*********************************** The SOCIETY for POPULAR ASTRONOMY *********************************** ==================================================== Electronic News Bulletin No. 268 2009 June 14 ==================================================== 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/ ETA AQUARIDS 2009 By Alastair McBeath, SPA Meteor Section Director Although the Eta Aquarids are always very difficult to observe visually from the UK, because their radiant rises in strong twilight shortly before dawn, radio observers here can still follow the shower's activity, as the radiant in early May is above the horizon for Britain from roughly 02h to 13:30 UT, culminating near 08h. Some visual and video reports have come in to the Section on the shower from other parts of the world in many years recently, and in combination, these can give an idea of how the shower behaved. This time, visual watchers were not helped by an almost-full Moon, though this was setting by the time the radiant could be usefully-seen. Data collected during the first half of 2009 May has been received so far from the following people. The radio results, "R" below, were mostly extracted from Radio Meteor Observation Bulletin 190 for May 2009 (RMOB, provided by Editor Chris Steyaert - see http://www.rmob.org ), while the visual details - "V" - were from North American Meteor Network watchers, NAMN ( http://www.namnmeteors.org ), whose data was helpfully summarized for us by NAMN leader Mark Davis. Contributing observers were: Salvador Aguirre (Mexico; NAMN; V), Enric Algeciras (Spain; RMOB; R), Orlando Benitez (Canary Islands; RMOB; R), Mike Boschat (Nova Scotia, Canada; RMOB; R), Jeff Brower (British Columbia, Canada; RMOB; R), Willy Camps (Belgium; RMOB; R), Gaspard De Wilde (Belgium; RMOB; R), David Entwistle (England; RMOB; R), Robert Lunsford (California, USA; NAMN; V), Mike Otte (Illinois, USA; RMOB; R), Andy Smith (England; RMOB; R), Chris Steyaert (Belgium; RMOB; R), Enrico Stomeo (Italy; video), Dave Swan (England; RMOB; R), Felix Verbelen (Belgium; RMOB; R), John Wardle (England; RMOB; R). The visual results were too limited for certainty, but suggested an Eta Aquarid maximum on May 6, about as expected. The "live" International Meteor Organization results from the shower (available at http://snipurl.com/k21bn ) struggled similarly due to few reports, but also found a peak on May 6, possibly with good activity lasting from at least 03h UT on May 5 to 19h on May 6. Zenithal Hourly Rates were probably up to 55-65, maybe higher, between these times, and were 30 or more when data were available between May 3 to 8. This protracted spell of better activity is quite typical for the shower, though the highest rates were perhaps a little down on the ~85 that might have been expected from recent theoretical studies. Conditions and the relatively few observers mean these values are quite tentative, however. Radio observers in places have been struggling of late, partly due to increasing numbers of commercial analogue TV and radio transmitters being turned off as the 'digital revolution' is forced upon various places. Early May also brought an unwelcome start to the northern summertime interference problems due to Sporadic-E, Es. Es sheets are made up of ionized particles that collect at random in the upper part of the main meteor ablation zone in the atmosphere, the E-layer, at approximately 120-100 km altitude. Radio signals reflecting from Es blanket-out those signals due to meteors alone, and render the observations worthless. Es predominantly happens between April to September. Despite this, the radio results as a whole indicated fairly typical better activity, when the Eta Aquarid radiant was above the horizon, between May 4 to 9 inclusive, with a midpoint on May 6. However, instead of a clear maximum on May 6, two peaks were supported by more radio systems, one on May 5, the other, possibly the stronger, on May 8. The May 5 peak may have been due to relatively brighter meteors, that on May 8 relatively fainter ones, though this pattern was not especially clear. While this did not strongly confirm the visual results, such a variance between the radio and visual data on this shower is not unknown, and may stem partly from the short observing window available visually. Grateful thanks go to all those named above for their contributions and comments. Further Eta Aquarid observations would be much appreciated! FIRST SUCCESS FOR OLD PLANET-HUNTING METHOD JPL Astrometry, a long-tried but hitherto unsuccessful method of fishing for planets outside the Solar System, has netted its first catch: a Jupiter-like planet orbiting one of the least-massive stars known. It involves measuring the position of a star on the sky as it orbits with an unseen planet. But the method requires very precise measurements over long periods of time, and until now has failed to turn up any exo-planets. For the past 12 years astronomers have been observing 30 stars with an astrometric instrument on the 200-inch telescope at Palomar. The newly found exo-planet, called VB 10b, is about 20 light-years away in the constellation Aquila. It is a gas giant, with a mass 6 times that of Jupiter and an orbit about as far from its star as Mercury is from the Sun -- far enough away from its star to be regarded as a 'cold Jupiter' analogous to our own. In reality, the planet's own internal heat might give it an Earthlike temperature. The planet's star, VB 10, is an M-type dwarf and has only one-twelfth the mass of our Sun, just barely big enough to fuse atoms at its core and shine with starlight. For years, VB 10 was the least massive star known; now it the least massive star known to host a planet. In fact, even though the star is much more