SPA ENB No. 251

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                 The SOCIETY for POPULAR ASTRONOMY
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         Electronic News Bulletin No. 251    2008 August 31
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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
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PERSEIDS 2008 PRELIMINARY REPORT
By Alastair McBeath, SPA Meteor Section Director

The unexpectedly late Perseid maximum on August 12-13 reported last
time has continued to attract most attention. Visual Zenithal Hourly
Rates (ZHRs) in the International Meteor Organization's (IMO's)
preliminary online overview from this maximum have gradually
decreased as more results have come through, and seem now to have
stabilized somewhat at between 110-130 from about 01:00-02:30 UT
on August 13. There may have been submaxima within this interval, with
ZHRs peaking at ~120-130 around 01:00, 01:30 and 02:00 UT within
that band, but the 30-minute "periodicity" these might suggest could
have been an artefact of the analysis, rather than a genuine effect. There
is little evidence now for an IMO visual results' peak on August 11-12
that could have been due to the predicted, early, strong maximum near
05:30 UT on August 12. However, ZHRs of 60+ were present from
roughly 01:00 UT on August 12 till at least 06:30 UT on August 13. The
"normal" maximum time on August 12, due between 11:30-14:00 UT,
was not well-observed, though ZHRs around then were ~80.

Poor weather over the British Isles has led to disappointing amounts of
usable visual data coming through to the Section, but the following
people have contributed something between helpful notes through to
fully-detailed observing reports from the Perseids to us so far,
including those on the SPA Forums, and the UK Weather World's
Space Weather Forum (address via the SPA Observing Forum's
"Perseids 2008" topic, at http://snipurl.com/3lisu  ): "Aristarchus"
(Greece), Andy Ball (Worcestershire), "blobrana" (no location given),
Jeff Brower (British Columbia, Canada; radio results), Willy Camps
(Belgium; radio results), "coldfieldboundary" (Belgium), Assistant
Meteor Director David Entwistle (Lancashire; radio results), Dave
Hancox (East Ayrshire), "Maddie" (Cornwall), Tony Markham
(Staffordshire), Conor McDonald (Co. Derry), Martin McKenna (Co.
Derry), Matthew Phipps (Hertfordshire), "Pro]v[etheus" (Surrey or
Derbyshire), Jonathan Shanklin (Cambridgeshire), Enrico Stomeo
(Italy; video results), Rich Taibi (Maryland, USA), and the Director
(Northumberland).  Most grateful thanks are due to all these people for
their efforts in observing and providing their results to us so quickly.
Naturally anyone else who still has data to submit is most welcome to
do so.

David Entwistle's assessment of the three sets of radio reports in now
has indicated a more likely strong, main peak between roughly 01:50-
02:05 UT on August 13. The graphs were often rather spiky, which could
suggest short-lived bursts of meteors from time to time, an effect which
lasted from maybe 01:00 or 01:30 UT through to 04:00 UT or a little
after, also on August 13. The possible minor early maximum around
05:20 UT on August 12, identified last time, still awaits confirmation.

From the SPA's visual results, Perseid ZHRs were between 15 to 30
from August 3-4 till 10-11, which concurred with the IMO details for the
same period, gradually rising with time. The limited data from August
11-12 and 12-13 though suggest ZHRs were between 60-75 on both
nights in what our observers recorded, and may actually have been a
little better overall on August 11-12. There were too few results for these
to be at all definitive, however. The sole set of video data was made
under trying conditions on August 12-13 especially, but things were far
from perfect even on August 11-12. After adjusting for sky conditions,
the Perseid video rate was overall somewhat higher on August 12-13,
and may have peaked especially around 01:51-01:56 UT, though this
is difficult to be sure of. Four of a hundred video Perseids were fireballs
on August 11-12, compared to none of 47 on 12-13, which could
suggest the Perseids were less bright overall on 12-13. While the early
casual reports reaching us suggested a visually bright showing of
Perseids on August 12-13, what full visual observations are available
now do not support this, aside from the video data. Perseid magnitudes
were no different on August 12-13 than other dates during the shower
in the details submitted now, with overall mean magnitudes corrected
to a limiting magnitude +6.5 sky for the Perseids and sporadics
respectively having fairly normal values of +2.6 and +3.5, with a
reasonably average 32% of Perseids leaving persistent trains.

