*********************************** The SOCIETY for POPULAR ASTRONOMY *********************************** ==================================================== Electronic News Bulletin No. 271 2009 July 26 ==================================================== 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/ PERSEID PROSPECTS By Alastair McBeath, SPA Meteor Section Director As the summer holidays get into full swing, by late July, many observers' thoughts turn to the Perseids. This year, the usual Perseid maximum is due between 17:30-20:00 UT on August 12. It should produce typically healthy Perseid numbers, with Zenithal Hourly Rates (ZHRs) around 80-100. However, Finnish meteor analyst Esko Lyytinen has suggested the Earth may encounter two denser meteoroid trails earlier on August 12, around 05:00 UT (laid down at parent comet 109P/Swift-Tuttle's 1862 return, though perhaps adding only 10 or less to the ZHR then) and 09:00 UT (from the comet's 1610 perihelion, adding between a few tens up to a hundred to the ZHR). Material scattered around these trails may lead to somewhat higher than normal Perseid activity for most of August 12 ahead of the ordinary maximum as well. If correct, these timings will be about as unhelpful as possible for British visual observers, though radio observers should still manage to follow something of the better activity, interference permitting. Visual watching throughout August 11-12 and 12-13, if clear, would be scientifically valuable, and potentially more rewarding certainly, despite the added drawback of the waning Moon (at last quarter on August 13). The Moon will visible nearly all night on both occasions, from before the Perseid radiant is first usefully- observable at about 22h UT for Britain. Even so, Perseid meteors should be relatively frequent enough, and many sufficiently bright, to mean some should still be seen around then. Keen watchers will need to observe as much clear sky as comfortably possible, without looking towards the Moon (hiding it behind a wall or roofline will help). Perseids are swift, often bright, and commonly trained meteors. Additional information, including a Perseid radiant chart and details on the others showers active simultaneously, is available on the August monthly meteor webpage of the SPA site, off the homepage at: http://snipurl.com/n2url Good luck & clear skies! NEW MAP HINTS AT VENUS' WET, VOLCANIC PAST ESA Venus Express has made the first map of Venus' southern hemisphere at infrared wavelengths. It hints that Venus may once have been more Earth-like, with both plate tectonics and a water ocean. The map comprises over a thousand individual images, recorded in 2006 and 2007. Because Venus is covered in clouds, normal cameras cannot see the surface, but Venus Express used a particular infrared wavelength at which it can see through them. Although radar systems have provided high-resolution maps of the surface, Venus Express is the first orbiting spacecraft to produce a map that hints at the chemical composition of the rocks. The new data are consistent with suspicions that the highland plateaus of Venus are ancient continents once surrounded by oceans and produced by past volcanic activity. The eight Russian landers of the 1970s and 1980s touched down away from the highlands and found only basalt-like rock beneath their landing pads. The new map shows that the rocks on the Phoebe and Alpha Regio plateaus are lighter in colour and look old in comparison with the majority of the planet. On Earth, such light-coloured rocks are usually granite and form continents. Granite is formed on the Earth when basalt rocks are driven deep below the surface by shifting tectonic plates. Water combines with the basalt to form granite, which is returned to the surface through volcanic eruptions. If there is granite on Venus, there must have been an ocean and plate tectonics in the past. Over time, Venus' water has been lost to space, but there might still be volcanic activity. Venus Express did not see any evidence of ongoing volcanic activity, but that does not rule it out. Venus is a big planet being heated by radioactive elements in its interior. It should have as much volcanic activity as the Earth. Indeed, some areas do appear to be composed of darker rock, which might be relatively recent volcanic flows. [Caveat by editor: not only does this item seem to build a very large castle on very thin air, since the only actual fact seems to be that when observed in the infrared some areas appear lighter than others, but it also seems doubtful whether the writer of the item understood what he was writing about. Granite is not made in the way that the article says; it exists as a magma, i.e. rock liquefied by heat at depth (nothing to do with water), and it is an igneous rock, that is to say it is emplaced, and solidifies, at considerable depth (NOT extruded at the surface volcanically) and is seen on the