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The SOCIETY for POPULAR ASTRONOMY
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Electronic News Bulletin No. 194 2006 April 16
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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/maaeIc1abpRyjcixLLVb/

Philip's, a publisher of astronomy books and planispheres for the
amateur astronomer, is sponsoring this bulletin. For information on
Philip's title see the end of this bulletin.



DOUBT CAST ON VENUS CATASTROPHE
BBC News

Previous speculations on how Venus evolved are upset by new
age dates for its surface. Massive vulcanism 500 million years ago
was thought to have covered over much of the planet's ancient
features. But work carried out at Imperial College suggests that a
volcanic catastrophe is not needed to explain the appearance of
Venus's surface. Scientists will be in a position to reconsider the
issue now that the Venus Express spacecraft has arrived at the planet.

On most planets and moons, impact craters tend to be clustered on very
old parts of the surface, owing to the heavy bombardment that is
believed to have taken place in the early days of the Solar System,
but craters on Venus are distributed randomly over the whole planet.
That has led some scientists to the conclusion that most of the
surface is of similar age. One way to arrive at that result is by
rapid re-surfacing -- a model that some planetary scientists have
found attractive. Now it is suggested that there is no need to invoke
massive outpourings of lava over a short period; instead, the planet's
present-day surface could be compatible with a slow decline of
volcanic activity.


LARGE NEBULA FOUND AT GALACTIC CENTRE
UCLA

Astronomers using the Spitzer space telescope have found an elongated
double-helix nebula, 80 light-years in length, approximately 300
light-years from the centre of our Milky Way galaxy. It is known that
the galactic centre has a strong magnetic field that is highly ordered
and that the magnetic field lines are oriented perpendicular to the
plane of the galaxy. If the magnetic field lines are twisted at their
base, a torsional wave will run up them. A strong, large-scale
magnetic field can affect the galactic orbits of molecular clouds by
exerting a drag on them. It can inhibit star formation, and can guide
a wind of cosmic rays away from the central region. The centre's
magnetic field is strong enough to cause activity that does not occur
elsewhere in the galaxy; the magnetic energy near the galactic centre
is capable of altering the activity of our galactic nucleus.

The magnetic field at the galactic centre, though 1,000 times weaker
than the magnetic field on the Sun, occupies such a large volume that
it has vastly more energy than the solar field. Some astronomers have
hypothesized that the magnetic field lines are anchored in a massive
disc of gas called the circumnuclear disc, which orbits the supposed
central black hole at a distance of several light-years. The orbital
period would be of the order of 10,000 years, which is just what would
be needed to explain the twisting of the field lines that could be
responsible for the shape of the double-helix nebula.


SIZES OF BROWN DWARFS
University of Wisconsin

Brown dwarfs -- failed stars that fall somewhere between the smallest
stars and the largest planets -- are faint, hard to find, and
difficult to characterize. Now astronomers report the discovery of a
pair of young brown dwarfs in mutual orbit, a discovery that has
enabled them to determine their masses and radii for the first time.
The pair is young, a mere 1 million years old, and resides just 1,500
light-years from Earth in the Orion Nebula, a rich star-forming region
of space. The objects are about the size that astronomers predicted.
Their radii are about 70% and 50% of the radius of the Sun, but their
masses are only 5.5 and 3.5% of the solar mass.


NEARBY BROWN DWARF
ESO

An international team of researchers has discovered a brown dwarf
belonging to the 24th-closest stellar system to the Sun. The object
is one of the coolest brown dwarfs so far discovered, having a
temperature of about 750 degrees Centigrade. It orbits a red dwarf
star named SCR 1845-6357 at about 4.5 times the mean distance between
the Earth and the Sun and is located at a distance of 12.7 light-
years. Its mass is estimated to be somewhere between 9 and 65 times
the mass of Jupiter.


