Newsletter from The Society for Popular Astronomy

*********************************** The SOCIETY for POPULAR ASTRONOMY *********************************** ==================================================== Electronic News Bulletin No. 287 2010 April 25 ==================================================== 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/ EARLY APRIL FIREBALLS By Alastair McBeath, SPA Meteor Section Director The first part of April this year has brought a healthy crop of fireballs (meteors of magnitude -3 or brighter) to the Section so far. Single- observer meteors were reported at 03:17 UT on April 6-7 (magnitude -7, seen from Edinburgh), about 04:25 UT on April 11-12 (very bright; Cornwall) and 21:31 UT on April 14-15 (-5/-7; Gwent), plus there were two others seen from more than one location. At 20:15 ± 5 minutes UT on April 9-10, a magnitude -10 or so meteor was spotted from Glasgow and Edinburgh. Reports from the witnesses suggested the object may have followed a roughly south to north trajectory over eastern Scotland north of the Fife peninsula, perhaps across part of the eastern Grampian Mountains of the "Aberdeen angle", or the North Sea offshore of there. On April 16-17 near 22:00 UT, a very bright, green fireball was seen from four locations in southern England - Gloucester, Surrey, Hampshire, and Devon. Two of the initial sightings can be found on the UK Weather World's Space Weather Forum (at: http://snipurl.com/vobdi ). This fireball seemed to have been out high above the western Channel, and part of its flight may have been some way offshore of the English coast between roughly Prawle Point in Devon and Lizard Point in Cornwall. Most observers were impressed both by its brilliance and its vivid green colour, though suggestions the colour may have been due to the volcanic ash cloud over and near the British Isles from Iceland, were without foundation. Bright green, though not common, does occur in meteors, particularly the brighter ones, without any such assistance. All further sightings of these, or other fireballs, made from the British Isles and nearby, would be welcomed by the Section. The minimum details required are: 1) Exactly where you were (give the name of the nearest town or large village and county if in Britain, or your geographic latitude and longitude if elsewhere in the world); 2) The date and timing of the event in UT (remember to subtract one hour from current clock time, BST, to get UT); and 3) Where the fireball started and ended in the sky, as accurately as possible, or where the first and last points you could see of the trail were if you did not see the whole flight. More advice and a fuller set of details to send (including an e-mail report form) are given on the "Making and Reporting Fireball Observations" page of the SPA website, at: http://snipurl.com/u8aer . The Easter break has also prompted another flurry of "sky lantern" sightings, sadly. These were last so problematic back in January (see ENB 280, at: http://snipurl.com/ucps9 ). In order not to miss genuine fireball observations, the Meteor Section is willing to receive reports of any unusual moving star-like light in the sky, where the witness could not be sure what the object was. However, it is very important to send as many details as possible - ideally completing the electronic Fireball Report Form fully - to enable the object's nature to be determined swiftly and accurately. An unhelpfully large number of the recent potential lantern sightings have had insufficient information provided initially to allow this, to the extent some could even have been genuine fireballs. Please remember this when sending in a possible fireball sighting, and help us to better help you! BRILLIANT IMAGED FIREBALL OVER THE USA By Alastair McBeath, SPA Meteor Section Director Around 22:05 local time on April 14, a spectacular fireball was seen from at least six states in the Midwest and Great Lakes regions of the USA, lighting-up the sky. Video recordings of the very slow, brilliant meteor were made by-chance, which were quickly picked-up by TV news stations and broadcast across the world. The videos are available online - try the BBC's News webpage at http://snipurl.com/voeg5 , for instance - while there are more comments, and links to additional Internet sites on the UK Weather World's Space Weather Forum topic at: http://snipurl.com/voekk . As too often, an immediate claim was made that the fireball had come from a meteor shower, this time the very minor Gamma Virginids, based on nothing more than a wild guess, because of when it had happened. Another week, and doubtless it would have been called a Lyrid! If the videos as broadcast were accurate to the object's appearance, especially its apparent speed, the actual fireball seemed to have been well below the slow-medium atmospheric velocity, around 30 km/sec, expected for the Virginids or any of the Antihelion Source meteors (as we currently term meteors from the many, very weak, radiants clustered near the ecliptic nearly opposite the Sun in the sky, and active for most of the year - see the April meteor activity webpage for notes on the Antihelion Source this month, at http://snipurl.com/vogzo ). None of this detracts from the magnificence of the fireball, of course! COMET McNAUGHT HAD UNUSUALLY LONG ION TAIL RAS British scientists have shown from Ulysses spacecraft data that Comet McNaught, which in early 2007 became the brightest comet seen for 40 years, disturbed a region of space much larger than that occupied by the visible tail. Analysis of magnetometer data suggests that the comet was surrounded by a shock wave created where the fast-flowing particles of the solar wind were slowed down abruptly when they impinged on the ionized gas emitted from the comet's nucleus. It was just by chance that Ulysses happened to pass through Comet McNaught's tail; it encountered the tail of ionized gas at a distance downstream of the comet's nucleus more than 1.5 times the distance between the Earth and the Sun -- much