It is currently Fri Dec 15, 2017 2:05 pm

Post new topic Reply to topic  [ 1 post ] 
Author Message
PostPosted: Wed Apr 29, 2009 9:51 pm 

Joined: Fri Dec 03, 2004 11:24 am
Posts: 4358
Location: Greenwich, London
Electronic News Bulletin No. 265 2009 April 12

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

Astronomica are proud to be sponsors of the SPA Electronic News

By Alastair McBeath, SPA Meteor Section Director

Determined not to be outdone by March, early April produced at least
one brilliant meteor for the British Isles, around 23:32 UT on April 4-5.
Three probable sightings from England have reached the Section so
far on it (there is some doubt over the date and timing of one, which
may have been partly in BST, not UT), from Merseyside,
Worcestershire and Buckinghamshire. However, Assistant Meteor
Director David Entwistle noted that a number of media sources had
picked-up on comments from Astronomy Ireland officers, which
suggested there had been more sightings from Ireland and Northern
Ireland. The media sources were not clear always whether this object
may have been on April 4-5 or 5-6, unfortunately. The fireball, if there
was just the one event, seems likely to have passed over part of
Ireland, but details remain sketchy on exactly where. The British
sightings were of an object to the west, which would be consistent
with this at least. The Astronomy Ireland website, at , may have more by now. For the initial
media comments, see for instance this BBC News page: .

Additional observations of this fireball, or any others spotted from
Britain and points adjacent (a fireball is any meteor of at least
magnitude -3), would be welcomed by the Meteor Section. See the
"Making and Reporting Fireball Observations" SPA webpage, at: , for information and a report form.

By Alastair McBeath, SPA Meteor Section Director

Regular Section correspondent and observer Jeff Brower in Canada
has kindly forwarded more news on the meteorites apparently fallen
from the predicted asteroidal fireball of October 7, 2008, following the
NASA press conference on March 25 (noted in the previous ENB).
Two field expeditions led by Dr Muawia Shaddad of Khartoum
University were carried out in December 2008, from the 5th to the
8th, and the 25th to the 30th, during which a total of 47 meteorites
were recovered, weighing in total 3.95 kg. The objects formed a
linear strewnfield about 29 km long, and the meteorites showed a
degree of mass-sorting within this zone which is typically found with
such meteorite showers. The location and nature of the zone certainly
strengthened greatly the likelihood that the meteorites did result from
this fireball, along with the freshness of the recovered material. The
find has been named "Almahata Sitta", Arabic for "Station Six", the
nearest man-made landmark to the remote fall region in Sudan, and
the objects themselves have been classified as ureilites, the unusual
achondrite meteorite class of very-coarse-grained stony bodies.
From earthly comparisons, such coarse-grained material likely
cooled only very slowly from a molten state originally, suggestive of
deep burial, or some similar insulation, within a larger planetary or
minor planetary body. Their chemistry is much as expected from
other ureilite examples, but their physical make-up is rather different,
with among other features, a more open, porous, structure than
expected. Some of these pores showed signs that olivine crystals in
them had possibly grown from a vapour, and cooled quickly, and it
has been suggested this crystal-growth pattern may have been
produced by a substantial impact on the original body, perhaps that
which freed asteroid 2008 TC3 and sent it on its collision-course
with Earth. My grateful thanks go to Jeff for much of this information.

By Alastair McBeath, SPA Meteor Section Director

The Lyrid meteor shower will be starting shortly. Its meteors are
usually seen from April 16-25, and the shower should peak on
Wednesday, April 22, perhaps around 11h UT, but almost certainly
at some stage between 03h-14h UT that day. This timing means only
radio-meteor observers in Britain will be able to follow whatever
happens during this predicted interval, till about midday at least.
However, the Moon is new on April 25, so skies should be dark for
anyone in the UK hoping to spot something of the rising Lyrid activity
overnight on April 21-22. The Lyrid radiant reaches a usefully-
observable elevation after 22h30m UT or so, and its visibility
improves throughout the night. The closer the shower's peak falls to
the ~11h timing, the higher its Zenithal Hourly Rates (ZHRs) are likely
to be, perhaps 20-25 or more. The average ZHR is 18, and it tends to
be lower the further the maximum happens away from this "ideal" time.
The maximum is typically quite short, lasting no more than a few
hours, but occasionally it can be more prolonged, such as in 2000
and 2001, when peak activity lasted for over eight hours. Rarely,
strong ZHRs up to 90 have occurred (last in 1982 over the USA).
Consequently, in years like 2009 with little or no Moon, the shower is
always one to watch, just in case. Lyrids are medium-fast meteors,
and can be very bright sometimes. More information is available in
the April 2009 meteor notes, off the Section's homepage at , as are full details on how to make and report
meteor watch observations. Good luck, and clear skies!