Thus apart from the oddly late, strong maximum, the Perseids appear
to have been quite similar to what we would usually expect. Why there
was such a peak on August 12-13 remains a mystery. The timing, if not
the strength, was not far from when the "tertiary peak", seen only in IMO
data from 1997-1999, might have recurred. The theoretical timing for
this peak would have been approximately 3 to 4 hours before the
August 12-13 peak, at ~21:30 UT on August 12. Professional
meteoricist Jeremie Vaubaillon, currently at the California Institute of
Technology, has recently indicated the unanticipated maximum may
have resulted from material that had spread unusually far from a
meteoroid stream laid down at parent comet 109P/Swift-Tuttle's 441
AD perihelion passage (as also reported in IAU Electronic Telegram
1480 of August 26; my thanks to SPA Vice-President Robin Scagell for
forwarding a copy). The Earth was expected to miss this trail, albeit
not by too great a distance, around 23:34 UT on August 12. This is not
an especially close match to what was observed, but no more
convincing candidate dust trails have been suggested as yet.
Investigations continue.


RECENT FIREBALLS
By Alastair McBeath, SPA Meteor Section Director

Two fresh fireball sightings have come through since ENB 250, an
event in daylight on August 19-20 at about 19:10 UT, probably of at
least magnitude -9 as spotted from just off the Hampshire coast, and
another at 21:08 UT on August 22-23, of roughly magnitude -4/-5, seen
from Cornwall. As ever, sightings of fireballs made from the UK or
nearby are always welcomed by the Section. Information on what to
record was given last time (see: http://snipurl.com/3litt ).


METEORITE--ASTEROID LINK SOLVED
MIT

For the last few years, it has seemed puzzling that the vast majority
of asteroids that come near the Earth are of a type that matches only
a tiny fraction of meteorites.  Since meteorites are supposed mostly
to be pieces of asteroids, that discrepancy was hard to explain, but a
solution has now been suggested.  The smaller rocks that most often
fall to Earth, it seems, come straight in from the main asteroid belt
between Mars and Jupiter, rather than from the 'near-Earth asteroid'
population.

In the main belt, the population is much more varied, and approximates
to the mix of types that is found among meteorites.  But why would the
things that most frequently hit us match that distant population
better than they match the objects that are right in our neighbourhood?
That's where the idea emerged of a fast track all the way from the
main belt to landing on the Earth's surface.  The fast track, it turns
out, is caused by a process called the Yarkovsky effect, which was
discovered long ago but whose significance has only recently been
recognized.

The Yarkovsky effect causes asteroids to change their orbits as a
result of absorbing the Sun's heat on one side and radiating it in a
different direction owing to their rotation.  That produces a small
thrust that operates continuously in one direction and can in time
alter the object's path.  Because the surface area of an object is
proportional to the square of its size whereas its mass goes as the
cube, the effect acts most strongly on the smallest objects.  Thus,
for rocks of boulder size and smaller, typical of meteorites, the
Yarkovsky effect can play a major role, moving them from anywhere in
the asteroid belt on to paths that can intersect the Earth.  For
larger asteroids a kilometre or so across, about which some people
have taken to worrying as potential threats to the Earth, the effect
is too weak to alter their orbits at all quickly.


'INNER OORT CLOUD' OBJECT?
Sloan Digital Sky Survey

A minor planet with the prosaic name 2006 SQ372 was discovered by the
Sloan Digital Sky Survey (SDSS) through the application of a searching
algorithm to data that were actually taken to look for supernovae --
an area of sky of about 200 square degrees was observed every clear
night in the autumns of 2005, 2006, and 2007.  2006 SQ372 has an
unusual orbit, an ellipse that is four times longer than it is wide;
the object is presently slightly closer to us than Neptune but is
beginning the outward leg of a 22,500-year journey that will take it
to a distance of something like 1600 times the distance from the Earth
to the Sun.  The only known object with a comparable orbit is Sedna --
a distant, Pluto-like dwarf planet discovered in 2003 -- but 2006
SQ372's orbit takes it more than half as far again from the Sun, and
its orbital period is nearly twice as long.

The new object is much smaller than Sedna, probably 30-60 miles across
instead of nearly 1,000.  It may be cometary in nature, but it never
gets close enough to the Sun to develop a long, bright tail of
evaporated gas and dust.  Even at its most distant turning point, 2006
SQ372 will be ten times closer to the Sun than the supposed main body
of the supposed Oort Cloud.  The existence of an 'inner' Oort cloud
has been suggested for many years, but SQ372 and perhaps Sedna are the
first objects found that could be deemed to have originated there.
2006 SQ372 was bright enough to find with the SDSS only because it is
near its closest approach to the Sun.  Since the SDSS survey observed
less than 1% of the sky, there could well be many similar objects to
be discovered.