surface only when and if the overburden has been eroded away.] COMET-LIKE OBJECTS IN THE ASTEROID BELT MAY HAVE FORMED BEYOND NEPTUNE Southwest Research Institute Many of the objects found today in the asteroid belt between the orbits of Mars and Jupiter *may* have formed in the outer part of the Solar System, according to an international team of astronomers. The team made numerical simulations that seemed to show that some comet-like objects that might have existed in a disc outside the original orbits of the planets could have been scattered across the Solar System and into the asteroid belt during a violent phase of planetary evolution. Usually, the Solar System is considered a place of relative permanence, with any changes occurring gradually over hundreds of millions to billions of years. New models of planet formation indicate, however, that at specific times, the architecture of the Solar System underwent dramatic upheaval. In particular, it now seems possible that approximately 3.9 billion years ago, the giant planets -- Jupiter, Saturn, Uranus and Neptune -- rearranged themselves. Evidence for such an event was first identified in the samples returned from the Moon by the Apollo astronauts. They tell us about an ancient cataclysmic bombardment where large asteroids and comets rained down on the Moon. [That's not the same thing as the giant planets rearranging themselves! And the evidence for there having been a rain of large objects onto the Moon was there for all to see just by looking through a telescope -- nobody needed to go there in person for such evidence. Remarkably, however, until the Apollo era many soi-disant astronomers clung contumaciously to the idea that the impact craters were volcanic. -- ED.] Scientists [and others] now recognize that that event was not limited to the Moon, but also affected the Earth and many other Solar-System bodies. The same dynamical conditions that devastated the planets also led to the capture of some would-be impactors in the asteroid belt. Once in the asteroid belt, the embedded comet-like objects collided with both themselves and the asteroids. The model shows that comets are relatively easy to break up when hit by something, at least when compared to typical asteroids. It is unavoidable that some of the debris went on to land on asteroids, the Moon and the Earth. In fact, some of the leftovers may still be arriving today. The team believes that the similarities between some micrometeorites landing on Earth and comet samples returned by the Stardust mission are no accident. Some of the meteorites that once resided in the asteroid belt show signs that they were hit 3.5 to 3.9 billion years ago. The scientists try to make the case for the hits to have been made by captured comets or perhaps their fragments. If that is so, they are telling us the same sort of story as the lunar samples, namely that there was a lot of dynamic activity in the Solar System about 4 billion years ago. Overall, the main asteroid belt contains a surprising diversity of objects ranging from primitive ice/rock mixtures to igneous rocks. The standard model used to explain it assumes that most asteroids formed in place from a primordial disc that had radical chemical variations. The model now being advanced, however, prefers the observed diversity of the asteroid belt not to be a direct result of an intrinsic compositional variation of the proto-planetary disc. IMPACT ON JUPITER Spaceweather.com Evidence is mounting that something hit Jupiter just a few days ago. The impact site (a dark 'scar' in Jupiter's clouds) was discovered on July 19 by Australian amateur astronomer Anthony Wesley, and other astronomers quickly confirmed the find. Infrared images posted on Spaceweather.com are consistent with an asteroid or comet strike. The debris zone in Jupiter's clouds is itself as wide as a small planet, making it an easy object for amateur telescopes. NEW KIND OF ASTRONOMICAL OBJECT PROPOSED Science Daily An international team of astronomers suggests that lots of 'hypercompact stellar systems', appearing as faint star clusters, might be detected at optical wavelengths in our immediate cosmic environment. Some such objects may already have been observed in astronomical surveys. Hypercompact stellar systems could supposedly result if a supermassive black hole were violently ejected from a galaxy, following a merger with another galaxy that also contained such a hole. The evicted hole would rip stars from the galaxy as it was thrown out. The stars closest to the hole would move in tandem with the massive object and become a permanent record of the velocity at which the kick occurred. It would be possible to quantify the kick by measuring how fast the stars move round the hole. Only stars orbiting faster than the kick velocity could remain attached to the hole after the kick. They carry with them a kind of fossil record of the kick, even after the