Number 9, November 20, 1990 by Phillip F. Schewe and Ben Stein HYPERBRANCHED MACROMOLECULES, huge tree-like molecules almost the size of a virus, can now be synthesized in the lab. Donald A. Tomalia of the Michigan Molecular Institute claims that some of his largest molecules, which he calls dendrimers (so named for their dendritic architecture), have molecular weights approaching one million units, much smaller than chromosomes (with weights in the billions), but much larger than previous molecules of comparable shape. Unlike conventional polymers, which are usually chainlike objects, the dendritic polymers resemble pom-poms. Tomalia believes that dendrimers are "molecular analogues of atoms," and that by varying dendrite growth patterns, one can create a "molecular-level periodic table." William A. Goddard at Caltech (818-356-6544) and others are studying possible uses for the molecules. (Science News, Nov. 10.) DID A METEOR IMPACT KILL THE DINOSAURS? An enhanced level of iridium and the presence of shocked materials in strata corresponding to the boundary between the Cretaceous and Tertiary eras some 65 million years ago has led some scientists to believe that the mass extinction of life forms at that time occurred in the aftermath of a large comet or asteroid impact. Alan Hildebrand of the University of Arizona (602-621-4655) reports evidence for such an event in the form of glass droplets (tektites) and shocked quartz crystals in an area centering around the Yucatan Peninsula. Hildebrand is also searching for large craters in the Colombian basin. (New Scientist, Nov. 17.) THE LARGEST AND MOST LUMINOUS GALAXY yet recorded was observed recently at the European Southern Observatory in Chile. Located in the galaxy cluster Abell 2029, the large galaxy has a diameter of 6 million light years (the Milky Way is only 100,000 light years across) and emits more than one fourth of the cluster's light. (Science News, Nov. 10.) CHAOTIC CONDITIONS DO NOT INSURE EFFICIENT MIXING of two viscous fluids. Julio Ottino of the University of Amherst (413-545-0593) has discovered that when spots of dye are injected into clear fluids and then stirred, some of the spots are quickly stretched out and folded in a chaotic fashion while other spots, located in certain islands of stability called "periodic points," will hold their shape. These studies are potentially important for certain industries (paint mixing and bread baking, for example) where thorough mixing of two or more fluids is crucial. The principles may apply also to tectonics, where the "fluids" are shifting and subducting crustal plates. (Science, Nov. 2.) THE TRAJECTORIES OF PARTICLES UNDERGOING A CHEMICAL REACTION can be mapped with a femtosecond time resolution. Scientists at Caltech (contact: Ahmed H. Zewail, 213-356-6536) use a femtosecond pulsed laser to follow the dissociation of sodium iodide. The reaction is described in terms of the time evolution of the internuclear separation, a coordinate which is measured with an experimental resolution of half an angstrom. (Nature, Nov. 15.)