WADHURST ASTRONOMICAL SOCIETY

DECEMBER NEWSLETTER 2008

INDEX: MEETINGS, OTHER NEWS, CONTACTS

NOVEMBER MEETING

            Phil Berry introduced the meeting by referring back to the two videos we watched last month, asking how members found them.  The general view was they were presented in an easy way to understand a difficult subject of Einstein’s theories.
            In reply to a question about recommending a good book in understanding Astronomy in general, Phil has particularly recommended “Universe” (paperback) by Roger A. Freedman and William J. Kaufmann.  There are 800 pages and the publisher is W. H. Freeman & Co Ltd – 8th edition (30th July 2007).  ISBN-9780716795643 available through Amazon or Waterstones book shops for £39.99.
            It was pointed out that subscription for the Society’s next session for 2009 remains unchanged at £15 per member and £20 for two members within the same family, and falls due on the first of January 2009.
            Boards were on the table, one for those requiring help in any aspect of astronomy with the hope that another member may be able to assist or at least point in the right direction for and answer, and the other asking for suggestions for talks.
            Phil then introduced John Murrell, our speaker for the evening.

Gamma Ray Bursts
by John Murrell

            John Murrell is a member of the Croydon Astronomical Society and he had travelled from Corydon to be with us tonight.  He has visited us before when he talked to us about “Virtual Astronomy”.  This time John was talking about the incredible emissions from Gamma Ray Bursters.
            Before he began he briefly reminded us of some basic physics such as the Inverse Square Law, Energy (1 joule = 10 million ergs, and it takes 340k calories to boil a litre of water), distance where 1 parsec = 3.26 light years.  He also defined “isotropic” as something exhibiting the same properties in all directions.  Wavelength = c/frequency. (c = speed of light in a vacuum)
            With this out of the way, John turned to gamma rays, saying that they had first been discovered by Villand, a French chemist and physicist, in 1900.  Unlike Cosmic Rays, which are particles, Gamma Rays are electromagnetic waves with a wavelength less than 10-10 metres compared with red laser light with a wavelength of about 700 nanometres.  They are very penetrating, high energy waves.
            Gamma rays can be occur from Meson Decay, Synchrotron generation and extreme temperatures when material falls into a black hole.  They can also result from atomic decay and can even result from thunderstorms on Earth.
            They can be detected using a Geiger counter or a proportional counter, both using an inert gas.  A scintillation detector uses a crystal or liquid that fluoresces when struck by ionising radiation.
            A spark detector produces a spark between two plates when a gamma ray passes through a layer of foil plates.
            John told us that the first satellite gamma ray detectors were used by the USA to detect any gamma radiation that could have been produced by the testing of an atom bomb by a foreign power during the partial Test Ban of the 60s.
            Pairs of VELA satellites would be used to find the coordinates of any source, but VELA detectors suffered from very poor resolution and were guilty of detecting random bursts that were suspected as being false warnings.
            Progressively more and more satellites were being launched that carried improved means of detecting gamma rays, such as WIND, Ulysses, Compton and more recently BEEPOSAX.  Gamma rays are more readily detected in space because many are stopped by our atmosphere, which is just as well because they can destroy living cells.  But never-the-less many do get through from gamma ray bursters.
            Now John introduced us to the Milky Way, saying that it is very flat.  At first, gamma rays were thought to be being radiated from sources in our own galaxy, but when data from the satellites were analysed, it was found that they were isotropic, meaning they were arriving from every direction and their intensity suggested great distances were involved.
            We were told that gamma ray bursts usually last a very short time, from less than a second to several seconds.  Following the detection of a gamma ray burst by instruments on a satellite, the source coordinates were investigated using earth bound instruments but nothing could be found.
            Over time, data was being collected and Gamma Ray Bursts, (GRBs), and it was necessary to find a way of recoding them, so a system was devised such that a burst GRB970228 represented an event in 1997 in February, (month 02) on the 28th day.
            We were shown the resultant images from ground based ROTSE (RObotic Transient Search Experiment) which combined four slightly displaced 0.45 metre f1.9 telescopes each with a field of view of 1.85o.   The output was being continually recorded so that now we could replay them and see the appearance of gamma ray burst GRB990123 before it faded, leaving nothing to be observed by any instruments.
            It has subsequently been suggested that the source is about 9 billion light years away!
            Also, short bursts radiate lower energy, but longer bursts can radiate energy in the order of 1047 joules.
            They are believed to be the result of the collapse of a very large star, and that we can only detect them because the narrow beam radiated points directly towards us.  On average, one is detected per day and Martin Rees, the Astronomer Royal, thinks that if the beam is about 100 wide it suggests that there are in fact 500 to 1,000 GRBs occurring per day.
            John told us of one remarkable GRB, GRB080319B (B making it the second event that day).  This seemed to consist of a narrow beam at the centre of a wider, less intense beam.  At maximum, the burst was optically as bright as magnitude +5.3 at a distance of 7.5 billion light years, making it by far the furthest object capable of being seen by the naked eye.
            Now we looked at the structure of Gamma Ray Bursters and first John talked about metalicity.  A Nova will create elements up to iron, but it takes a Supernova to produce higher elements.  The biggest explosions known are called hypernovae where a very large star throws off its outer layers and then the core collapses to a neutron star, emitting huge amounts of gamma rays in a very narrow beam.
            Are we safe?  John said these massive stars are 100 times bigger than our own Sun.
            Then spoke of a star binary – Wolf Rayet 104 - at a distance of 8,000 light years.  It is a candidate for becoming a supernova at some time.  What is worrying is that its axis is within 12o of the Earth…!  The effect of such an event on life on earth could be catastrophic.
            Finally John ended his excellent talk with a video of a thunderstorm just over the horizon, and from the tops of the thunderstorm, following lightening strikes we could see Red Sprites radiating out into space and from these gamma rays had been detected.
            For any member wishing to know more John suggests searching Google for Gamma Ray Bursts.  I have looked and there is a vast amount at all levels of comprehension.  He also recommends “Flash!: The Hunt for the Biggest Explosions in the Universe” by Govert Schilling and published by Cambridge University Press.  Amazon has it on sale at £17.82 instead of £27.00 RRP.  This covers the history up to the year 2000 in a lot of detail but John says it is presented in not too technical a way.