RIVERS OF STARS ACROSS THE SKY
New Scientist

Three 'rivers' of stars have been discovered stretching thousands of
light-years across the northern sky. Last month astronomers
discovered a trail of stars covering at least 45° of the sky lying
between the Plough and Arcturus. The trail is about 80000 light-years
away and is thought to consist of thousands of stars travelling ahead
of and behind the cluster NGC 5466. Now, two more, even longer ones,
have been found. The trails are far too distant to be seen with the
unaided human eye, and their light is normally obscured by the huge
number of Milky Way stars between them and Earth. The arcs were
discovered by analysis of data published last year by the Sloan
Digital Sky Survey. The data related specifically to stars in a
portion of the sky known as the north galactic cap. To locate the
streams, the researchers looked at the brightnesses and colours of
millions of stars in the survey. Stars that originate in the same
cluster are typically thought to be born at the same time and under
the same conditions, so by looking for similar distributions of star
brightness and colour, astronomers identified stars that had been
stripped away from the outer regions of a parent cluster.
Verification of a cluster origin must await spectroscopic measurements
of the stars' radial velocities. If the stars did once belong to a
cluster, their velocities should all be much the same as one another.


COMETS FOUND IN MAIN ASTEROID BELT
New Scientist

A number of ice-containing comets lies disguised as main-belt
asteroids between Mars and Jupiter, claim astronomers from the
University of Hawaii who have dubbed the new population 'main belt
comets'. They describe three objects, with near-circular orbits in
the asteroid belt, that stream volatile materials, producing
observable tails for weeks and months at a time. The finding may
revive a theory that ice-bearing asteroids may have played an
important role in forming the Earth's oceans.


COMET FRAGMENTS APPROACHING EARTH
BBC Online

In 1995, Comet 73P/Schwassmann-Wachmann 3 broke up into at least three
components stretched out like a string of pearls. Astronomers watched
with interest, but the comet was 150 million miles away. Next month
the fragments are going to fly past Earth closer than any comet has
come in almost 80 years. The closest fragment will be about six
million miles away and will pass through the constellations Cygnus and
Pegasus on May 12, 13 and 14. The largest fragments are expected to
appear like 3rd- or 4th-magnitude stars, dimly visible to the unaided
eye. The number of fragments is constantly increasing. When the
break-up began in 1995 there were three, but now there are at least
twenty. It appears that some of the fragments are themselves forming
their own sub-fragments, so the number could multiply further as the
comet approaches.


PROBE RETURNS FIRST MARS PICTURES
BBC

Scientists are studying the first pictures taken of the surface of
Mars by the Mars Reconnaissance Orbiter. The spacecraft arrived at
the planet a fortnight ago with a mission to map it in unprecedented
detail. The test images released by NASA show a 50-km swathe of land
in the planet's mid-latitude southern highlands. The smallest objects
that can be made out are about 8 m across, but once the orbiter
descends to a lower orbit the camera will be able to pick out objects
one metre across.


CHANDRA FINDS EVIDENCE FOR QUASAR IGNITION
Chandra X-Ray Center

Since the discovery of quasars over 40 years ago, astronomers have
been trying to understand the conditions surrounding the birth of
those immensely powerful objects. New data from the Chandra X-ray
observatory may help. Hot, X-ray-producing regions around two distant
quasars, 4C37.43 and 3C249.1, observed by Chandra are thought to have
formed during their initial activation. The features are located tens
of thousands of light-years from the central supermassive black holes
thought to power the quasars. The best explanation for the
observations is that a burst of star-formation, or the activation of
the quasars themselves, has driven an enormous amount of gas away from
the host galaxies at extremely high speeds. It is speculated that the
quasars had their origins in galactic collisions. Computer
simulations suggest that the merger of galaxies drives gas toward the
central regions where it triggers a burst of star-formation and
provides fuel for the growth of a central black hole. The inflow of
gas into the black hole releases a tremendous amount of energy, and a
quasar is born. The power output of the quasar dwarfs that of the
surrounding galaxy and expels gas from the galaxy in what has been
termed a galactic super-wind. Over a period of about 100 million
years, the super-wind would drive all the gas away from the central
regions of the galaxy, quenching both star-formation and further
black-hole growth. The quasar phase would accordingly end and the
galaxy would settle down to a relatively quiet life.


