=========================================================================<br /><br /> * * * * SKY & TELESCOPE's WEEKLY NEWS BULLETIN - June 28, 2002 * * * *<br /><br />=========================================================================<br /><br />OMEGA CENTAURI'S CHECKERED PAST<br /><br />The great globular star cluster Omega Centauri is unique. It's the biggest<br />and brightest globular cluster in the sky and a treasured sight for<br />amateur observers who are far enough south to see it well. It's the<br />physically largest globular in our Milky Way galaxy, containing millions<br />of stars. And in recent years, astronomers have been finding other unique<br />properties about it that are more mysterious....<br /><br />Omega Cen is the only globular in the Milky Way containing at least two<br />different populations of stars born at different eras. Most of its stars<br />have only about 1/40 the Sun's proportion of heavy elements, a sign of<br />great age. But there's a second population of younger stars mixed in that<br />have 10 times more heavy-element content and also have clearly aged less.<br />These stars amount to 5 percent of the total. And there's an intermediate<br />group of stars having various ages in between....<br /><br />Three astronomers in Italy and Germany have just published a study of the<br />three populations' motions. Although "the global scenario for the<br />formation and evolution of Omega Centauri still remains a mystery," write<br />Francesco R. Ferraro and Michele Bellazzini (Bologna Observatory) and<br />Elena Pancino (European Southern Observatory), their work is helping to<br />sort the situation out.<br /><br />The youngest stars, the astronomers find, are clearly moving as a separate<br />group with respect to the rest. Moreover, they don't share in the<br />cluster's general rotation. And the intermediate-age group of stars also<br />has a distinct motion of its own.<br /><br />The astronomers say the young stars are the tattered remains of a<br />different cluster that Omega Cen has swallowed and nearly assimilated....<br /><br />> http://SkyandTelescope.com/news/current/article_651_1.asp<br /><br />- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -<br /><br />THE CASE OF THE MISSING COMETS<br /><br />Comets come in two flavors: those that originate from the Kuiper Belt<br />about 30 to 60 astronomical units away, and the nearly isotropic comets<br />(NICs) that come from the far more distant Oort Cloud more than 10,000<br />a.u. away. Oort Cloud comets fly through the inner solar system at a rate<br />of about a dozen per year. However, what becomes of these icy bodies once<br />they stop producing their easy-to-spot gas comas and tails has remained<br />somewhat of a mystery.<br /><br />Assuming Oort Cloud comets eventually go dormant, the solar system should<br />be brimming with burnt-out objects that have run out of the frozen gases<br />that make them glow. Comets from the Kuiper Belt do often settle into<br />asteroid-like cinders. But whether Oort Cloud objects undergo a similar<br />fate was uncertain. If Oort Cloud comets evolve to cinders, search<br />programs such as the Lincoln Laboratory Near Earth Asteroid Research<br />project (LINEAR) should have uncovered truckloads of the spent bodies. But<br />they simply aren't there.<br /><br />A new study published in last week's Science assigns a grim fate to these<br />dormant passersby. According to Harold F. Levison (Southwest Research<br />Institute, Boulder) and his colleagues, 99 percent of the Oort cloud<br />bodies that come to our solar system disrupt and dissolve into dust....<br /><br />> http://SkyandTelescope.com/news/current/article_650_1.asp<br /><br />- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -<br /><br />VENUS SPOKES: AN EXPLANATION AT LAST?<br /><br />Percival Lowell's name is synonymous with canals on Mars and the discovery<br />of Pluto. But as reported in the July issue of Sky & Telescope, about a<br />century ago Lowell also made some infamous observations of Venus.<br />Throughout his career, Lowell insisted on the existence a central dark<br />spot and a spindly spoke pattern on the cloud enshrouded planet. Lowell<br />called the dark blemish the "subsolar spot," believing it to be fixed in<br />position as required by the synchronous rotation he thought the planet<br />maintained relative to the Sun. On the other hand, Lowell's spokes seemed<br />to remain fixed not with respect to the Sun but the Earth.<br /><br />Generally dismissed as illusion, Lowell's spokes have haunted planetary<br />observation historians for more than a century. But even illusions have to<br />have explanations.<br /><br />Now there's a new theory, independently suggested by several astute<br />readers of the recent S&T article, which may finally lay the century-old<br />mystery to rest. Their interest was piqued by the unusual observing setup<br />Lowell used in making his observations. Although he used the famous<br />24-inch Clark refractor, Lowell found that the markings on Venus showed up<br />best when the instrument was stopped down to a mere 3 inches. With that<br />setup, Lowell shrank the Clark telescope's exit pupil to a pinhole of<br />diameter less than 0.5 millimeter in front of his eye....<br /><br />Philip C. Steffey points out that an exit pupil of 0.5 mm would be about<br />the size of the fovea or less. The resulting light beam in the eye would<br />have been "capable of casting shadows of blood vessels and 'floaters' in<br />the vitreous body on the retina." Instead of mapping the surface of Venus,<br />Lowell would have been mapping the structures in his eye!