Number 23, February 28, 1991 by Phillip F. Schewe and Ben Stein AN ERUPTION ON COMET HALLEY has taken astronomers at the European Southern Observatory (ESO) by surprise. Having passed near the sun in 1986, Halley is by now more than 2 billion km from the sun (halfway between Saturn and Uranus) and the light reflected from this dirty snowball is meager. An observation on 12 February 1991 by an ESO telescope in Chile showed, however, that Halley had suddenly become 300 times brighter than it was supposed to be. No comet had ever before undergone such an outburst so far from the sun. Three explanations have been tentatively offered: the outburst may have been the release of energy stored inside the comet nucleus; Halley may have collided with another body; or there may have been some interaction with particles in the solar wind. (ESO press release, 22 Feb. 1991.) AN X-RAY SURVEY OF THE LARGE MAGELLANIC CLOUD (LMC) by the ROSAT satellite has revealed 45 individual x-ray sources, 15 of which were never seen before. The ROSAT observations are part of the first-ever all-sky survey at x- ray wavelengths with a true imaging telescope. Among previous x-ray satellites, EXOSAT and the Einstein Observatory were imaging devices but, in that mode, only looked at individual sources. (Nature 14 Feb. 1991.) In the same issue of Nature, members of the ROSAT team also published the first soft x-ray images of the moon. NEUROMAGNETISM has become increasingly important in the cognitive sciences. Starting with work in 1975 at New York University (Samuel Williamson, 212-998-7692), the number of labs worldwide that use the brain's magnetism to map sensory functions has grown to 50 or more. Researchers address such questions as: where in the brain is the "mind's eye" or the "mind's ear"? What part of the brain responds when we see a face? The instruments used to measure neural magnetism, magnetometers fashioned from superconducting quantum devices (SQUIDS), are now commercially available from such companies as Siemens (Germany) and BTI (San Diego). The SQUID arrays, usingup to 37 sensors, still employ the older 4-K superconductors since the 77-K superconductor technology is not yet ready. (Scientific American, March 1991.) OLBERS' PARADOX asks why, if the universe contains an infinite number of stars distributed approximately evenly, is the night sky not lit up by starlight coming from all directions. Astronomers have grappled with this issue for some time. The two chief explanations seem to be (a) that light from galaxies is limited by their finite life and the finite speed of light and (b) that the expansion of the universe shifted much of the light coming toward us out to longer wavelengths. Paul S. Wesson of the University of Waterloo in Ontario (519-885-1211) has performed new studies which favor the finite-age argument. Basically the universe is not old enough for light from certain distant stars to have reached us yet. (Science News 23 Feb. 1991.)