Photoluminescence (PL) measurements of size-selected silicon clusters thin films generated by laser pyrolysis of SiH4 in a flow reactor and deposited at low energy after a mechanical velocity selection are presented. Several parameters have been investigated such as the mean size of the clusters, (in the range ∼3–5 nm) and the dispersion of their size distribution. Particular attention has been paid to the determination of the absolute value of the PL efficiency. Measurements show that such silicon nanocrystallites demonstrate high external PL efficiency (up to 12%) and strong dependence of the PL properties on the cluster size. Results have been applied to the physical and chemical interpretation of spectroscopic observations in many astronomical objects, of a broad, red emission band. This so called extended red emission (ERE) is attributed to the PL of an interstellar dust grain component when excited by the UV radiation emitted by a nearby star. Hence it has been shown that nanometer-size crystals of pure silicon are the best candidate materials as regard PL efficiency, peak wavelength and bandwidth. This demonstrates that size effects in interstellar grains play a dominant role in the understanding of this astrophysical phenomenon.