ENB No. 242 April 27 2008

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ENB No. 242 April 27 2008

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Electronic News Bulletin No. 242 2008 April 27

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://tinyurl.com/kogyx

By Alastair McBeath, SPA Meteor Section Director

Problems in updating the Section's webpages have continued this past
month, though in fact only a few UK fireball sightings have arrived since
the previous ENB anyway. Details on these are as follows:

Date Time (UT) Magnitude and Notes Observed from
08/04/6-7 ~21:00 Bright; in clouds Cheshire
08/04/9-10 20:34 -4/-6? Cheshire
08/04/9-10 ~23:10 Very bright; green Essex
08/04/14-15 ~21:00 Brilliant; 2 reports Dorset

Rather further afield, Assistant Meteor Director David Entwistle spotted
news of a probable meteorite fall in Argentina on April 6-7, around
22:03 local time (01:03 UT). David noted that media coverage, as so
often, had been rather confused about it, but found one website which
seemed to give more factual information at
http://www.astroentrerios.com.ar/site/ , for those interested in testing
their Spanish language skills.

Fireball reports made from the British Isles and points adjacent (a
fireball is any meteor of at least magnitude -3) are always welcomed
by the SPA Meteor Section. Details to send are outlined on the "Fireball
Observing" page of the SPA website, at: http://tinyurl.com/l62fh .

By Alastair McBeath, SPA Meteor Section Director

Longer-term ENB readers may recall we featured notes on meteors
observed in the Martian atmosphere a couple of times back in 2005,
most recently in No.185 ( http://tinyurl.com/4g2sux ), where a number
of supposed "meteor images" taken from the surface of Mars turned
out to have been due to cosmic rays impacting the CCD imaging chips
instead. In early April at the RAS National Astronomy Meeting, this year
held in Belfast, professional researcher Apostolos Christou of Armagh
Observatory, presented findings cross-referencing predictions for when
meteoroidal dust trails produced by Comet 79P/du Toit-Hartley could
have occurred, with observations of Mars' ionosphere made by
NASA's Mars Global Surveyor (MGS) spacecraft. MGS was not able
to record meteors or the state of the Martian ionosphere directly.
However, Christou's team were able to use evidence from the craft's
radio signals passing through the Martian atmosphere to determine the
density of electrons in it, and thus infer the likely status of the Red
Planet's ionosphere. From that, they believed they could suggest
whether it may have been affected by increased meteor activity or not.

When meteor ablation happens in the Earth's atmosphere, some
elements from the incoming meteoroids can become ionized, to form a
hot, ionized gas, a plasma. We can sometimes see this plasma for a
few seconds after a brighter, swifter-moving meteor as a persistent
train. In a meteor shower, this can create a temporary, thin, plasma
layer in addition to the normal ionospheric plasma layer produced by
the continuous rain of sporadic meteors. On Earth, this layer lies around
95-100 km altitude, in the typical meteor ablation region. On Mars, this
theoretical layer should occur around 80-95 km altitude according to the
work at Armagh.

Six potential Martian meteor showers due to Comet 79P were
predicted since MGS entered orbit in 1997, but information on the state
of the planet's ionosphere was available only for two, in 2003 April and
2005 March. The 2003 data showed an ionospheric disturbance
coincident in time and at the predicted height, allowing the researchers
to make an association with the theoretical meteor shower then. The
characteristics seen in the disturbance were suggested as similar to
those found from the annual Perseid meteor shower visible from Earth
each July-August.

