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The SOCIETY for POPULAR ASTRONOMY
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Electronic News Bulletin No. 231 2007 November 11
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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
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/maaiFhCabCEXNcixLLVb/



BELT OF MOONLETS IN SATURN'S RINGS
University of Colorado

A narrow belt harbouring moonlets that may be up to nearly half a
kilometre across has been discovered in Saturn's outermost (A) ring.
Images taken by Cassini have revealed a series of eight propeller-
shaped 'wakes' in a thin belt of the A ring, indicating the presence
of corresponding moonlets. The propeller wakes highlight tiny areas
of the belt where ring material has been perturbed by the
gravitational forces caused by individual moonlets. The belt is about
2,000 miles across, only about 1/80th of the overall diameter of the
ring system. The moonlets may be the result of the break-up of a
object similar to Pan, Saturn's innermost 20-mile-diameter moon.
The finding supports the theory that Saturn's rings were created in a
'collisional cascade' of debris begun by a catastrophic break-up of an
even larger satellite.


MASSIVE BLACK HOLE FOUND
NASA

The companions to certain stars which are known to be members of
binary systems because of variations in their radial velocities are
believed to be black holes. A particularly massive one has recently
been identified in the dwarf galaxy IC 10, 1.8 million light-years
away in the constellation Cassiopeia. The observed star is ejecting
gas in the form of a wind. Some of the material spirals toward the
black hole, heats up, and gives off powerful X-rays before crossing
the point of no return. Last November, astronomers using the Chandra
X-ray observatory discovered that IC 10's brightest X-ray source,
IC 10 X-1, exhibits sharp changes in X-ray brightness that suggested
eclipses. The Swift satellite soon confirmed the eclipses and
revealed details about the star's orbit, to which the application of
Kepler's Laws showed that the companion, supposed to be a black hole,
has a mass of at least 24 Suns.

The progenitor star probably started with 60 or more solar masses, and
like its host galaxy was probably deficient in elements heavier than
hydrogen and helium. In massive, luminous stars with a high fraction
of heavy elements, the extra electrons of elements such as carbon and
oxygen increase the effect of outwardly-directed radiation pressure
and the loss of mass through stellar winds. But with a low fraction of
heavy elements, the IC 10 X-1 progenitor may have shed comparatively
little mass before it exploded, so it could leave behind a massive
black hole.


DISTANT SUPERNOVAE MARGINALLY BRIGHTER?
University Of Toronto

Supernovae are not as powerful or bright, on average, as they once
were, according to a study by Toronto astronomers, who compared
supernovae in 'nearby' galaxies with those that exploded up to nine
billion light-years away, and found that the distant ones were an
average of 12% brighter.


LARGE BINOCULAR TELESCOPE
BBC Online

Almost 20 years after it was first conceived, what will be the world's
most powerful optical telescope is about to be commissioned. The £55m
Large Binocular Telescope in Arizona consists of two 8.4-m mirrors,
each weighing nearly 16 tons, close together on a single mounting.
It has a light-gathering power equivalent to that of a single 11.8-m
instrument and the resolution of a single telescope of 22.8 m in
diameter. Certain features of its construction are testing the
practical implementation of new technologies which may be critical for
next-generation large telescopes.


PLIOCENE SEA MUD RECORDS SUPERNOVA BLAST
New Scientist

Sea-floor sediments have indicated that a supernova exploded during
the Pliocene era and may have caused a minor extinction event on
Earth. Levels of radioactive iron-60 suggest that the supernova was
between 60 and 300 light-years away. Radiation from the blast could
have weakened the Earth's atmosphere, exposing organisms to more of
the Sun's ultraviolet radiation. There is a timing coincidence with
an extinction peak, but no direct evidence of a link.


COMET OUTBURST CONTINUES TO MYSTIFY
New Scientist

The comet that suddenly became about a million times brighter nearly
two weeks ago continues to appear remarkably bright, leaving
observers puzzled over what caused the outburst and whether the comet
will perform an encore in the coming months. Comet 17P/Holmes is
normally very faint, about 25,000 times too faint to be seen
with the unaided eye. But since its dramatic brightening on October 23,
the comet's coma, a surrounding shell of gas and dust, has been
expanding at a rate of about 500 m/s, making the comet appear as a
fuzzy 'star' that can be clearly seen with the unaided eye in the
constellation Perseus.

