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 Post subject: ENB No. 197 May 28 2006
PostPosted: Sun May 28, 2006 6:23 pm 
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The SOCIETY for POPULAR ASTRONOMY
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Electronic News Bulletin No. 197 2006 May 28
====================================================


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
using 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://popastro.c.topica.com/maaeQzOabqTR0cixLLVb/

Philip's, a publisher of astronomy books and planispheres for the
amateur astronomer, is sponsoring this bulletin. For information on
Philip's titles see the end of this bulletin.


POSSIBLE SW3 METEORS?
By Alastair McBeath, SPA Meteor Section Director

The famous fragmenting comet, 73P/Schwassmann-Wachmann 3 (SW3)
reaches perihelion around June 7, having been readily available for
British
observers since April till quite recently. After it first began breaking
up
violently at its 1995 return, model simulations suggested that May 2001
might bring some meteor activity due to the relative proximity of the
comet
then. As reported in ENB 71, nothing meteoric was detected due to SW3
that year, probably because the miss-distance to the nearest dust trail
from
the comet, laid down in 1941, was just too great, at 0.0026 astronomical
units (a.u.). Investigations then indicated that the next possible
meteor
activity from SW3 might happen in 2011. A paper in this February's
International Meteor Organization (IMO) journal WGN, by physicists
Rainer Arlt and Jeremie Vaubaillon, showed it was very unlikely that
anything meteoric would occur this spring from this source, according to
our current understanding of the comet and its dust trails, but
observers
were urged to be alert just in case.

Any potential SW3-associated meteors might be seen between 2006 May
28 to June 6, with the Earth passing closest to the main clump of dust
trails
laid down since 1801 around May 31 to June 2 (though these dates
should not be treated as the shower's potential maximum). The minimum
distance from the Earth to this set of trails is around 0.05 a.u.
however, a
lot further than in 2001. The theoretical radiant should be near RA
13h48m,
Dec +31 degrees, in south-east Canes Venatici, some degrees west (to
the right) of the middle of Bootes' kite-shape, well on-view all night.
For
visual observers, another key determinant is the SW3 meteors' apparent
velocity, which is exceptionally slow, at 16 km/sec (the range for
natural
meteors near the Earth is 11 to 72 km/sec). This is far slower than any
other shower meteors active at this time of year. New Moon was on May
27, waxing to first quarter on June 3, and pulls steeply up the sky for
the
UK, so will cause some problems additional to the twilight till after
midnight UT by June 1, and will not set until after 01h UT by June 6.

Reports of observations over this time, whether any suspected SW3
meteor activity is seen or not, should be sent to the SPA Meteor Section
as soon as possible afterwards. Suspected meteors from this source
should have their type labelled as "SW3", and along with all the usual
meteor details, be sure to give each meteor's apparent speed as
accurately
as possible. Those with access to suitable gnomonic-projection star maps
should plot all potential SW3 meteors for later analysis. More advice on
what to report and where to can be found via the meteor homepage at:
http://popastro.c.topica.com/maaeQzOabqTR1cixLLVb/ .


OTHER METEOR PROSPECTS THIS JUNE
By Alastair McBeath, SPA Meteor Section Director

Aside from the possibility of some SW3 meteors, most of the predicted
main meteor activity in twilight June happens in the daytime sky, so is
accessible only to radio meteor observers. There are two other potential
visual showers to check on however, the June Lyrids and June Bootids.

Searching for any June Lyrids between June 11-21, perhaps at maximum
on June 16, will benefit from some Moon-free skies in 2006, with
moonrise about ten minutes either side of midnight UT in the UK for the
suggested peak. Theoretically, the radiant then lies near Vega, at about
RA 18h32m, Dec +35 degrees, well on-view all night, though not all
previous results agreed with this location, and on-going investigations
by
the IMO have failed to find it in recent observations. At most, low
rates of
slow-medium speed meteors might be seen, but only with difficulty
because of the bright sky, albeit some of the earliest observations were
made from Britain back in the late 1960s. Very little has been seen of
the
shower since the late 1970s, except for some probable low rates
independently reported by several people in 1996. The source may be
active only in some years.