Curiously, for the 2005 event, no such signature was found. Christou
suggested this was perhaps because the meteors ablated lower in the
Martian atmosphere in 2005, though without suggesting any mechanism
for why this should have been so then, but not in 2003. Oddly, no
checking of solar activity at either time was mentioned, though on Earth,
2003 produced the most recent peak in observed auroral activity, a very
strong one, while 2005 was a more "normal" auroral year. Solar wind
and auroral ions help produce several plasma layers above the meteor
layer in the Earth's atmosphere. Consequently, while the findings from
2003 are interesting, it will need more evidence to confirm that the
ionospheric disturbance method is viable for making meteor
observations over Mars, perhaps by checking the MGS data for times
when other showers may have encountered the planet. More theoretical
work needs to be done using other comet orbits to establish when those
might have happened first. By then, different Mars probes may be
available to make alternative types of meteor observation, possibly
even imaging from the surface. For more details, see the links from the
topic at http://tinyurl.com/3mnzxeon the SPA's General Chat Forum.

By Alastair McBeath, SPA Meteor Section Director

British visual meteor watchers tend to find May an increasingly difficult
month, as twilight lengthens, becoming omnipresent from the more
northerly parts before its end. In general, meteor rates tend to be low,
the usual sporadic smattering plus those from the Antihelion Source.
Antihelion meteors radiate from a roughly oval region, about 30
degrees in RA (two hours) by 15 degrees in Dec, which lies roughly
opposite the Sun in the sky, hence its name. The actual Source's centre
is about 12 degrees east of this point. Video results suggest it is active
virtually throughout the year, producing generally low rates. Radiant
centre positions for this area are provided in the Section's Meteor
Showers List, available via the meteor homepage, at:
http://tinyurl.com/2m335d .

In May, the Antihelion Source's centre tracks along the most southerly
part of the ecliptic, from eastern Libra to Ophiuchus-Sagittarius, an
area that is on-view for most of the shortening nights, culminating
around 01h UT. Commonly, one or two meteors per hour is a good
level of Antihelion activity, but the start and end of May were identified
in the latest International Meteor Organization (IMO) analyses as
being some of the better times of year for such rates. Even so, this
"improved" activity is liable to bring Zenthial Hourly Rates (ZHRs) no
better than 3 or 4, which, given the deep southerly radiant Declination,
may reduce observed numbers from the UK to negligible proportions for
most of the month. Despite this, observations to check on it are
recommended whenever there is no Moon after mid-evening. Antihelion
meteors are of medium speed.

Adding interest to the month, and continuing their activity from April,
May 3-10 should see the strongest, swift-moving, Eta Aquarid rates,
with ZHRs of perhaps 30+ on every night during this time, rising to
maybe 70 or more at best on May 5, according to the most recent IMO
long-term report. This suggested a previously-suspected 12-year
periodicity in activity, governed by Jupiter's orbit, may be real, with the
next high-point in ZHRs (possibly up to 85) falling between this year
and 2010. Unfortunately, the shower's radiant, at RA 22h32m, Dec -1
degree, in the little "Y"-shaped Water Jar asterism of Aquarius
(Gamma, Zeta, Eta and Pi Aquarii) on the UK mornings of May 4-5
and 5-6, rises only shortly before dawn, so it is rare to glimpse more
than one or two Eta Aquarids a night here, and even this low rate is
not guaranteed. New Moon on May 5 is ideal for all observers, but
morning twilight will still be the greatest problem. From the southern
hemisphere, this is one of the strongest, reliable, annual meteor
showers, and anyone planning a trip to places at latitude 35 degrees
south or so in early May can enjoy over two hours of dark sky before
dawn to observe Eta Aquarids in. It is from such regions that our
knowledge of the shower primarily comes. It has long been known that
it shows a variable number of submaxima around its main peak, which
helps produce the week-long period of increased rates, and this is
similar to their sibling shower of the Orionids of October-November.
Both are linked to Comet 1P/Halley, last nearby in 1986. The Earth
passes closer to the denser parts of the Halley meteor streams in
May, hence the Eta Aquarid rates are usually two to three times
stronger than the Orionid ones. Both showers seemed to show a
nearly in-phase relationship between coincident higher and lower
activities on their potential 12-year cycle. However, Eta Aquarid rates
in 2007 were somewhere between average to below par (see ENB
222; http://tinyurl.com/4o3pyn ), despite the unexpectedly strong
Orionid rates the previous October, when peak ZHRs reached around
50-60, two to three times their normal level (see ENB 207). It will be
interesting to see what happens in 2008, following another strong
Orionid return last year (see ENB 230, http://tinyurl.com/2jkskc ). From
Britain, Eta Aquarids are slightly easier to see later in May, when their
radiant (by May 15 at RA 23h00m, Dec +3 degrees, very near Beta
Piscium) rises a little earlier before dawn, but rates are of course lower
then, though the waxing Moon still favours post-midnight watching. The
shower ends around May 28.

