*********************************** The SOCIETY for POPULAR ASTRONOMY *********************************** ==================================================== Electronic News Bulletin No. 265 2009 April 12 ==================================================== Here is the latest round-up of news from the Society for Popular Astronomy. The SPA is Britain's liveliest astronomical society, with members all over the world. We accept subscription payments online at our secure site and can take credit and debit cards. You can join or renew via a secure server or just see how much we have to offer by visiting http://www.popastro.com/ Astronomica are proud to be sponsors of the SPA Electronic News Bulletin APRIL 4-5 BRITISH ISLES FIREBALL By Alastair McBeath, SPA Meteor Section Director Determined not to be outdone by March, early April produced at least one brilliant meteor for the British Isles, around 23:32 UT on April 4-5. Three probable sightings from England have reached the Section so far on it (there is some doubt over the date and timing of one, which may have been partly in BST, not UT), from Merseyside, Worcestershire and Buckinghamshire. However, Assistant Meteor Director David Entwistle noted that a number of media sources had picked-up on comments from Astronomy Ireland officers, which suggested there had been more sightings from Ireland and Northern Ireland. The media sources were not clear always whether this object may have been on April 4-5 or 5-6, unfortunately. The fireball, if there was just the one event, seems likely to have passed over part of Ireland, but details remain sketchy on exactly where. The British sightings were of an object to the west, which would be consistent with this at least. The Astronomy Ireland website, at http://www.astronomy.ie , may have more by now. For the initial media comments, see for instance this BBC News page: http://snipurl.com/fop87 . Additional observations of this fireball, or any others spotted from Britain and points adjacent (a fireball is any meteor of at least magnitude -3), would be welcomed by the Meteor Section. See the "Making and Reporting Fireball Observations" SPA webpage, at: http://snipurl.com/fopsh , for information and a report form. ALMAHATA SITTA, SUDAN, METEORITE UPDATE By Alastair McBeath, SPA Meteor Section Director Regular Section correspondent and observer Jeff Brower in Canada has kindly forwarded more news on the meteorites apparently fallen from the predicted asteroidal fireball of October 7, 2008, following the NASA press conference on March 25 (noted in the previous ENB). Two field expeditions led by Dr Muawia Shaddad of Khartoum University were carried out in December 2008, from the 5th to the 8th, and the 25th to the 30th, during which a total of 47 meteorites were recovered, weighing in total 3.95 kg. The objects formed a linear strewnfield about 29 km long, and the meteorites showed a degree of mass-sorting within this zone which is typically found with such meteorite showers. The location and nature of the zone certainly strengthened greatly the likelihood that the meteorites did result from this fireball, along with the freshness of the recovered material. The find has been named "Almahata Sitta", Arabic for "Station Six", the nearest man-made landmark to the remote fall region in Sudan, and the objects themselves have been classified as ureilites, the unusual achondrite meteorite class of very-coarse-grained stony bodies. >From earthly comparisons, such coarse-grained material likely cooled only very slowly from a molten state originally, suggestive of deep burial, or some similar insulation, within a larger planetary or minor planetary body. Their chemistry is much as expected from other ureilite examples, but their physical make-up is rather different, with among other features, a more open, porous, structure than expected. Some of these pores showed signs that olivine crystals in them had possibly grown from a vapour, and cooled quickly, and it has been suggested this crystal-growth pattern may have been produced by a substantial impact on the original body, perhaps that which freed asteroid 2008 TC3 and sent it on its collision-course with Earth. My grateful thanks go to Jeff for much of this information. LYRID PROSPECTS By Alastair McBeath, SPA Meteor Section Director The Lyrid meteor shower will be starting shortly. Its meteors are usually seen from April 16-25, and the shower should peak on Wednesday, April 22, perhaps around 11h UT, but almost certainly at some stage between 03h-14h UT that day. This timing means only radio-meteor observers in Britain will be able to follow whatever happens during this predicted interval, till about midday at least. However, the Moon is new on April 25, so skies should be dark for anyone in the UK hoping to spot something of the rising Lyrid activity overnight on April 21-22. The Lyrid radiant reaches a usefully- observable elevation after 22h30m UT or so, and its visibility improves throughout the night. The closer the shower's peak falls to the ~11h timing, the higher its Zenithal Hourly Rates (ZHRs) are likely to be, perhaps 20-25 or more. The average ZHR is 18, and it tends to be lower the further the maximum happens away from this "ideal" time. The maximum is typically quite short, lasting no more than a few hours, but occasionally it can be more prolonged, such as in 2000 and 2001, when peak activity lasted for over eight hours. Rarely, strong ZHRs up to 90 have occurred (last in 1982 over the USA). Consequently, in years like 2009 with little or no Moon, the shower is always one to watch, just in case. Lyrids are medium-fast meteors, and can be very bright sometimes. More information is available in the April 2009 meteor notes, off the Section's homepage at http://snipurl.com/fp0cf , as are full details on how to make and report meteor watch observations. Good luck, and clear skies! SPOTLESS SUN Spaceweather.com NASA has asserted that the Sun has plunged into the deepest solar minimum in nearly a century. Sunspots have all but vanished. In 2008, the Sun had