STRONGEST MAGNETIC FIELDS IN UNIVERSE
RAS

The first computer simulation to model the collision of two magnetized
neutron stars shows that the impact generates the strongest magnetic
fields known in the Universe. The gigantic fields are more than 10 to
the power 15 times stronger than the magnetic field of the Earth and
are thought to launch the violent gamma-ray-burst explosions. It has
been suspected that such collisions may be at the heart of some of the
brightest explosions in the Universe since the Big Bang, the so-called
short gamma-ray bursts.


NEW OUTBURST OF RECURRENT NOVA RS OPHUICHI
RAS

On 2006 February 12, amateur astronomers reported that a faint star in
the constellation of Ophiuchus had suddenly become clearly visible to
the unaided eye. The star, RS Oph, is a recurrent nova, having had five
previous outbursts in the last 108 years, most recently in 1985. The
latest explosion has been observed in unprecedented detail by an
armada of space- and ground-based telescopes.

RS Oph is about 5,000 light-years away. It consists of a white-dwarf
star in close orbit with a much larger red giant. The two stars are
so close together that hydrogen-rich gas from the outer layers of the
giant is continuously falling onto the dwarf. After around 20 years,
enough gas has been accreted that a runaway thermonuclear explosion
occurs on the white dwarf's surface; in less than a day, its energy
output increases to over 100,000 times that of the Sun, and the
accreted gas (several times the mass of the Earth) is ejected into
space at speeds of several thousand km/s. An unusual feature of
RS Oph is that the red giant is losing a lot of gas in a wind that
envelops the whole system. As a result, the explosion on the white
dwarf occurs inside its companion's extended atmosphere, and the
ejected gas then slams into it at very high speed. There are some
analogies with certain supernovae which also eject high-velocity
material which interacts with the surroundings. However, the
evolution of a supernovae takes tens of thousands of years, whereas
in RS Oph such evolution is occurring literally before our eyes.


PLANET BIRTH AROUND DEAD STAR
NASA

The Spitzer infrared space telescope has been observing a pulsar
called 4U 0142+61, about 13000 light-years away in Cassiopeia. The
pulsar was once a large, bright star with a mass between 10 and 20
times that of our sun, but burnt out and underwent a supernova
explosion about 100,000 years ago. Spitzer has seen a disc, probably
consisting of debris from the explosion, around the pulsar that
remains. The disc orbits at a distance of about 1 million miles and
probably contains about 10 Earth-masses of material. It seems
possible that some of the material in the disc could ultimately
coagulate into planets. In fact the first planets ever discovered
outside our solar system, in 1992, were three that were found to be in
orbit around a pulsar called PSR B1257+12.


MERCURY FORMATION IMPACT MIGHT HAVE SPLATTERED EARTH
RAS

The planet Mercury has a mean density almost as high as the Earth's,
which is perhaps surprising since it is a much smaller body. In an
effort to explain why Mercury has such a high mean density,
experiments have been tried on a computer to see whether a major
collision between two bodies of comparable size could have been
responsible. It seems that, indeed, in such a collision a lot of the
less-dense material from the outer layers of the colliding bodies
would be lost to them.


ICE LAYERS RECORD COMET CREATION
BBC News

The Deep Impact mission is casting new light on how comet Tempel 1
formed. The probe sent a 370-kg projectile crashing into Tempel 1 and
then studied the plume of debris. Images from last July's encounter
appear to show as many as seven different layers on the comet's
surface, a sign that Tempel 1 was built up from lesser objects. It is
suggested that, in the outer parts of the early Solar System, smaller
bodies called cometesimals collided and merged, gradually piling up to
form the larger objects we know as comets.