NASA has asserted that the Sun has plunged into the deepest solar
minimum in nearly a century. Sunspots have all but vanished. In
2008, the Sun had no spots 73% of the time, a 95-year low. In 2009,
sunspots are even scarcer, with the 'spotless rate' currently 87%.
The situation is unusual but not unprecedented: similarly deep
solar minima were common in the late-19th and early-20th centuries,
and each time the Sun recovered with a fairly robust solar maximum.

Cosmos Online

New high-resolution maps of Jupiter have provided evidence that the
Great Red Spot -- the biggest storm in the Solar System -- is
shrinking. Scientists at Berkeley have collected data over a number
of years from space probes such as Galileo and Cassini and have used
them to create detailed maps of wind speeds in the Great Red Spot.
They have shown that, from 1996 to 2006, the spot's diameter shrank at
an average rate of a kilometre a day. The Red Spot is twice as large
as the Earth; it has lasted for at least the 300 years since
observations began, and may be much older. The Spot is made up of
gases such as hydrogen, helium, ammonia, methane and water vapour,
just like the rest of the Jovian atmosphere. What gives it its
distinctive red colour is still not agreed, but some scientists
believe it may result from material drawn up from deeper in Jupiter's
atmosphere, below the ammonia clouds.

Previously, changes in the size of the Spot were estimated by looking
at cloud patterns created by the storm. To get a more accurate
measure, software has been developed to follow the movement of cloud
patterns over long periods of time. The shrinking of the Spot has
relevance to the energy balance in the surrounding atmosphere. The
amount of energy leaving the Red Spot does not appear to be balanced
by the energy the storm is gaining, but the reason for any discrepancy
is not obvious and neither is its significance.


Astronomers using the Hubble telescope have identified a star that
was a million times brighter than the Sun before it exploded as a
supernova in 2005. According to current theories of stellar
evolution, the star ought not to have self-destructed so early in its
life. The star, which is estimated to have had about 100 times the
Sun's mass, was not mature enough, according to theory, to have
evolved a massive iron core of nuclear-fusion ash, which is supposedly
the prerequisite for the core collapse that triggers a supernova blast.
The explosion, called supernova SN 2005gl, was seen in the barred-
spiral galaxy NGC 266 on 2005 October 5. Pre-explosion pictures from
the Hubble archive, taken in 1997, reveal the progenitor as a very
luminous point source with an absolute visual magnitude of -10.3.
That is so bright that the star seems likely to have belonged to the
class of stars called Luminous Blue Variables (LBVs). As an LBV
evolves it sheds much of its mass through a violent stellar wind.
Only at that point does it develop a massive iron core, and then it
explodes as a core-collapse supernova. Extremely massive and luminous
stars of more than 100 solar masses, such as Eta Carinae in our own
Milky Way, are expected to lose their entire hydrogen envelopes prior
to their ultimate explosions as supernovae. The new progenitor
identification shows that, at least in that case, the star exploded
before losing most of its hydrogen envelope, suggesting that the
evolution of the core and the evolution of the envelope are less
coupled than previously seemed to be implied by stellar-evolution
theory. One possibility is that the progenitor to SN 2005gl was
really a binary system that merged, making it look more luminous and
less evolved than it really was. The observations indicated that
only a small part of the star's mass was flung off in the explosion;
most of the material was probably drawn into the collapsing core that
may now be a black hole estimated at least 10 to 15 solar masses.

Chandra X-ray Center

There is believed to be a class of black holes, with masses in the
approximate range 7--25 solar masses, that has arisen in binary
systems. Some such objects emit powerful jets of particles and
radiation, rather analogous to those seen in quasars, and are called
'micro-quasars'. A new study looks at a famous micro-quasar in our
own Galaxy, and regions close to its event horizon. The system, GRS
1915+105, contains a black hole about 14 times the mass of the Sun
that is feeding off material from a nearby companion star. As the
material swirls toward the black hole, an accretion disc forms. The
system shows remarkably unpredictable and complicated variability on
time-scales ranging from seconds to months, including 14 different
patterns of variation.

Since its launch in 1999, the Chandra X-Ray observatory has observed
GRS 1915+105 eleven times. The studies indicate that its jet may be
periodically choked off when a hot wind, seen in X-rays, is driven off
the accretion disc around the black hole. The wind is believed to
shut down the jet by depriving it of the matter that would otherwise
have fuelled it. Conversely, once the wind dies down, the jet can
re-emerge. The latest Chandra results also show that the wind and the
jet carry about the same amount of matter away from the black hole,
possibly suggesting that the black hole is somehow regulating its
accretion rate, which may be related to the toggling between mass
expulsion via either a jet or a wind from the accretion disc.

