Mid-IR Spitzer photometry of the H ii region N33 fitted using YSO models (Robitaille et al. 2007). The majority of the ten best-fit models suggest the presence of only envelope around the central stellar source of N33. The curve in black represents the best-fit. The derived total luminosity of the nebula according to these models varies between 5  ×  10³ and 10⁴ L_⊙. One should however be cautious in interpreting these results since N33 is not a YSO; see Sect. 3.8 for details.