<br /><br />> http://SkyandTelescope.com/news/current/article_648_1.asp<br /><br />- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -<br /><br />STELLAR WATER FOUNTAINS<br /><br />A star known as W43A in Aquila is on the verge of becoming a planetary<br />nebula. Last week Japanese and British astronomers announced that the<br />star, lying some 8,500 light-years away, is beginning to show signs of<br />things to come, in the form of two water-rich jets streaming from it.<br /><br />W43A is in its final days. It is believed to have once been about the size<br />of our Sun and since used up its hydrogen fuel and ballooned into a red<br />giant, surrounding itself with a disk of dust. The giant will eventually<br />shed its outer atmosphere and form a planetary nebula, while its core will<br />collapse into a white dwarf. One of the enduring puzzles about planetary<br />nebulae is why they have the shapes they do. A star's spherical outer<br />atmosphere somehow is cast away in two lobes of material. W43A's twin<br />water fountain may provide some clues....<br /><br />> http://SkyandTelescope.com/news/current/article_643_1.asp<br /><br />- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -<br /><br />ASTRO NEWS BRIEFS<br /><br />Contour Set to Launch<br /><br />On July 3rd, NASA's Comet Nucleus Tour (Contour) is scheduled to launch<br />from Cape Canaveral, Florida. The original launch date of July 1st was<br />scrubbed due to the discovery of particulate matter on a solar panel. The<br />mission will encounter at least two comets during its planned four-year<br />lifespan. Included on the agenda are flybys of Comets Encke (November 12,<br />2003) and Schwassmann-Wachmann 3 (June 19, 2006). Contour will also have<br />the flexibility to examine any serendipitous comets that may be discovered<br />during its lifetime. The craft will analyze the chemical composition of<br />the comets and will take high-resolution images of the icy wanderers. The<br />mission is NASA's sixth Discovery mission, coming in with a budget of $159<br />million.<br /><br />Does Gravity Brighten the Farthest Quasars?<br /><br />Among the many fruits of the Sloan Digital Sky Survey is a bumper crop of<br />newfound quasars, some of which have redshifts around 6, implying that<br />they -- and the billion-solar-mass black holes that apparently power<br />them -- were born within a billion years of the Big Bang. In some minds,<br />these quasars' stupendous luminosities challenge the Cold Dark Matter<br />theory of cosmology, which predicts that massive structures grow slowly.<br />(A quasar's luminosity is proportional to the mass of its central black<br />hole.) But the challenge is lessened, writes Princeton University expert<br />Edwin L. Turner in tomorrow's Nature, if gravitational lensing has boosted<br />the apparent brightness of many high-redshift quasars, as many astronomers<br />have conjectured. That may well be happening, write Harvard University<br />astronomers J. Stuart B. Wyithe and Abraham Loeb in the same issue of<br />Nature: after crunching the numbers they conclude that up to one-third of<br />the highest-redshift quasars "will have had their observed [brightness]<br />magnified by a factor of ten or more, as a consequence of gravitational<br />lensing by galaxies along the line of sight." But fellow Harvard<br />astronomer Joshua N. Winn cautions that the effect is sensitive to the<br />still-unknown distribution of early quasar luminosities and may well be<br />considerably weaker. Astronomers should know the answer once a Hubble<br />Space Telescope snapshot survey of Sloan quasars, just starting now, is<br />completed.<br /><br />Chandra Spots Pulsar Bull's-eye<br /><br />Astronomers using the Chandra X-ray Observatory have uncovered a bright<br />ring of high-energy particles surrounding the supernova remnant SNR<br />G54.1+0.3. The image shows the bright central pulsar, the encompassing<br />ring (presumably a shock front), and an extended nebula. Taken as a whole,<br />the scene is a snapshot from a star's afterlife -- revealing the radiation<br />and particles that spew out from a supernova's corpse.<br /><br />> http://SkyandTelescope.com/news/current/article_646_1.asp<br /><br />- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -<br /><br />HIGHLIGHTS OF THIS WEEK'S SKY<br /><br />* Venus is in the western sky at sunset.<br />* Last-quarter Moon is on July 2nd<br />* The Earth is at aphelion, the farthest point from the Sun, on July 6th<br /><br />For details, see This Week's Sky at a Glance and Planet Roundup:<br /><br />> http://SkyandTelescope.com/observing/ataglance/<br /><br />=========================================================================<br /><br />Copyright 2002 Sky Publishing Corp. S&T's Weekly News Bulletin is provided<br />as a free service to the astronomical community by the editors of SKY &<br />TELESCOPE magazine. Widespread electronic distribution is encouraged as<br />long as our copyright notice is included, along with the words "used by<br />permission." But this bulletin may not be published in any other form<br />without written permission from Sky Publishing; send e-mail to<br />permissions@SkyandTelescope.com or call +1 617-864-7360. More astronomy<br />news is available on our Web site at http://SkyandTelescope.com/news/.