Another shower not present in most years is the June Bootids, whose
most recent activities since 1927 were in 1998 (Zenithal Hourly Rates -
ZHRs - of 50-100 for one night only), and on 2004 June 23, a date
before the shower was previously thought detectable, with ZHRs of
20-50. Till these latest returns, the previous outbursts had happened in
years when the shower's parent comet, 7P/Pons-Winnecke, had reached
perihelion, as it does next in 2008, but that close link does not apply
at
present, as the comet's orbit has shifted too far from the Earth's
(about
0.24 a.u.) to have a direct influence. Though no predictions are
in-force
yet for possible activity in 2006, watchers are urged to be alert for
any
unusual meteor rates from roughly June 21 to July 1, just in case. A
repeat
of the strong 1998 peak timing would be due within six hours of 14h UT
on June 27. Helpfully, new Moon on June 25 means this whole spell will
have no extra problems beyond the twilight for UK observers, assuming
skies are clear. Very slow meteors (14 km/sec) are characteristic,
emanating from a diffuse radiant in northern Bootes centred at RA
14h56m,
Dec +48 degrees, an area nicely available for watching throughout the
short nights. Note that despite the relative proximity of their
radiants, and
similarly low velocities, the June Bootids and possible SW3 meteors are
not related to one another.

Further notes on June's meteor sources can be found on the Meteor
Section's monthly webpage. Good luck, and clear skies!


ORIGIN OF NEPTUNE'S PRINCIPAL MOON
University of California

Triton is unique among the large moons in the Solar System because
it orbits Neptune in a retrograde orbit. It is unlikely to have
formed in that configuration and was probably captured from elsewhere.
It is being proposed that Triton was originally a member of a binary
pair of objects orbiting the Sun; it may have come from a binary very
similar to Pluto and its moon Charon. In a sufficiently close
encounter with a giant planet like Neptune, such a system can be
disrupted by the planet's gravitational forces, and the result can be
a permanent change of orbital companions. That mechanism, known as an
exchange reaction, could have delivered Triton to any of a variety of
different orbits around Neptune.

With properties similar to the planet Pluto and about 40% more
massive, Triton has an inclined, circular orbit that lies between a
group of small inner moons with prograde orbits and an outer group of
small satellites with both prograde and retrograde orbits. There are
other retrograde moons in the Solar System, including the small outer
moons of Jupiter and Saturn, but all are tiny compared to Triton (less
than a few thousandths of its mass) and have much larger and more
eccentric orbits about their parent planets.


TRIO OF EXTRA-SOLAR 'NEPTUNES'
ESO

Astronomers have discovered that a nearby star is host to three
Neptune-mass planets. During more than two years, the astronomers
studied HD 69830, a 6th-magnitude star slightly less massive than the
Sun and located 41 light-years away in the constellation Puppis.
Radial-velocity measurements showed the presence of three low-mass
companions orbiting their parent star in 8.67, 31.6 and 197 days. The
detected velocity variations are between 2 and 3 metres per second,
corresponding to about 9 km/h, the speed of a person jogging gently.
The newly found planets have minimum masses between 10 and 18 times
the mass of the Earth. Theoretical simulations favour a rocky
composition for the inner planet, and a rocky/gas structure for the
middle one; the outer planet has probably accreted some ice during its
formation, and may be made of a rocky/icy core surrounded by a quite
massive envelope. Calculations have also shown that the system is in
a dynamically stable configuration. The outer planet appears to be
located near the inner edge of the so-called habitable zone, where
liquid water can exist at the surface of rocky/icy bodies.


IDEA THAT DWARFS GALAXIES GAVE WAY TO GIANTS
Harvard-Smithsonian Center for Astrophysics