Radio meteor observers should be able to catch the Eta Aquarids quite
well, often finding them in generally elevated echo-count rates
throughout the morning daylight hours particularly, in the first half of
May. Indeed, they herald the busiest time of year for stronger daytime
meteor showers overall. The first of these daylight streams producing
usually radio-detectable rates, the Omicron Cetids, was initially
detected using radar at Jodrell Bank in Cheshire in 1950. Observations
from there in 1951 and 1953 suggested the shower's activity could be
variable, both from day to day, and year to year. Radar observations
elsewhere were carried out only erratically after this, with further shower
detections made during the 1960s, but only very occasionally since
1969. The implied overall picture was that activity lasted between
roughly May 5 to June 2, from a radiant near RA 01h52m, Dec -4
degrees at the maximum on May 20, some 7 to 8 degrees west of Mira
(Omicron Ceti). Since this radiant position is about 45 degrees
south-west of the Sun, a very few visual Omicron Cetids have been
claimed shortly before dawn from tropical and southern hemisphere
sites, and anyone at such places this year might be lucky too, though
May's full Moon on the 20th offers no assistance. Radio observations
analysed from 1994-2004 by the SPA suggested the shower seemed
to still peak at about this time, but no particularly strong returns were
registered during that period. Radio, or rare visual, reports of this
shower would be most welcome.

Good luck to all meteor observers in May, and clear skies!


Omega Centauri is the largest and brightest globular cluster in the
sky, but is too far south to be seen from Britain. Although it is
17000 light-years away, it appears almost as large as the Full Moon
when seen from a dark site. Perceptions of its nature have changed a
number of times. It was first listed in Ptolemy's catalogue nearly
two thousand years ago as a single star. Edmond Halley reported it as
a nebula in 1677. In the 1830s John Herschel was the first to
recognise it as a globular cluster. Now, there is suggestion that
Omega Centauri is not really a globular cluster after all, but a dwarf
galaxy stripped of its outer stars. Omega Centauri has several
characteristics that distinguish it from other globular clusters: it
rotates faster than most such clusters, its shape is considerably
flattened, and it consists of several generations of stars -- typical
globular clusters usually consist of just one generation of old stars.
Moreover, Omega Centauri is about 10 times as massive as other big
globular clusters, almost as massive as a small galaxy.

Recent images from the Hubble telescope and spectra taken with the
Gemini South telescope in Chile suggest that Omega Centauri may have
an 'intermediate-mass' black hole in its centre. Astronomers measured
the motions of the stars near the centre of Omega Centauri. The
measured velocities are related to the total mass of the cluster, and
those of stars near the centre were appreciably higher than expected
from the mass deduced from the number and type of stars seen. It is
fashionable nowadays to attribute any suggestion of excess mass in an
astronomical object to a black hole; in this case one of about 40,000
solar masses would fit the observations, but that is not the only
possible explanation. The astronomers conclude their article by
saying, "While our preferred model is the existence of a central black
hole, detailed numerical simulations are required to confidently [sic]
rule out the other possibilities."


A team of Japanese astronomers using X-ray satellites has discovered
that our Galaxy's central black hole had a powerful flare-up three
centuries ago. The black hole, known as Sagittarius A*, is thought
to be about 4 million times the mass of our Sun, but the energy
radiated from its surroundings is billions of times weaker than the
radiation emitted from the vicinities of the supposed central black
holes in certain other galaxies.