XMM-NEWTON FINDS MASSIVE CLUSTER OF GALAXIES
ESA

The orbiting X-ray observatory XMM-Newton has discovered a very
massive cluster of galaxies in the distant Universe.  The newly
discovered cluster, called 2XMM J083026+524133, is estimated to
contain as much mass as a thousand large galaxies; much of it is in
the form of 100-million-degree gas.  It was first observed by chance
as XMM-Newton was studying another celestial object, and was placed
in a catalogue for a future follow-up.  Astronomers discovered the
cluster as they were analysing the catalogue, which is based on 3,500
X-ray images which together cover about 1% of the sky, and contains
more than 190,000 individual X-ray sources.  The team was looking for
extended patches of X-rays that could either be nearby galaxies or
distant clusters of galaxies.  J083026+524133 stood out because it was
so bright.  Images made in visible light by the Sloan Digital Sky
Survey did not show any obvious nearby galaxy in that location, but
when the team obtained a deep exposure with the Large Binocular
Telescope in Arizona they found a cluster of galaxies calculated to be
at a distance of 7.7 billion light-years.


DO GALAXIES HAVE A MINIMUM MASS?
New Scientist

The Milky Way's galactic companions all seem to have much the same
amount of mass in their cores.  A collection of at least 22 dwarf
galaxies orbits the Milky Way; the brightnesses range over a factor of
10,000.  Astronomers analysed the motions of stars in the innermost
1000 light-years, where they might expect common properties to emerge
if any existed, in 18 of the dwarf galaxies.  They measured the
velocities of hundreds of stars in orbit around the galaxies' centres,
which allowed them to calculate the masses of the galaxies' cores.
Surprisingly, the masses all came out about the same -- roughly 10
million solar masses.  What the 'New Scientist' is claiming is that
most of the mass is dark matter, and that the dimmest galaxies had
10,000 times more dark matter than visible matter.  It says that that
is an unusual ratio, and by way of an example says that the Milky Way,
for example, contains only roughly 10 times as much dark matter as
ordinary matter.  The similarity of the masses hints that the dwarf
galaxies must have at least that much mass in order to form.

The 'New Scientist' article goes on to speculate about the nature of
dark matter and to draw various conclusions that do not necessarily
follow from the observational facts.  There is a misunderstanding
whereby differences in the ratio of the mass to the luminosity between
different objects are assigned (as they are in the paragraph above) to
ratios of 'dark matter' to 'visible matter'.  A (normally unspoken)
assumption is that the proper ratio of mass to light is that exhibited
by the Sun.  If the Milky Way has ten times as much total mass in
relation to its total brightness it is said to have ten times as much
dark matter as visible matter.  But that is not a sensible way of
describing the situation.  A star of early-M spectral type is about
five magnitudes (a factor of 100) fainter than the Sun but has about
half as much mass, so it has a mass-to-light ratio of 50 in solar
units.  It does not seem helpful to conclude, as by analogy with the
assertions above one would be obliged to do, that an early-M star has
fifty times as much dark matter as visible matter, or that there is
anything mysterious about the former.


FIRST LIGHT FOR THE FERMI SPACE TELESCOPE
NASA

NASA has announced that its new gamma-ray telescope, formerly known as
GLAST, has passed its orbital checks and has been formally renamed the
'Fermi Gamma-ray Space Telescope' in honour of Prof. Enrico Fermi
(1901-1954), a pioneer in high-energy physics.  Scientists expect
Fermi, by observing gamma-rays, to make many discoveries involving
highly energetic sources such as pulsars and black holes.  Since the
spacecraft's launch on June 11, scientists have tested and calibrated
its two instruments, the Large-Area Telescope (LAT) and the GLAST
Burst Monitor (GBM).

The large-area telescope has obtained in four days an all-sky image
similar to the one that the now-defunct Compton gamma-ray observatory
took years of observations to produce.  It scans the entire sky every
three hours when operating in survey mode, which will occupy most of
the telescope's observing time during the first year of operations, to
allow scientists to monitor rapid changes.  The telescope is sensitive
to photons with energies ranging from 20 MeV (million electron volts)
to over 300,000 MeV. The high end of the range, which corresponds to
energies more than 5 million times greater than dental X-rays, is
little explored.  The spacecraft's secondary instrument, the GBM,
detected 31 explosions of the sort known as gamma-ray bursts in its
first month of operations.  The GBM is sensitive to less-energetic
gamma-rays than the LAT, giving it a complementary view of the broad
gamma-ray spectrum.


The SPA Electronic News Bulletins are sponsored by the Open University.

Bulletin compiled by Clive Down


(c) 2008 the Society for Popular Astronomy