no spots 73% of the time, a 95-year low. In 2009, sunspots are even scarcer, with the 'spotless rate' currently 87%. The situation is unusual but not unprecedented: similarly deep solar minima were common in the late-19th and early-20th centuries, and each time the Sun recovered with a fairly robust solar maximum. JUPITER'S RED SPOT Cosmos Online New high-resolution maps of Jupiter have provided evidence that the Great Red Spot -- the biggest storm in the Solar System -- is shrinking. Scientists at Berkeley have collected data over a number of years from space probes such as Galileo and Cassini and have used them to create detailed maps of wind speeds in the Great Red Spot. They have shown that, from 1996 to 2006, the spot's diameter shrank at an average rate of a kilometre a day. The Red Spot is twice as large as the Earth; it has lasted for at least the 300 years since observations began, and may be much older. The Spot is made up of gases such as hydrogen, helium, ammonia, methane and water vapour, just like the rest of the Jovian atmosphere. What gives it its distinctive red colour is still not agreed, but some scientists believe it may result from material drawn up from deeper in Jupiter's atmosphere, below the ammonia clouds. Previously, changes in the size of the Spot were estimated by looking at cloud patterns created by the storm. To get a more accurate measure, software has been developed to follow the movement of cloud patterns over long periods of time. The shrinking of the Spot has relevance to the energy balance in the surrounding atmosphere. The amount of energy leaving the Red Spot does not appear to be balanced by the energy the storm is gaining, but the reason for any discrepancy is not obvious and neither is its significance. UNUSUAL SUPERNOVA PROGENITOR STAR STScI Astronomers using the Hubble telescope have identified a star that was a million times brighter than the Sun before it exploded as a supernova in 2005. According to current theories of stellar evolution, the star ought not to have self-destructed so early in its life. The star, which is estimated to have had about 100 times the Sun's mass, was not mature enough, according to theory, to have evolved a massive iron core of nuclear-fusion ash, which is supposedly the prerequisite for the core collapse that triggers a supernova blast. The explosion, called supernova SN 2005gl, was seen in the barred- spiral galaxy NGC 266 on 2005 October 5. Pre-explosion pictures from the Hubble archive, taken in 1997, reveal the progenitor as a very luminous point source with an absolute visual magnitude of -10.3. That is so bright that the star seems likely to have belonged to the class of stars called Luminous Blue Variables (LBVs). As an LBV evolves it sheds much of its mass through a violent stellar wind. Only at that point does it develop a massive iron core, and then it explodes as a core-collapse supernova. Extremely massive and luminous stars of more than 100 solar masses, such as Eta Carinae in our own Milky Way, are expected to lose their entire hydrogen envelopes prior to their ultimate explosions as supernovae. The new progenitor identification shows that, at least in that case, the star exploded before losing most of its hydrogen envelope, suggesting that the evolution of the core and the evolution of the envelope are less coupled than previously seemed to be implied by stellar-evolution theory. One possibility is that the progenitor to SN 2005gl was really a binary system that merged, making it look more luminous and less evolved than it really was. The observations indicated that only a small part of the star's mass was flung off in the explosion; most of the material was probably drawn into the collapsing core that may now be a black hole estimated at least 10 to 15 solar masses. ERRATIC BLACK HOLE Chandra X-ray Center There is believed to be a class of black holes, with masses in the approximate range 7--25 solar masses, that has arisen in binary systems. Some such objects emit powerful jets of particles and radiation, rather analogous to those seen in quasars, and are called 'micro-quasars'. A new study looks at a famous micro-quasar in our own Galaxy, and regions close to its event horizon. The system, GRS 1915+105, contains a black hole about 14 times the mass of the Sun that is feeding off material from a nearby companion star. As the material swirls toward the black hole, an accretion disc forms. The system shows remarkably unpredictable and complicated variability on time-scales ranging from seconds to months, including 14 different patterns of variation. Since its launch in 1999, the Chandra X-Ray observatory has observed GRS 1915+105 eleven times. The studies indicate that its jet may be periodically choked off when a hot wind, seen in X-rays, is driven off the accretion disc around the black hole. The wind is believed to shut down the jet by depriving it of the matter that would otherwise have fuelled it. Conversely, once the wind dies down, the jet can re-emerge. The latest Chandra results also show that the wind and the jet carry about the same amount of matter away from the black hole, possibly suggesting that the black hole is somehow regulating its accretion rate, which may be related to the toggling between mass expulsion via either a jet or a wind from the accretion disc. MOST DETAILED MAP OF NEARBY UNIVERSE Anglo-Australian Observatory A survey of galaxies, called the 'Six-degree-field galaxy survey', has been carried out with the 1.2-m UK Schmidt telescope (now part of the Anglo-Australian Observatory). The Schmidt's wide field of view -- 5.7 degrees, or 11 times the width of the Full Moon -- enabled the survey to cover as much as 80% of the southern sky in a reasonable time. From conception to delivery, the survey has taken almost a decade. It has recorded the positions of more than 110,000 galaxies over more than 80% of the southern sky, out to about two billion light-years (a redshift of 0.15). As well as participating in the overall expansion of the Universe, galaxies have their own individual 