Anglo-Australian Observatory

A survey of galaxies, called the 'Six-degree-field galaxy survey', has
been carried out with the 1.2-m UK Schmidt telescope (now part of the
Anglo-Australian Observatory). The Schmidt's wide field of view --
5.7 degrees, or 11 times the width of the Full Moon -- enabled the
survey to cover as much as 80% of the southern sky in a reasonable
time. From conception to delivery, the survey has taken almost a
decade. It has recorded the positions of more than 110,000 galaxies
over more than 80% of the southern sky, out to about two billion
light-years (a redshift of 0.15). As well as participating in the
overall expansion of the Universe, galaxies have their own individual
'peculiar' motions. Assessment of the peculiar velocities may be
possible for about 10% of the galaxies surveyed. It is done by
comparing a galaxy's distance predicted by its redshift with its
distance estimated from its internal properties. The technique
depends upon measuring the width of spectral lines in the galaxy
concerned. That has been done with a purpose-built spectrograph, the
'Six-degree-field' instrument, which allows 150 spectra to be taken
simultaneously. The light from each of the 150 individual galaxies is
brought to line up on the entrance slit of the spectrograph by means
of a set of flexible optical fibres whose front ends are positioned on
the galaxy images by a robotic fibre-positioner.


A new study from the Spitzer space telescope finds that the abundances
of certain molecules in planet-forming discs around young stars differ
according to the luminosities of the stars concerned. Researchers
examined planet-forming discs around 44 Sun-like stars and 17 cooler
ones (M dwarfs and brown dwarfs). The stars are all about 1-3 million
years old, an age when planets are thought to be growing. The
astronomers tried to measure the abundance of hydrogen cyanide with
respect to that of a baseline molecule, acetylene. They found that
the cool stars, both M-dwarf stars and brown dwarfs, showed no
hydrogen cyanide at all, while 30% of the Sun-like stars did. They
speculate that ultraviolet light, which is much stronger around the
Sun-like stars, may drive the production of the hydrogen cyanide.

By Professor Roger Griffin, Bulletin moderator

I apologise for having allowed the following item to appear in the
mentioned Bulletin. It seemed to me very doubtful at the time,
because there is an implicit assumption that was not stated, let alone
justified, that the blobs of material that reflect the light of the
star all lie 'in the plane of the sky' -- the same distance from us as
the star. It seemed a very unlikely situation, but I let it pass,
imagining that authors, referees and Editors must have known what they
were doing better than *I* did. It is something of a relief to find
that that was not actually so: a paper has recently appeared in
'Astronomy & Astrophysics' demolishing the distance determination on
exactly the grounds that I indicate above. The authors of the new
paper also know that the 'reflecting' material (actually 'scattering'
is a more accurate characterisation of its action) is not just like a
white surface but scatters much more effectively in a forward
direction, so the material that is best able to scatter starlight
towards us lies preferentially on *this* side of the nebula, wholly
falsifying the principle upon which the 'distance determination' was

It goes almost without saying that the organisation that was keen to
trumpet the brilliance of the original investigation has not seen fit
to issue a disclaimer and retraction now that that investigation has
been convicted of what seemed, even at the time, like an obvious flaw.

For convenience, the original article in ENB 238 is reproduced below.
It is acknowledged that the wording is not exactly the same as in the
press report put out by the European Southern Observatory, since
(as happens to all the items in these Bulletins) an effort was made
to improve the logical order, wording and clarity of the presentation.


"Taking advantage of the presence of light echoes, astronomers have
measured accurately the distance of a Cepheid -- a class of variable
stars that constitutes one of the first steps in the cosmic distance
ladder. Cepheids are pulsating stars that have been used as distance
indicators for almost a hundred years. The new measurement is
important as, unlike most others, it is purely geometrical and does
not rely on hypotheses about the physics of the star itself.

"The astronomers studied RS Puppis, a Cepheid bright enough to be
easily visible with binoculars. RS Pup varies in brightness by almost
a factor of five every 41.4 days. It is 10 times more massive than the
Sun, 200 times larger, and on average 15 000 times more luminous. It
is the only Cepheid known to be embedded in a large nebula, which is
made of very fine dust that reflects some of the light emitted by the
star. Because the luminosity of the star changes in a very
distinctive pattern, the presence of the nebula allows the astronomers
to see light echoes and use them to measure the distance of the star.

"The light that travels from the star to a dust grain and then to the
telescope arrives a bit later than the light that comes directly from
the star, so any isolated dust blob in the nebula will show a 'light
echo' that varies in brightness in the same manner as the Cepheid,
but after a delay. The astronomers monitored the variations of the
brightnesses of several blobs in the nebula, and thereby derived the
distance of each blob from the star -- it is simply the measured time
delay multiplied by the velocity of light. Knowing those distances
and the apparent separations on the sky between the star and each
blob, one can compute the distance of RS Pup. From the observations
of the echoes from several nebular features, the distance of RS Pup
was found to be 6500 +/- 90 light years."

Owing to holidays, the next edition of the bulletin will be issued on
May 1.

Bulletin compiled by Clive Down

(c) 2009 the Society for Popular Astronomy

 Profile Send private message  
Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 1 post ] 

All times are UTC

Who is online

Users browsing this forum: No registered users and 0 guests

You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You can post attachments in this forum

Search for:
Jump to:  
Powered by phpBB® Forum Software © phpBB Group