further away than the visible dust tail extended. Ulysses took 8 days to traverse the shocked solar wind surrounding Comet McNaught, compared to 2.5 days in shocked wind surrounding Comet Hyakutake in 1996. The Giotto spacecraft's encounter with Comet Grigg-Skjellerup in 1992 took less than an hour from one shock crossing to another; to cross the shocked region at Comet Halley took a few hours. The comparisons show that Comet McNaught was not only spectacular from the ground but was an unusually large obstacle to the solar wind. VENUS IS VOLCANICALLY ACTIVE ESA Venus Express has returned the clearest indication yet that Venus is still active. Relatively young lava flows have been identified by their emission of infrared radiation. The finding suggests that the planet remains capable of volcanic eruptions. The sparseness of craters on Venus suggests that something is wiping the planet's surface clean. That something is thought to be volcanic activity, but the question is whether it happens quickly or slowly -- whether there is some sort of cataclysmic volcanic activity that resurfaces the entire planet with lava, or a gradual sequence of smaller volcanic eruptions. The latter are suggested by maps of the infrared brightness or 'emissivity'. Astronomers concentrated on three regions that are analogous to Hawaii, well-known for its active vulcanism. Those regions on Venus have higher emissivities than their surroundings, indicating different compositions. On Earth, lava flows react rapidly with oxygen and other elements in the atmosphere, changing their composition. On Venus, the process should be similar, though more vigorous because of the hotter, denser atmosphere. The researchers interpret the areas of high emissivity as lava flows that have not undergone as much weathering as their surroundings, implying that they are relatively recent, possibly even still forming. PLUTO SHOWS CHANGES NASA A comparison of Hubble Telescope images of Pluto obtained in 1994 and 2003 shows that the northern hemisphere has brightened while the southern hemisphere has dimmed. Ground-based observations suggest that Pluto's atmosphere doubled in mass during approximately the same interval. Pluto gets so cold that its atmosphere can actually freeze and fall to the ground. If the Earth's atmosphere did that, it would make a layer 30 feet thick, but Pluto has far less atmosphere. When it is on the ground, Pluto's entire blanket of air is no more than a frosty film of nitrogen and methane. Until the mid-1980s, Pluto's northern hemisphere had been tilted away from the Sun for over 100 years, accumulating a substantial amount of frost. Now the northern hemisphere is coming into sunlight and appears, as shown in the Hubble images, to have been growing brighter. The atmosphere might also be changing in response to Pluto's highly eccentric orbit. During the late 1980s, Pluto approached as close to the Sun as it ever gets and was consequently warming. Surface frosts exposed to such 'warmth' may be subliming -- that is, changing back into gas. DUSTY DISCS IN PLANETARY SYSTEMS RAS Two stars observed in the infrared with the MIDI interferometer, which combines the light from the 8-m units of the VLT in Chile, appear to have discs of rocky and dusty material at distances comparable to that from the Earth to the Sun. The stars concerned, both considerably younger than the Sun, are HD 69830, of spectral type K0 V, in the constellation Puppis and thought to have three planets with masses comparable to Neptune, and Eta Corvi, type F2 V. Earlier observations had indicated that both stars had discs; Eta Corvi is known to have cold material around it at a distance of 150 Astronomical Units (Earth--Sun distances; 1 AU is about 150 million km). With MIDI a relatively small dusty disc around HD 69830 was clearly seen; it lies between 7.5 and 360 million km from the star. A similar disc was found close to Eta Corvi, lying between 24 to 450 million km out. Those results represent the first resolution of dusty discs so close to their parent stars. COOLEST BROWN DWARF FOUND NEAR SUN University of Hertfordshire Brown dwarfs are bodies with masses in the range between those of giant planets and the faintest stars. Some are isolated, while others orbit normal stars or exist in star clusters. Astronomers have now discovered a previously unknown brown dwarf just 2.9 parsecs (9 light- years) away -- the seventh-closest star, and the first to be found so close since Luyten 726-8 was discovered in 1948. The star, UGPS J0722-05, has a temperature of 400-500 K and is far less luminous and significantly cooler than previously known objects. The Jupiter-sized object emits only 0.000026% as much energy as the Sun. Since 1995, more than 100 methane brown dwarfs, or T dwarfs, have been found, with spectra similar to that of the planet Jupiter and with effective temperatures in the range 500-1300 K. The detection of even cooler bodies will open a new arena for atmospheric physics and may help to determine the formation rate of stars and brown dwarfs in our Galaxy as a function of both mass and time. ROCKY PLANETS MAY BE COMMON IN THE MILKY WAY RAS Astronomers have found evidence that rocky planets are commonplace in our Galaxy. A survey of white dwarfs, the compact remnants of stars that were once like our Sun, found that many show signs of contamination by heavier elements and possibly water. White dwarfs are the endpoint of stellar evolution for the vast majority (>90%) of all stars in the Milky Way. Because they ought to have almost pure hydrogen or helium atmospheres, if heavier elements such as calcium, magnesium and iron are found then they are interpreted as external pollutants. For decades, it was believed that the interstellar medium (the tenuous gas between the stars) was the source of the metals in the polluted white dwarfs. The team used data from the Sloan Digital Sky Survey (SDSS), a project that aims to survey the sky in infrared light, imaging more than 100 million objects and following up 1 million of them by