massive than the planet, it is not a great deal bigger. BETELGEUSE IS SHRINKING AAS The red supergiant star Betelgeuse, in the constellation Orion, has steadily shrunk over the past 15 years, according to University of California, Berkeley, researchers. Long-term monitoring by an interferometer on Mount Wilson in California shows that Betelgeuse, which is so big that in our Solar System it would reach to the orbit of Jupiter, has shrunk in diameter by more than 15% since 1993. Despite the star's diminished size, its visual brightness has shown no significant decrease during that period. Betelgeuse was the first star (after the Sun) to have its angular size directly measured, and even today it is one of only a handful of stars that can be identified by the Hubble telescope as being slightly extended rather than an unresolved point of light. In 1921, Francis Pease and Albert Michelson used optical interferometry to estimate that its diameter was equivalent to the orbit of Mars. Last year, new ideas of the distance to Betelgeuse raised it from 430 light-years to 640, which increased the diameter derived from it from about 3.7 to about 5.5 astronomical units (Earth--Sun distances). Since 1921, its size has been re-measured by many different interferometer systems. At a given wavelength the variation has been scarcely beyond the measurement uncertainties, but different wavelengths give diameters differing by up to 30%, because the tenuous gas in the outer regions of the star emits light at some wavelengths and absorbs it at others -- the star looks bigger in the light of an emission line than it does in an absorption line. The interferometer built at Berkeley in the early 1990s sidesteps the confounding emission and absorption lines by observing in the mid- infrared with a narrow bandwidth that can be tuned between spectral lines. The interferometer combines signals from pairs of telescopes in order to determine path-length differences between light that originates at the star's centre and light that originates at the star's edge. Its observing wavelength of about 11 microns, in the mid-infrared, penetrates the dust, and the narrow bandwidth avoids any spectral lines, so the star is seen relatively undistorted. The first measurements showed the size to be quite close to Pease and Michelson's result, but over 15 years it has decreased about 15%, changing smoothly but faster as the years have progressed. CLUSTER IN CROWDED NEIGHBOURHOOD PROVES TO BE NORMAL Science Daily Using ESO's Very Large Telescope, astronomers have been observing the Arches Cluster, which is about 25 000 light-years away towards the constellation Sagittarius and contains about a thousand young, massive stars, less than 2.5 million years old. It is close to the centre of our Milky Way, where it must be subject to unusual disturbances from the stars, gas and the super-massive black hole there. The Arches Cluster is ten times more massive than typical young star clusters scattered throughout the Milky Way, and it has unusually high abundances of chemical elements heavier than helium. Observing the Arches Cluster is challenging because of the huge quantities of absorbing dust, which visible light cannot penetrate, between the Earth and the Galactic Centre. The new study, made in the infrared, confirms the Arches Cluster to be the densest cluster of massive young stars known. It is about three light-years across, with more than a thousand stars packed into each cubic light-year -- an extreme density a million times greater than in the Sun's neighbourhood. Astronomers studying clusters of stars have always found that high-mass stars are rarer than less-massive ones, and their relative numbers are the same everywhere. but the Arches Cluster has seemed to be a striking exception. With the extreme conditions in the cluster, one might indeed imagine that stars would not form in the same way as in our quiet solar neighbourhood. However, the new observations show that the distribution of masses in the cluster actually does follow the same universal law. The most massive star has a mass of about 120 times that of the Sun. Astronomers conclude from that that, if stars more massive than 130 solar masses exist, they must live for less than 2.5 million years and end their lives without exploding as supernovae, as massive stars usually do. The total mass of the cluster seems to be about 30 000 times that of the Sun, much more than was previously thought. NEW CLASS OF DIM SUPERNOVAE Science Daily Core-collapse (or gravitational) supernovae are the final tremendous explosions that end the life-cycles of stars more massive than approximately 8 times the Sun. After running out of fuel, the core of such a star collapses and forms a neutron star or a black hole. At the same time, the outer layers are ejected at high velocity and briefly shine as brightly as billions of stars together. The total energy suddenly released by such a supernova exceeds the total energy released by the Sun during its whole past and future life time of 10 billion years. However, some core-collapse supernovae are up to 100 times less energetic and luminous than usual. Such 'low'-power explosions normally show the presence of hydrogen gas, but a recent event, supernova SN 2008ha, is the first dim supernova in which no hydrogen could be detected. Taken together, the dimness and lack of hydrogen leave room for at least two possibilities for the origin of SN 2008ha. One is that the progenitor star may have been a moderately massive star in a binary system, and lost its outer layers through interaction with the companion. Alternatively, it may have been a very massive star which shed its envelope through stellar winds. CARBON STAR (?) EXPLODES ScienceDaily Astrophysicists at the