'peculiar' motions. Assessment of the peculiar velocities may be possible for about 10% of the galaxies surveyed. It is done by comparing a galaxy's distance predicted by its redshift with its distance estimated from its internal properties. The technique depends upon measuring the width of spectral lines in the galaxy concerned. That has been done with a purpose-built spectrograph, the 'Six-degree-field' instrument, which allows 150 spectra to be taken simultaneously. The light from each of the 150 individual galaxies is brought to line up on the entrance slit of the spectrograph by means of a set of flexible optical fibres whose front ends are positioned on the galaxy images by a robotic fibre-positioner. DISCS AROUND COOL STARS HAVE DIFFERENT CHEMICAL MIXES JPL A new study from the Spitzer space telescope finds that the abundances of certain molecules in planet-forming discs around young stars differ according to the luminosities of the stars concerned. Researchers examined planet-forming discs around 44 Sun-like stars and 17 cooler ones (M dwarfs and brown dwarfs). The stars are all about 1-3 million years old, an age when planets are thought to be growing. The astronomers tried to measure the abundance of hydrogen cyanide with respect to that of a baseline molecule, acetylene. They found that the cool stars, both M-dwarf stars and brown dwarfs, showed no hydrogen cyanide at all, while 30% of the Sun-like stars did. They speculate that ultraviolet light, which is much stronger around the Sun-like stars, may drive the production of the hydrogen cyanide. CORRECTION AND RETRACTION OF AN ITEM IN ENB 238 (2008 FEBRUARY 17) By Professor Roger Griffin, Bulletin moderator I apologise for having allowed the following item to appear in the mentioned Bulletin. It seemed to me very doubtful at the time, because there is an implicit assumption that was not stated, let alone justified, that the blobs of material that reflect the light of the star all lie 'in the plane of the sky' -- the same distance from us as the star. It seemed a very unlikely situation, but I let it pass, imagining that authors, referees and Editors must have known what they were doing better than *I* did. It is something of a relief to find that that was not actually so: a paper has recently appeared in 'Astronomy & Astrophysics' demolishing the distance determination on exactly the grounds that I indicate above. The authors of the new paper also know that the 'reflecting' material (actually 'scattering' is a more accurate characterisation of its action) is not just like a white surface but scatters much more effectively in a forward direction, so the material that is best able to scatter starlight towards us lies preferentially on *this* side of the nebula, wholly falsifying the principle upon which the 'distance determination' was based. It goes almost without saying that the organisation that was keen to trumpet the brilliance of the original investigation has not seen fit to issue a disclaimer and retraction now that that investigation has been convicted of what seemed, even at the time, like an obvious flaw. For convenience, the original article in ENB 238 is reproduced below. It is acknowledged that the wording is not exactly the same as in the press report put out by the European Southern Observatory, since (as happens to all the items in these Bulletins) an effort was made to improve the logical order, wording and clarity of the presentation. "LIGHT ECHOES REVEAL DISTANCE TO STAR ESO "Taking advantage of the presence of light echoes, astronomers have measured accurately the distance of a Cepheid -- a class of variable stars that constitutes one of the first steps in the cosmic distance ladder. Cepheids are pulsating stars that have been used as distance indicators for almost a hundred years. The new measurement is important as, unlike most others, it is purely geometrical and does not rely on hypotheses about the physics of the star itself. "The astronomers studied RS Puppis, a Cepheid bright enough to be easily visible with binoculars. RS Pup varies in brightness by almost a factor of five every 41.4 days. It is 10 times more massive than the Sun, 200 times larger, and on average 15 000 times more luminous. It is the only Cepheid known to be embedded in a large nebula, which is made of very fine dust that reflects some of the light emitted by the star. Because the luminosity of the star changes in a very distinctive pattern, the presence of the nebula allows the astronomers to see light echoes and use them to measure the distance of the star. "The light that travels from the star to a dust grain and then to the telescope arrives a bit later than the light that comes directly from the star, so any isolated dust blob in the nebula will show a 'light echo' that varies in brightness in the same manner as the Cepheid, but after a delay. The astronomers monitored the variations of the brightnesses of several blobs in the nebula, and thereby derived the distance of each blob from the star -- it is simply the measured time delay multiplied by the velocity of light. Knowing those distances and the apparent separations on the sky between the star and each blob, one can compute the distance of RS Pup. From the observations of the echoes from several nebular features, the distance of RS Pup was found to be 6500 +/- 90 light years." Owing to holidays, the next edition of the bulletin will be issued on May 1. Bulletin compiled by Clive Down (c) 2009 the Society for Popular Astronomy The Society for Popular Astronomy has been helping beginners to amateur astronomy -- and more experienced observers -- for more than 50 years. If you are not a member then you may be missing something. Membership rates are extremely reasonable, starting at just £16 a year in the UK. You will receive our bright quarterly magazine Popular Astronomy, regular printed News Circulars, help and advice in pursuing your hobby, the chance to hear top astronomers at our regular meetings, and other benefits. 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