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Wednesday, 27 October 2010

SPA ENB No. 297

                 The SOCIETY for POPULAR ASTRONOMY
         Electronic News Bulletin No. 297   2010 October 24
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
By Richard Bailey, SPA Solar Section Director
The Solar Section's September Report is now on view on the SPA web
site at   .  Two pages of photographs and
drawings by Section members are included in the report.
The Hubble telescope has returned images of a suspected asteroid
collision.  They show a bizarre X-shaped object at the head of a
comet-like trail of material.  Last January, astronomers began using
Hubble to track the object, named P/2010 A2.  At first they thought
they had witnessed a fresh asteroid collision, but then were surprised
to discover that the collision occurred in early 2009.  They supposed
that the debris field would expand dramatically, as if from an
explosion, but instead they found the object to be expanding very
P/2010 A2 is located in the asteroid belt, where it has been estimated
that there may be about one collision a year between modest-sized
asteroids.  Catching asteroids in the act of colliding, however, is
difficult because large impacts are rare while small ones, such as the
one that probably produced P/2010 A2, are exceedingly faint.  The
Lincoln Near-Earth Research (LINEAR) Program Sky Survey first observed
the object's comet-like tail last January, and indeed some astronomers
thought it might be a comet.  But the Hubble images revealed a point-
like object, with a long, flowing dust tail behind a never-before-seen
X pattern.  Estimates based on the brightnesses of the point source
and the tail suggest that the asteroid is something like 120 metres
across and the tail contains enough dust to make a ball 20 metres in
diameter.  It is suggested that a smaller rock, perhaps 3 to 5 metres
across, collided with the larger one, perhaps at a speed of about 5
km/s, smashing and vaporizing the small asteroid and stripping
material from the larger one.  There is no explanation so far for the
X shape.
Ohio State University
An explosion of a giant star in a galaxy some 3 billion light-years
away recently appeared to be muted by surrounding dust rather than
appearing as a typical supernova.  It was found in 2007 August in the
course of a survey with the Spitzer space telescope, which observes in
the infrared.  Astronomers would not expect to find a supernova in
that way, since supernovae release much of their energy as light,
rather than in the infrared.  Enormous heat flared from the object for
a little over six months, then faded away in 2008 March -- a clue that
the object was a supernova.  Over the six months, it released more
energy than our Sun will produce in its entire existence.  If the
source were a supernova, the extreme amount of energy it emitted would
qualify it as a big one, or 'hypernova'.  The temperature of the
object was around 700°C -- only a little hotter than the surface of
Venus.  Astronomers decided that there must have been, around the
star, thick clouds of dust that absorbed so much light energy and
dissipated it as heat, muffling as it were the explosion.  They think
that the star was probably at least 50 times the mass of our Sun; such
stars typically eject clouds of dust as they near the end of their
existence.  It appears that the star must have made at least two such
ejections -- one about 300 years before the supernova, and one only
about 4 years before it.  The dust and gas from both ejections formed
slowly expanding shells.  Astronomers believe the outer shell to be
nearly opaque, so it absorbed any light energy that passed through the
inner shell and converted it to heat -- which is why the supernova
appeared in the Spitzer survey as a hot dust cloud.  It is suggested
that we might see the star brighten a decade or so from now,when the
shock wave from the exploding star has pushed the inner dust shell
into the outer one.
Science Daily
At the South Pole there is a telescope that is making a survey of 2500
square degrees of the sky at millimetre wavelengths -- between the
infrared and radio regimes.  The people concerned with it have been
reviewing the results from the first 200 square degrees, and have been
using the 'Sunyaev-Zel'dovich effect' to identify massive clusters of
galaxies.  That effect is a small distortion of the cosmic microwave
background (a pervasive all-sky glow left over from the Big
Bang).  Such distortions are created as background radiation passes
through a large galaxy cluster.  Surveying for that effect works just
as well for very distant clusters as for 'nearby' ones, and provides
good estimates of the masses of clusters.  One particular cluster,
found at a distance of about 7 billion light-years in the
constellation Pictor, appears to have a mass of about 8 x 10 to the 14
Suns, nearly as much as the well-known Coma cluster; it is of course
seen as it was all that time ago, and could be expected to have grown
much more by now.  Once the cluster was found, the team studied it
with the infrared camera on the Spitzer space telescope to resolve
the individual galaxies within the cluster; then radial-velocity
measurements with the Magellan telescopes in Chile gave the redshift
(z) as 1.07 and enabled the distance to be estimated, as well as
confirming the mass of the cluster from the velocity dispersion of the
The first galaxies formed well before the Universe was one billion
years old and were much smaller than some of the systems (including
the Milky Way) that we see today.  One important growth mechanism is
certainly the collision and merging of galaxies, but some observations
made with ESO's Very Large Telescope indicate that young galaxies
could also grow by accreting hydrogen and helium gas from their
surroundings.  The observers began by selecting three very distant
galaxies, being careful to choose specimens that appeared undisturbed
by interactions with other galaxies.  The selected galaxies were very
regular, smoothly rotating discs, similar to the Milky Way, and they
were seen about two billion years after the Big Bang (at a redshift of
about three).  In galaxies in the modern Universe the heavy elements
are more abundant close to the centre.  But in all three of the
selected distant galaxies the VLT found that, close to the centre,
there was a patch with fewer heavy elements but showing vigorous
star-formation, suggesting that the material fuelling the star-
formation was coming from the surrounding pristine gas that was low in
heavy elements.
Astronomers using the VLT have measured the redshift, 8.6, of the most
remote galaxy so far identified.  The galaxy is seen as it was when the
Universe was only about 600 million years old (a redshift of 8.6).
It is difficult to observe such distant objects -- not only do they
appear very faint, but the redshift moves most of their light far into
the infrared.  Moreover, at that early time, less than a billion years
after the Big Bang, the Universe was not fully transparent -- much of
it was filled with a hydrogen fog that absorbed the fierce ultraviolet
light from young galaxies.  The period when the fog was still being
cleared by that ultraviolet light is known as the era of reionisation.
Despite such difficulties, the Hubble telescope discovered last year
several candidate objects that were thought to be galaxies shining in
the era of reionisation.  Confirming the redshifts of such objects can
reliably be done only by spectroscopy with very large ground-based
telescopes; in this case it required a total integration time of 16
hours on the VLT. 
Bulletin compiled by Clive Down
(c) 2010 the Society for Popular Astronomy
Good Clear Skies
Colin James Watling
Real Astronomer and head of the Comet section for LYRA (Lowestoft and Great Yarmouth Regional Astronomers) also head of K.A.G (Kessingland Astronomy Group) and Navigator (Astrogator) of the Stars (Fieldwork)

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