the Huygens Region, a significant massive (triple) star is the O9.5IV star θ 2 Ori A, located just to the southeast of the Orion Bar. The radiation of this star dominates the ionization structure of the gas towards the south-east of the HuygensRegion ( O’Dell et al. 2017 ). It also possesses strong winds. The most dominant star in the Orion Nebula is the O7V star θ 1 Ori C, the most massive Trapezium star. It is itself a binary, with possibly a third companion ( Lehmann et al. 2010 ). While its exact peculiar velocity is controversial, the θ 1 Ori C is plowing away from the molecular cloud, the site of its birth, and will have travelled some 25 pc before it explodes as a supernova ( O’Dell et al. 2009 ; Kraus et al. 2009 ; Pabst et al. 2019 ). While the small, 0.5 pc sized Huygens Region has been extensively studied, studies on the much fainter EON are less numerous. O’Dell Harris (2010) determine the temperature of the ionized gas within the large EON H II region to be T ~8.3 × 10 3 K, while electron densities decrease from n e ~ 3000 cm −3 at the Trapezium stars to about n e ~ 30 cm −3 20′ away, roughly as d −2 , with d the projected distance. Güdel et al. (2008) report that the EON exhibits significant X-ray emission, emanating from hot ( T ~ 2 × 10 6 K), dilute ( n e ~ 0.1−0.5 cm −3 ) gas. While, in principle, ionization followed by thermal expansion of the H II region can create a bubble of ~ 2 pc size, the hot gas is the tell-tale signature that the stellar wind of massive stars, rather than stellar radiation, is driving the expansion and forming the EON cavity. As Güdel et al. (2008) note, the emission characteristics in conjunction with its structure and young age render it unlikely that the bubble is a supernova remnant. While the observed morphology is in qualitative agreement with simple models for stellar