hole has slowed down. Finding such objects would be like discovering DNA from a long-extinct species. The best place to find hypercompact stellar systems, the authors argue, is in clusters of galaxies like the Coma and Virgo clusters, dense regions containing thousands of galaxies that have been merging for a long time. Merging galaxies result in interactions between their black holes (if they have any), a pre-requisite for the kicks. Even if a hole got kicked out of a galaxy, it would still be gravitationally bound to the whole cluster of galaxies and would still be there somewhere in the cluster. The scientists would like to think that hypercompact stellar systems may already have been seen and nobody has realized it. The objects would be easy to mistake for common star systems like globular clusters. The key signature of hypercompact stellar systems would be a high internal velocity dispersion. It would be detectable only by measuring the velocities of stars, which would be obliged to circulate far more quickly around a massive hole than in the self-gravity of a normal cluster. Determination of the velocity dispersion in a star cluster is a difficult measurement that would in most cases require a lot of observing time on a large telescope. At rare intervals, a hypercompact stellar system would make its presence known in a much more dramatic way, when one of the stars was tidally disrupted by the hole, causing a beacon-like flare. The only evidence of such floating black holes would be their armada of stars, with a display of stellar fireworks to signal their existence more obviously perhaps once in a million human lifetimes. FERMI FINDS GAMMA-RAY GALAXY SURPRISES NASA/Goddard Space Flight Center In 1991, just before the launch of the Compton gamma-ray observatory, astronomers knew of gamma-rays from exactly one galaxy beyond our own. To their surprise and delight, the satellite observed similar emissions from dozens of other galaxies. Now its successor, the Fermi space telescope, is filling in the picture with new finds of its own. Compton showed us that two classes of active galaxies emitted gamma rays -- blazars and radio galaxies. Fermi has found a third, and opened a new window in the field. Active galaxies are those with unusually bright centres that show evidence of particle acceleration to speeds approaching that of light itself. In 1943, astronomer Carl Seyfert described the first two types of active galaxy on the basis of the widths of their spectral lines, a measure of the rapidity of gas motion in their cores. Today, astronomers recognize many additional classes, but they now believe that the types represent the same essential phenomenon seen at different viewing angles. They think that, at the centre of each active galaxy, there is a black hole of upwards of a million times the Sun's mass. Through processes not understood, some of the matter headed for the black hole blasts outward in fast, oppositely-directed particle jets. For the most luminous active-galaxy classes -- blazars -- astronomers are looking right down the particle beam. Fermi has detected gamma-rays from a 'Seyfert 1' galaxy catalogued as PMN J0948+0022, which lies 5.5 billion light-years away in the constellation Sextans. Its spectrum shows narrow lines, which indicates slower gas motions which had been taken to argue against the presence of a particle jet. But, unlike most narrow-line Seyfert 1 galaxies, PMN J0948 also produces strong and variable radio emission which suggests that the galaxy is indeed producing such a jet. The gamma-rays seen by Fermi confirm the existence of particle acceleration to near the speed of light in that type of galaxy. Another case where Fermi sees something new involves NGC 1275, a massive Seyfert galaxy much closer to us. Also known as Perseus A, one of the sky's loudest radio sources, NGC 1275 lies at the centre of the Perseus cluster of galaxies, about 225 million light-years away. Compton's EGRET instrument did not detect gamma-rays from NGC 1275, although some were detected by another instrument sensitive to lower energies. But Fermi clearly shows the galaxy to be a gamma-ray source at the higher energies for which EGRET was designed, with a flux about seven times higher than the sensitivity threshold of EGRET. If NGC 1275 had been that bright when EGRET was operating, it would have been seen. The implied change in the galaxy's output suggests that its particle beam was either inactive or much weaker a decade ago. The time-scales of such changes indicate the maximum sizes of the emitting regions. If gamma-rays can switch on in NGC 1275 in ten years they must arise from a source no more than ten light-years across. That means that the radiation must come from a small region in the galaxy -- presumed to be near its supposed black hole -- rather than being emitted by hot gas throughout the galaxy. 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