When gas spirals inward toward the black hole, it heats up to millions
of degrees and emits X-rays. If more matter piles up near the black
hole, the X-ray output increases. A large outburst sends out a pulse
of X-rays in an ever-expanding shell that successively illuminates
everything in its path, creating what are known as light echoes. In
particular, there is a large cloud known as Sagittarius B2 that is 300
light-years from the centre, so the cloud responds to events that
occurred 300 years earlier. The observations that have now been
reported were collected between 1994 and 2005, and show that a region
in Sagittarius B2 only 10 light-years across varied considerably in
brightness in just 5 years. By observing how the cloud lit up and
faded over 10 years, the astronomers could trace the black hole's
activity 300 years ago. It appears that the hole was a million times
brighter three centuries ago than it is now. That the increased
brightness definitely came from X-rays and not sub-atomic particles
was established by the presence of a diagnostic spectral signature of
ionised iron atoms.


Two clusters of Red Supergiant Stars (RSGs) have been discovered
close to one another in the Milky Way's Galactic Bar. RSGs represent
the final brief stage in a massive star's life-cycle before it becomes
a supernova. They are very rare objects, so to find two clusters of
them close together is remarkable. Together the clusters contain 40
RSGs, which is nearly 20% of the total known in the Milky Way. The
clusters are on the edge of the Galactic Bar which is ploughing
through the disc of the Milky Way. It is likely that an interaction
between the bar and the disc was what triggered the star-formation
event that created the clusters, which are in the constellation
Scutum, about 20 000 light-years from Earth and separated from each
other by 800 light-years. Cluster 1 contains 14 RSGs and is 12
million years old; Cluster 2 contains 26 RSGs and is 17 million years
old. The clusters were initially identified in the mid-infrared
Galactic Plane survey (GLIMPSE), a huge data-base of images taken by
the Spitzer space telescope, and the properties of their stars were
subsequently investigated with the Keck telescope in Hawaii.


UK astronomers have produced the most sensitive infrared map of the
distant Universe so far obtained. Combining data over a period of
three years, they have produced an image containing over 100,000
galaxies over an area four times the angular size of the Full Moon.
As is well known, the light we are receiving now from very distant
galaxies started out a long time ago, when the Universe was
comparatively young. By observing in the infrared, astronomers can
now see further back in time, since light from the most distant
galaxies is excessively red-shifted. The new ultra-deep image allows
astronomers to observe galaxies evolving at different stages in cosmic
history, all the way back to just 1 billion years after the Big Bang.
One of the key aims is to understand when the rarest, most massive
galaxies formed. Galaxies ten times the mass of the Milky Way were
already in place at very early epochs, but it is not understood how
they managed to form so quickly.


A team of astronomers has found evidence for a dramatic surge in star
birth in a newly discovered population of massive galaxies in the
early Universe. The galaxies are so distant that the light we detect
from them has been travelling for more than 10 billion years. The
recent discovery of a new type of extremely luminous galaxy at that
epoch -- one that is very faint in visible light, but much brighter at
longer, radio wavelengths -- is the key to the new results. A related
type of galaxy was first found in 1997 with the James Clerk Maxwell
telescope on Mauna Kea in Hawaii and a new camera called SCUBA that
detects radiation emitted at sub-millimetre wavelengths. In 2004 the
Cambridge-led team proposed that the 'sub-millimetre galaxies' might
represent only part of the picture of rapid star-formation in the
early Universe, because SCUBA is biased towards colder objects. They
suggested that a population of similar galaxies with slightly higher
temperatures could exist but have gone largely unnoticed. They
searched for such galaxies with radio and optical observations, found
them, measured their distances and then confirmed their star-forming
nature through the detection of vastly extended clouds of gas and dust.
The newly found galaxies have prodigious rates of star formation, far
higher than anything seen in the present-day Universe.

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

Bulletin compiled by Clive Down

(c) 2008 the Society for Popular Astronomy

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