Left: time evolution of the opacity exponent β, where κ_ν ∝ ν^β, and the values are derived from the fits to the adjacent SEDs. Right: SEDs of the point sources SM 1, VLA 1623, GSS 30-IRS1, LFAM 1, LFAM 3 AND GSS 32, belonging to different evolutionary classes (see Fig. 2). The β-values decrease from 2 for the starless dense dust core SM 1 to 0.2 for the visible T Tauri star GSS 32. Colour coding of the symbols is light blue: ISO-CVF 5.5 μm and 16 μm. Red: WISE 22 μm or Spitzer 24 μm. Blue: Herschel-PACS 100 μm and 160 μm. White: PACS-spectra at 63, 73, 145 and 189 μm. Green: APEX 350 and 870 μm. Black refers to literature data: Wilson et al. (1999, 450 μ and Motte et al. (1998, 1.3 mm).