a

. “Given that the cluster apparently comprises few galaxies, yet contains a large amount of iron, a new type of astronomical object is implied by our results. A revision of theoretical models of the metal [heavy element] enrichment process in galaxy clusters may therefore be required,”  M. Hattori et al., “A Dark Cluster of Galaxies at Redshift z=1,” Nature, Vol. 388, 10 July 1997, p. 146.

b

. Lennox L. Cowie and Antoinette Songaila, “Heavy-Element Enrichment in Low-Density Regions of the Intergalactic Medium,” Nature, Vol. 394, 2 July 1998, pp. 44–46.

c

. “In both cases, the scatter of the observed values [of heavy hydrogen] is quite large and seems to reach a factor of 10. Although it is already surprising to see such variations within ~1000 pc from the sun, this looks unbelievable within only 30 pc from the sun.” [1 pc (or parsec) = 3.258 light-years] A. Vidal-Madjar, “Interstellar Helium and Deuterium,” Diffuse Matter in Galaxies, editors J. Audouze et al. (Boston: D. Reidel Publishing Co., 1983), pp. 77–78.