DECEMBER MEETING

            Wednesday 10th December 2008 Please note that as it is December, the Society meets on the Second Wednesday instead of the third as in other months.
            Paul Treadaway, who is a well known member of the Society gives a talk about “How Stars Work”.
            Members will remember the talk Paul gave us last year and will recall the intriguing scenarios he posed.
            Christmas is with us at the end of the month so there will be plenty of mince pies to be eaten during the meeting.
            The meeting begins at 1930 although members are invited to arrive anytime after 1900 as this is a good time to exchange ideas and discuss problems.
            The venue as always is in the Upper Room of the Methodist Church at the east end of Wadhurst Lower High Street, opposite the entrance to Uplands College.  (For those with SatNav – the post code is TN5  6AT)

FUTURE MEETINGS

            Wednesday 21st January 2009 This is the Annual General Meeting.  That should not take too long and then Phil Berry gives a fascinating talk, bringing us up to date with the impressive progress he has achieved with his observatory following on from his previous talk earlier in 2008.  This time Phil calls his talk “The Further Trials and Tribulations of an Amateur Astronomer”.
            Wednesday 18th February 2009 This is the date of the February meeting.
            Wednesday 18th March 2009 This is the date of the March meeting.

OTHER NEWS AND INFORMATION

Amateur imaging of Pluto-Charon separation

I have just learnt from the European Astrosky Network that Italian amateur astronomer, Antonello Medugno has managed to image the separation between Pluto and Charon.  I believe this is the first time this has been achieved by an amateur
The separation between Pluto and Charon is only 0.7" arcsec.  Antonello used his Meade 14" LX200GPS with about 9 metre focal length and a R+Ir filter, Starlight Xpress SXV-H9 CCD, and a median of 21 frames exposed for 6 seconds.  The image can be seen at:
www.astroimaging.it
and selecting “Couple Pluto-Charon”.  It is worth a look if you get a chance
Ed.

