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Unformatted text preview: Understanding differences among dust properties in various galaxies is still pretty primitive. We do see differences between the Milky Way, LMC, and SMC, likely driven by metallicity. Andromeda and a few other luminous spirals with decent data show reddening curves much like the Milky Way, with weak evidence for different strengths of the 2200-Angstrom feature between our galaxy and M31 (Bianchi et al. 1996 ApJ 471, 203). One study has used differential reddening between the images of gravitational lenses to look at reddening curves, and even to extract redshift estimates from the slopes (Falco et al. 1999 ApJ 523, 617). Whether grey (wavelength-independent) extinction exists is important for the issue of a nonzero cosmological constant, since such dust would dim supernovae without the expected accompanying reddening. The universality (or otherwise) of the extinction curve makes a huge difference for how strongly we can correct for dust effects. Baade is said to have been asked late in his life whether he would we can correct for dust effects....
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