Comet Holmes was actually discovered during a similar, but less
spectacular, brightening event in 1892 November. It faded after a few
weeks, only to brighten again the following January. The comet orbits
the Sun every seven years on a path that takes it from the distance of
Jupiter's orbit to about twice that of the Earth's. Interestingly, in
both the 1892 event and the recent one, it initially brightened about
five months after reaching perihelion (its closest approach to the
Sun). The comet is still actively releasing gas -- mostly water
vapour, but the spectrum also shows ethane, acetylene and hydrogen
cyanide in trace amounts. The standard explanation of comet outbursts
is that a sudden 'event' exposes fresh ices within the nucleus to
solar radiation which causes them to vaporise, dragging dust along
with them. Sunlight reflected by the dust then makes the comet appear
brighter. There is no agreement among astronomers as to the nature of
the 'event' that could have produced such a cataclysmic increase in
brightness. The comet is at present in the main asteroid belt, at a
distance from the Sun of 2.5 astronomical units. Water-ice can no
longer sublimate beyond about 3 astronomical units, so if the comet is
still active past that point, something more volatile than water-ice
must be driving its activity.


FIFTH PLANET DISCOVERED ORBITING 55 CANCRI
New Scientist

A fifth planet has been discovered around 55 Cancri, a star about 41
light-years away and slightly cooler and dimmer than our own Sun. The
55 Cancri system was already known to include four planets, including
three giant planets that orbit closer than Mercury orbits the Sun and
one that is four times as massive as Jupiter and orbits at about
Jupiter's distance from the Sun. The newly discovered fifth planet is
comparable in mass with Saturn and lies between the hot, close-in
planets and the frigid distant one. It orbits the star at a distance
of 117 million kilometres, about 8% farther than Venus is from our
Sun.


HIGHEST-ENERGY COSMIC RAYS LINKED WITH AGN'S
DOE/Fermi National Accelerator Laboratory

Using the Pierre Auger Observatory in Argentina, the largest
cosmic-ray observatory in the world, a team of scientists from 17
countries found that the sources of the highest-energy particles are
not distributed uniformly across the sky but are linked to nearby
galaxies that have active nuclei in their centres. Active Galactic
Nuclei (AGN) are thought to be powered by supermassive black holes
that swallow matter from their host galaxies and spew out particles
and energy. While most galaxies may have black holes at their
centres, only a fraction of them has an AGN. The mechanism whereby
AGNs might accelerate particles to energies 100 million times higher
than the most powerful particle accelerator on Earth is still a
mystery.

Cosmic rays are protons and atomic nuclei that travel at close to the
speed of light. When they smash into our upper atmosphere, they create
a cascade of secondary particles called an air shower that can spread
across 40 or more square kilometres as they reach the Earth's surface.
The Pierre Auger Observatory records cosmic-ray showers through an
array of 1,600 particle detectors placed 1.5 kilometres apart in a
grid spread across 3,000 square kilometres. Twenty-four specially
designed telescopes record the emission of fluorescence light from the
air shower. While the observatory has recorded almost a million
cosmic-ray showers, only the rare, highest-energy cosmic rays can be
linked to their sources with sufficient precision. The Auger
collaboration discovered that the directions of the sources of the 27
highest-energy events, with energy above 57 EeV (5.7 x 10*19
electron-volts), correlated well with the locations of AGNs in some
'nearby' galaxies such as Centaurus A.


CATCH A STAR 2008 - an astronomical competition for school students
Douglas Pierce-Price, ESO Education Officer

'Catch a Star' and win fantastic prizes! This astronomical
competition, run by ESO and the European Association for Astronomy
Education, is open now. School students around the world can enter,
writing astronomical projects or creating astronomical artwork, to win
prizes such as astronomy software, posters, and exclusive 'Catch a
Star' T-shirts. In addition, students from Europe and Chile can win
exciting travel prizes, such as a once-in-a-lifetime trip to visit
ESO's Very Large Telescope on Cerro Paranal in Chile, or to other
observatories in Europe. The deadline for entries is 2008 February 29.
Find out more on the web at: ttp://popastro.c.topica.com/maaiFhCabCEXOcixLLVb/
Printable flyers about the competition are available as PDF files at:
http://popastro.c.topica.com/maaiFhCabCEXPcixLLVb/ If you have any uestions about the
competition, please contact eduinfo@eso.org.



Bulletin compiled by Clive Down

(c) 2007 the Society for Popular Astronomy


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