obtaining their spectra. By examining the positions, motions and spectra of the white dwarfs identified in the SDSS, the team shows that an interstellar origin for the metals is no longer a satisfactory theory. Instead, rocky planetary debris is probably the usual culprit. The new work indicates that at least 3% and perhaps as much as 20% of all white dwarfs are contaminated in this way, with the debris most likely in the form of rocky minor planets with a total mass of about that of an asteroid 140 km in diameter. That implies that a similar proportion of stars like our Sun, as well as stars that are somewhat more massive, like Vega and Fomalhaut, build terrestrial-type planetary systems. The scientists also measured the composition of the pollutants through their spectroscopic signatures, which stand out in the otherwise pure atmosphere of the white dwarfs. It appears that a significant fraction of the stars are polluted with material that contained water, with implications for the frequency of habitable planets around other stars. BLACK HOLES AND GALAXY DEATH RAS Black holes are thought to reside at the centre of almost every galaxy, with some growing to more than a billion times the mass of the Sun. Now a team of UK astronomers has proposed that such super- massive black holes are commonplace, release more than enough energy to strip their host galaxies apart, and in the process shut down star formation in their galaxies for good. For many years black holes have fascinated scientists and the public alike, with their peculiar ability to warp space and time and their sinister tendency to devour everything they encounter. Before matter falls in, as it swirls around the black hole it forms an 'accretion disc', where it heats up and radiates energy. The super-massive black holes have such strong gravitational fields that the infalling matter releases a vast amount of energy, making each accretion disc far brighter than the combined output of the billions of stars in the galaxy around it. One of the consequences of such outpouring of energy is that it drives away cool gas and dust, the raw ingredients of new stars. That permanently shuts down star-formation in the surrounding galaxy, whose remaining stars age, end their lives, and are never replaced. The new study considered the role of super-massive black holes in the development of galaxies. To search for them, the team used the Hubble telescope and the Chandra X-ray observatory to observe in optical, near-infrared and X-ray light. In particular, the astronomers looked for galaxies which have a very high emission of X-rays, a probable signature of black holes devouring gas and dust. From the space telescopes' data astronomers find that at least 1/3 of all the massive galaxies they observed not only contain super-massive black holes, but that at some point in their histories the emission from the holes' accretion discs far outshines the galaxies themselves. The energy output of regions around the black holes is high enough to strip apart every massive galaxy in the cosmos 25 times over, whilst the X-ray emission from them turns out to dwarf that from every other source in the Universe put together. POSSIBLE MICROQUASAR IN STARBURST GALAXY M82 RAS Radio astronomers at Jodrell Bank have discovered a strange new object in M82, a galaxy that is 10 million light-years away and is forming new stars at a prodigious rate, many of them massive stars that die quickly, a supernova explosion occurring every 20 to 30 years. The new object, which appeared last May, has perplexed astronomers, who have never seen anything quite like it before. The object turned on very rapidly within a few days and has shown no sign of decaying in brightness over the first months of its existence. The new young supernova explosions that astronomers expect to see in M82 brighten at radio wavelengths over several weeks and then decay over several months, so that explanation seems unlikely. The plausibility of a supernova explanation was further undermined when very accurate positional monitoring by the UK network of radio telescopes, MERLIN, tentatively detected a change in position for the object over the first 50 days. It was equivalent to an apparent motion of over four times the speed of light. Such large apparent velocities are not seen in supernova remnants and are usually only found with relativistic jets ejected from accretion discs around massive black-hole systems. The nucleus of M82 may contain a super-massive black hole. The new detection lies at a position close to, but several arcseconds away from, the dynamical centre of M82 -- far enough away that it would seem unlikely that this object is associated with the central collapsed core of the galaxy. The new source could be the first radio detection of an extragalactic 'micro-quasar'. Examples of such systems within the Milky Way are found as X-ray binaries with relativistic jets ejected from an accretion disc around a collapsed star fuelled with material dragged from a close binary companion. However, this object would be brighter than any Galactic example yet detected, has lasted months longer than any known X-ray binary, and lies at a position in M82 where no variable X-ray source has been yet been detected. LOFAR OPENS UP LOW-FREQUENCY UNIVERSE RAS The Low Frequency Array (LOFAR), a new pan-European radio-astronomy instrument, has started mapping the Universe at very long wavelengths, a part of the electromagnetic spectrum that is relatively unexplored. Astronomers hope LOFAR will allow them to study cosmic rays, pulsars, and the magnetic field within our own and nearby galaxies. LOFAR will also compile a census of radio-emitting galaxies from the very early Universe, which may help us to understand how galaxies formed and evolved over cosmic time. NEXT BULLETIN Owing to holidays, the next scheduled bulletin will be issued on May 16. Bulletin compiled by Clive Down (c) 2010 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. 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