SKY NOTES FOR DECEMBER

Planets

Mercury is poorly placed for observation this month but may possibly be glimpsed low down in the south west after sunset. It will be close to the crescent Moon and Jupiter on the evening of the 29th.

Venus is a bright evening object in the west at magnitude -4.1 by mid month although it is still fairly low in the sky.

Mars is not suitably placed for observation this month due to a solar conjunction on the 5th.

Jupiter is still an early evening object in Sagittarius at magnitude -2.0, although by the end of the month it will be setting only an hour and a half after the Sun.

Saturn currently lies in the constellation of Leo at magnitude 1.0. At the start of the month it rises around midnight but by the end it rises at 22.30 with its rings all the time becoming more edge on as seen from Earth.                                                                                                                                                                                                                                 

Lunar Occultations
As usual in the table I’ve only included events for stars down to around magnitude 7.5 that occur before midnight. DD = disappearance at the dark limb and RD = re-appearance at the dark limb. Times are all GMT.

Lunar Occultation of Venus
In the afternoon of December 1st Venus will be occulted by the dark limb of a thin crescent Moon and will later emerge on the bright limb. The disappearance should be a fairly easy event as Venus will be at magnitude -4.1 and the Sun will be setting around the time that the planet is occulted. One thing to bear in mind is that the Moon will be quite low in the sky (only 14° high at disappearance) so a good south western horizon is required.

Phases of the Moon for December

Meteors

Geminids The Geminid meteor shower is active from December 7th through to the 16th with maximum occurring on the night of the 13th when the ZHR (zenithal hourly rate) can be expected to be above 100. The bad news is that the Moon will have been full the day before and will be above the horizon from late afternoon and remain so all night. Next year the Moon will pose no such problems.

Ursids The Ursids is another December shower, and one that may be worth watching. The radiant lies in the constellation of Ursa Minor (and is therefore circumpolar) and on average is expected to produce a ZHR of around 10, although recent years have seen this rise to 30 to 50. Activity lasts from December 17th to the 25th with maximum occurring on the 22nd when the waning crescent Moon will not rise until the early hours of the morning (03.20).

ISS
There are a large number of passes of the ISS this month as seen from Wadhurst but many are low in the sky or occur in the early hours of the morning. I have only included those that are the brightest, attain reasonable altitude and occur before midnight. The information given is for when the ISS is at maximum altitude, so it is best to look some minutes before this time. Full details of all passes can be found at: - www.heavens-above.com  Times are all GMT.

Iridium Flares
For some reason this month all the flares seem to occur low down in the sky. However out of the 74 events visible from Wadhurst I’ve picked the best in terms of brightness, altitude and time of day. If you wish to see a complete list, go to www.heavens-above.com

The Night Sky
This is the time of year when we realise that winter is coming as the most well known constellations that we associate with cold, crisp and frosty nights make an appearance. Taurus with its two naked eye clusters (Pleiades and Hyades) is high in the sky with the Pleiades in particular striking in a pair of binoculars. The square of Pegasus (which is a misnomer because the top left star is in fact in Andromeda) is moving towards the western horizon. Andromeda itself is still fairly high and on a reasonably clear night the giant spiral galaxy M31 can be picked out with the naked eye. Its magnitude is around 3.5 and it lies at a distance of some three million light years being about twice as massive as our own galaxy.  In binoculars it is just possible to see M32 which is a satellite galaxy of M31. If you continue the line through Andromeda eastwards you come to Perseus. There you will find an open cluster M34 visible to the naked eye but better seen in a pair of binoculars. Towards the east Orion has risen, with his two dogs Canis Major (containing Sirius – the brightest star in the sky) and Canis Minor not far behind. Gemini also is just coming into view. In the west the summer triangle has gone with the exception of Deneb in the constellation of Cygnus. It can be rewarding to locate and identify some of the fainter constellations that lie below Pegasus and Andromeda. For example Pisces, Aries, Triangulum and Cetus all have stars that are not particularly striking but between them these constellations cover a considerable area of the sky.

