  The near UV spectra of OB stars in our galaxy are often dominated by a very
broad extinction band peaking at ~2175 Angstroms. Forty years after its
discovery, the origin of this band remains unknown, although interstellar dust
particles are generally assumed to be the carriers. Here we report that
two-photon absorption by hydrogen molecules in gaseous clouds enveloping OB
stars can lead to a band peaking at ~2175 Angstroms. We present astronomical
spectral evidence supporting our proposal that this nonlinear absorption
mechanism accounts for the lambda2175 feature.