University of Warwick think that a star that appeared temporarily in 2006 may have been a supernova explosion of a carbon star -- a type of cool giant star with an excessive abundance of carbon in its atmosphere. The object, known as SCP 06F6, was first noticed in 2006 in images taken with the Hubble telescope; it appeared, and faded away again beyond detection, over the course of 120 days. The distance of the object was indeterminate. According to the new research, identifying the event as a carbon-star supernova, it must have been at a distance of about 2 billion light-years. It would be an unusual type of supernova in several aspects: SCP 06F6 is located in a blank part of the sky, with no visible host galaxy. If the star did explode as a type-II supernova, why then did it take up to four times as long to brighten and fade as other such supernovae, and why did it emit up to 100 times more X-ray energy than would be expected? There is no answer to those questions, but the leader of the research at Warwick, Boris Gänsicke, remarks that several telescopes are now being designed and built to monitor the entire sky continuously for short appearances of new stars, and he is sure that SCP 06F6 will not remain alone in puzzling astronomers over the coming years. NEW MASS ESTIMATE FOR M87 BLACK HOLE AAS To try to understand how galaxies form and grow, astronomers need to start with basic census information about today's galaxies. What are they made of? How big are they? What are their masses? Astronomers assess that last category, galaxy masses, from the speeds of stars orbiting within a galaxy. Studies of the total mass are important, but so is the question as to whether the mass is in a black hole, in the stars, or in a dark halo. A new computer model has been made of M87, one of the largest 'nearby' galaxies; it is more complicated than previous ones because, in addition to modelling its stars and black hole, it takes into account the galaxy's dark halo -- a spherical region surrounding it that extends beyond its main visible structure, and contains dark matter -- matter with a substantially smaller ratio of luminosity to mass than the Sun's. The new model suggests that the black hole at the heart of M87 is two to three times more massive than previously thought; at 6.4 billion times the Sun's mass, it is the most massive black hole estimated by a supposedly robust technique, There is a suggestion that the 'accepted' masses of black holes in other large galaxies may be off by similar factors. That would put them more into line with the masses that have been attributed to the black holes that are supposed to power quasars at the centres of extremely distant galaxies seen at a much earlier cosmic epoch. GHOST REMAINS AFTER BLACK-HOLE ERUPTION Chandra X-ray Center The Chandra X-ray Observatory has found what has been called a cosmic 'ghost', because it is supposed to be all that is left to mark the site of an earlier quasar outburst after other radiation from the presumed outburst has died away, It is in the Chandra Deep Field North, one of the deepest X-ray images taken by Chandra. The source, called HDF 130, is over 10 billion light-years away and existed at a time 3 billion years after the Big Bang. Chandra astronomers think that the X-ray glow from HDF 130 is evidence for a powerful outburst from a central black hole in the form of jets of energetic particles travelling at almost the speed of light. When the eruption was ongoing, it would have produced prodigious amounts of radio and X radiation, but after several million years the radio signal would have faded as the electrons radiated away their energy. However, even less-energetic electrons can still produce X-rays by interacting with the pervasive sea of photons remaining from the Big Bang (the 'cosmic background radiation'). Collisions between the electrons and the background photons can impart enough energy to the photons to boost them into the X-ray energy band. That process can produce an extended X-ray source that lasts for another 30 million years or so. If the observations have been correctly interpreted, this is the first X-ray ghost seen after the demise of radio-bright jets. Astronomers have observed extensive X-ray emission of similar origin, but only from galaxies with radio emission on large scales, signifying continuing eruptions. In HDF 130, only a point source is detected in radio images, coinciding with the elliptical galaxy seen in its optical image. The radio source is taken to indicate the presence of a growing super-massive black hole. The details of Chandra's data on HDF 130 helped to identify its nature. In X-rays, HDF 130 has a cigar-like shape that extends for some 2.2 million light-years. The linear shape of the X-ray source is consistent with the shape of radio jets and not with that of a cluster of galaxies, which is expected to be circular. The energy distribution of the X rays is also consistent with the interpretation of an X-ray ghost. BAA OUT-OF-LONDON WEEKEND By Rod Levene BAA Out-of-London Weekend with Leeds AS, at University of Leeds Dept. of Music Clothworkers Hall; September 4, 5 and 6. Theme: Dynamic Stars. Details: http://britastro.org/baa/ Calendar page and http://www.leedsastronomy.org.uk/ Meetings page. Leeds AS is 150 years old this year and is hosting a BAA Out-of- London weekend. A civic reception with the Lord Mayor on Friday evening precedes a Saturday of talks at the University of Leeds, relating to stars. There will also be trade stands, and tickets include lunch. Dinner on Saturday evening features a talk asking "Are we alone" and Sunday morning is spent in the Astrophysics Dept. 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. 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