Advance Warning for 2009

Jan 7th – the Moon passes through the Pleiades cluster.
Jan 7th – Alcyone (the brightest of the Pleiades at magnitude 2.9) undergoes a graze      occultation as seen from the South East.
Early April to Early May – best evening views of Mercury
July 18th – the Moon passes through the Pleiades cluster.
Late September to Mid October – best morning views of Mercury.
October 21st – there is a Lunar occultation of Antares (magnitude 1.2).
December 14th – maximum of the Geminid meteor shower.
December 31st – partial Lunar eclipse.

There are also three penumbral eclipses of the Moon in 2009. Unfortunately the change in brightness as the Moon passes through the penumbral shadow (as opposed to the umbral shadow for a partial or total eclipse) is something that could easily me missed.

NASA SPACE PLACE

What Happened to Comet Holmes?
by Dr. Tony Phillips

            One year after Comet 17P/Holmes shocked onlookers by exploding in the night sky, researchers are beginning to understand what happened. 
            “We believe that a cavern full of ice, located as much as 100 meters beneath the crust of the comet’s nucleus, underwent a change of phase,” says Bill Reach of NASA's Spitzer Science Centre at the California Institute of Technology.  “Amorphous ice turned into crystalline ice” and, in the transition, released enough heat to cause Holmes to blow its top. 
            Anyone watching the sky in October 2007 will remember how the comet brightened a million-fold to naked-eye visibility.  It looked more like a planet than a comet—strangely spherical and utterly lacking a tail.  By November 2007, the expanding dust cloud was larger than Jupiter itself, and people were noticing it from brightly-lit cities. 
            Knowing that infrared telescopes are particularly sensitive to the warm glow of comet dust, Reach and colleague Jeremie Vaubaillon, also of Caltech, applied for observing time on the Spitzer Space Telescope—and they got it.  “We used Spitzer to observe Comet Holmes in November and again in February and March 2008,” says Reach.
            The infrared glow of the expanding dust cloud told the investigators how much mass was involved and how fast the material was moving. “The energy of the blast was about 1014 joules and the total mass was of order 1010 kg.” In other words, Holmes exploded like 24 kilotons of TNT and ejected 10 million metric tons of dust and gas into space. 
            These astonishing numbers are best explained by a subterranean cavern of phase-changing ice, Reach believes. “The mass and energy are in the right ballpark,” he says, and it also explains why Comet Holmes is a “repeat exploder.”
            Another explosion was observed in 1892.  It was a lesser blast than the 2007 event, but enough to attract the attention of American astronomer Edwin Holmes, who discovered the comet when it suddenly brightened.  Two explosions (1892, 2007) would require two caverns.  That’s no problem because comets are notoriously porous and lumpy.  In fact, there are probably more than two caverns, which would mean Comet Holmes is poised to explode again.
            When?
            “The astronomer who can answer that question will be famous!” laughs Vaubaillon.
            “No one knows what triggered the phase change,” says Reach.  He speculates that maybe a comet-quake sent seismic waves echoing through the comet’s caverns, compressing the ice and changing its form.  Or a meteoroid might have penetrated the comet’s crust and set events in motion that way.  “It’s still a mystery.”
            But not as much as it used to be.

See more Spitzer images of comets and other heavenly objects at:
www.spitzer.caltech.edu
            Kids and grownups can challenge their spatial reasoning powers by solving Spitzer infrared “Slyder” puzzles at
http://spaceplace.nasa.gov/en/kids/spitzer/slyder

            This article was provided by the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration.

FEDERATION OF ASTRONOMICAL SOCIETIES NEWSLETTER

            The new issue of the Federation of Astronomical Societies will be available for each member at the December meeting.

SAGAS web-site                                                                        www.sagasonline.org.uk

Any material for inclusion in the January 2009 letter should be with the Editor by 28th December 2008

CONTACTS

Chairman   John Vale-Taylor 

Phil Berry  01892 783544

Treasurer  Mike Wyles  01892 542863

Newsletter Editor  Geoff Rathbone  01959 524727

SAGAS                                          

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