Some astronomers have been presenting their views on the early history
of the Universe. They think that fewer, larger galaxies, rather than
more numerous, smaller ones, dominated the Universe by the time it was
a billion years old; dwarf-galaxy formation almost stopped 'only' a
few hundred million years after the Big Bang. They see the sequence
of events as follows. Nearly 14 billion years ago, the Big Bang
filled the Universe with hot matter in the form of negatively charged
electrons and positively charged hydrogen and helium ions. As space
expanded and cooled, the electrons and ions combined to form neutral
atoms. Those atoms efficiently absorbed light, yielding a pervasive
dark fog throughout space, which lasted during an era dubbed the 'Dark
Ages'. Then the first generation of stars began clearing that fog by
bathing the Universe in ultraviolet radiation, which split the atoms
again into electrons and ions in a process called ionization. Since
the Big Bang created an ionized Universe that later became neutral,
that second phase of ionization by stars is known as the 'epoch of
re-ionization'. It took place in the first few hundred million years
of existence. During that epoch, gas was not only ionized but also
heated. Cool gas clumps together to form stars and galaxies more
easily than hot gas. The hotter the gas, the more massive a galactic
'seed' must be to attract enough matter to become a galaxy. Before
the epoch of reionization, galaxies containing 'only' 100 million
solar masses of material could form easily, but afterwards galaxies
required more than 10 billion solar masses of material to be
assembled. The astronomers admit that confirmation of the suppression
of dwarf-galaxy formation will require the use of a future generation
of more powerful telescopes.


ASTEROID RISK NEGLIGIBLE
New Scientist

An Earth impact by Apophis, an asteroid about 320 metres in diameter,
in 2036 has been shown to be even more unlikely than was previously
thought, following new radar measurements taken at the Arecibo
Observatory in Puerto Rico. On May 6 the speed of the asteroid was
found to be 6 millimetres per second different from expectation.
Astronomers say that it's just a small correction to the orbit that
propagates forward in time enough to reduce the probability of impact.
Before the observation, it was thought that in 2029 Apophis would
approach within about 5.86 Earth radii, but now the figure is 5.93
Earth radii. That seemingly small difference is significant, because
if the asteroid's path carries it through a certain 'keyhole' -- in
this case, a specific region of near-Earth space just 600 metres
across -- Apophis's orbit could be perturbed enough to put it on a
collision course with Earth in 2036. It is now even clearer than it
was previously that Apophis will miss the keyhole. The asteroid will
next be in a position favourable to radar observation in 2013.


FIVE-IMAGE GRAVITATIONAL LENS
ESA

The Hubble telescope has obtained the first picture of a distant
quasar 'lensed' into five images. The picture also includes a lot of
other lensed galaxies and even a supernova. Although plenty of
multiply-lensed quasars have been seen previously, this is the only
case so far in which multiple quasar images are produced by an entire
galaxy cluster acting as a gravitational lens. When the background
quasar's light passes through the cluster of galaxies on its way to
us, the light is bent by the space-warping gravity field in such a way
that five separate images of the object are produced surrounding the
cluster's centre. The cluster also creates a cobweb of images of
other distant galaxies gravitationally lensed into arcs.

The galaxy cluster creating the lens is seven billion light-years
away, and is seen when the Universe was half its present age. A
gravitational lens always has to produce an odd number of lensed
images, but one image is usually very weak and masked by the light of
the lensing object itself. In this case the fifth image is far enough
from the core of the central imaging galaxy to be visible, as well as
the other four. The galaxy hosting the background quasar is at a
distance of 10 billion light-years and can be seen in the image as
faint red arcs; it is the most highly magnified quasar host galaxy yet
seen. The Hubble picture also shows a large number of stretched arcs
that are images of galaxies lying behind the cluster, each of which is
split into multiple distorted images. By comparing the picture with
one taken a year earlier, the researchers also discovered a supernova
exploding in one of the cluster galaxies.




PHILIP'S RECENT TITLES:

Philip's The Sky at Night Volume 2 is the latest volume in Sir
Patrick Moore's series of essays written to accompany the television
series of the same name. It tracks developments in astronomy,
astrophysics and space exploration in the period from 2001 November
to 2005 March -- £9.99
Philip's Stargazing 2006 by Heather Couper and Nigel Henbest.
A month-by-month guide, now partly otiose - £6.99
Published last year:
Philip's Solar System Guide by Peter Grego describes how to observe
not only the planets but also the Moon, Sun, comets, meteors,
asteroids and other objects in our Solar System -- £9.99
Philip's Solar System Observer -- a pack for the amateur Solar-System
observer. It contains three items: Philip's Solar Observer's Guide,
Philip's Map of the Solar System and Philip's Solar System Phenomena
poster -- £12.99

For more information on those and other Philip's titles please visit
www.philips-maps.co.uk or call 020 7644 6935 for a catalogue.



Bulletin compiled by Clive Down

(c) 2006 the Society for Popular Astronomy


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