Number 37, June 5, 1991 by Phillip F. Schewe and Ben Stein TRANSPARENCY IN GASES CAN BE INDUCED with lasers. Stephen E. Harris of Stanford (415-497-0224) and his colleagues use "population trapping" to make strontium gas transparent to light that is normally absorbed (Physical Review Letters, 20 May 1991). In this process, discussed by theorists for years but not until now applied to gases, the absorption of laser light---corresponding to the transition of atoms from their ground state into an excited state---is inhibited by destructive interference of the laser light with light at a second wavelength corresponding to a transition from an intermediate energy state to the upper excited state. Thus strontium was made transparent to light at a wavelength of 337 nm by imposing a second beam at a wavelength of 570 nm. Harris believes the technique can be applied to materials other than gases. (Science News, 1 June 1991.) THE DEEPEST IMAGE OF THE UNIVERSE ever recorded has been made at the New Technology Telescope in La Silla, Chile, operated by the European Southern Observatory. The picture, showing a field of distant galaxies in the constellation of Sextans, exhibits objects fainter than magnitude 29, one full magnitude (a factor of 2.5) fainter than any previous work. (ESO news release, 31 May 1991.) QUASARS MAY BE HIDDEN in certain radio galaxies, such as Cygnus A, not previously known to harbor quasars. In some theories, all active galactic nuclei are thought to contain quasars surrounded by donut-shaped clouds of dust and gas which (if the viewing angle from Earth were edge-on) would obscure the quasar at visible wavelengths. Stanislav Djorgovski of Caltech has studied Cygnus A at several infrared wavelengths. Speaking at the recent meeting of the American Astronomical Society in Seattle, he said that the longer wavelength observations reveal a compact source---perhaps a quasar, considering the amount of energy emitted from there at radio wavelengths---at the galactic center. (Science News, 1 June 1991.) MANTLE PLUMES , flows of hot rock rising to the Earth's surface from the mantle, may be responsible for various volcanic hot spots around the globe, including Hawaii and Iceland. At a meeting at Caltech last month, several scientists, such as Charles Langmuir of the Lamont Doherty Geological Observatory, suggested that some plumes originate not in the upper mantle (670 km thick) but at the very bottom of the mantle, more than 2000 km farther down. (Science, 24 May 1991.) MARS, VENUS, AND JUPITER will be very close to each other in the evening sky in June, particularly on June 17. Venus's profile will reach "maximum elongation" (it will be 45 degrees east of the sun in our sky) on June 13, when it will set three hours after the Sun. (Astronomy, June 1991.)