MASSIVE AMOUNTS OF WATER RELEASED BY COMET HIT
RAS

Observations by the Swift satellite have shown that comet Tempel 1
grew brighter and brighter in X-ray light after the Deep Impact probe
collided with it, and that the X-ray outburst lasted a total of 12
days. The X-rays provide a means of estimating how much material was
ejected as a result of the impact, because the X-rays were emitted by
the newly liberated water as it interacted with the high-energy solar
wind. The X-ray power output depends on both the water production
rate from the comet and the flux of subatomic particles streaming out
of the Sun as the solar wind. Data from the ACE satellite, which
constantly monitors the solar wind, enabled the two quantities to be
separated. The result of the observation and calculation is that far
more water was liberated, and over a longer period, than was
previously realised. Over the duration of the outburst, the total
mass of ejected water attributable to the impact was 250,000 tons --
of the order of a million times the mass of the impacting projectile.


SUPERMASSIVE BLACK HOLES IN COLLISION
University of Virginia

An international team of astronomers using the Chandra X-ray
observatory has observed the cluster of galaxies called Abell 400,
where astronomers had previously suggested that a pair of supermassive
black holes might be colliding. The team found that the two black
holes appear to be swallowing gas from their host galaxy, and each is
ejecting a pair of oppositely-directed jets of radio-emitting plasma.
As the holes fall through the gas in the cluster, the plasma jets are
swept back behind them. The jets are bent together and intertwined,
indicating that the black holes are bound and moving together, and may
eventually collide and merge.


URANUS MOON EMBEDDED IN BLUE RING
University of California

The outermost ring of Uranus, discovered just last year, is bright
blue, making it only the second known blue ring in the Solar System.
Perhaps not coincidentally, both blue rings are associated with small
moons. The outer ring of Saturn is blue and has Enceladus at its
brightest spot, and Uranus is strikingly similar, with its blue ring
right on top of Mab's orbit. Astronomers suspect both rings owe their
blue colour to subtle forces acting on dust in the rings that allow
smaller particles to survive while larger ones are recaptured by the
relevant moon.


XENA AS BRIGHT AS FRESH SNOW
New Scientist

New Hubble images show that Xena is only slightly larger than Pluto;
the trans-Neptunian object was previously thought to be 25% to 30%
larger. Intriguingly, the down-sizing implies that Xena must be
whiter than almost any other object in the Solar System, suggesting
that it is constantly being re-surfaced. Xena, officially called 2003
UB313, was first announced in 2005 July and lies about three times as
far from the Sun as Pluto. Its brightness indicated that it was
larger than Pluto, but it was not clear by how much because
astronomers did not know how much sunlight its surface reflected.
Analysis of images taken last December show Xena's diameter to be 2400
km -- about 5% larger than Pluto's 2290 km. Its size and brightness
show that it must reflect about 86% of the light that falls on it,
making it about as bright as fresh snow and brighter than every other
Solar-System body except Saturn's moon Enceladus. Spectral
observations suggest that its surface is covered with frozen methane,
like Pluto's. But unlike Pluto, which is mottled by bright and dark
splotches, Xena is so uniformly bright that photometry does not
indicate any rotational period. Xena moves in a very elongated orbit
that stretches from 38 to 97 astronomical units; when it is near the
Sun in its 560-year orbit it may have a gaseous atmosphere, but when
it moves away it receives so little sunlight that any atmosphere would
freeze onto the surface, leaving it fresh and white.


PHILIP'S RECENT TITLES:

Philip's The Sky at Night Volume 2 is the latest volume in Sir Patrick
Moore's series of essays written to accompany the BBC television
series of the same name. It tracks developments in astronomy,
astrophysics and space exploration in the period from 2001 November
to 2005 March -- £9.99
Philip's Stargazing 2006 by Heather Couper and Nigel Henbest - £6.99
Published last year:
Philip's Solar System Guide by Peter Grego contains an abundance of
information and images, and is a practical and colourful introduction to
our corner of the Universe. It describes how to observe not only the
planets but also the Moon, Sun, comets, meteors, asteroids and other
objects found within our Solar System -- £9.99
Philip's Solar System Observer -- a pack for the amateur Solar-System
observer. It contains three items for exploring and enjoying our
corner of the Universe: Philip's Solar Observer's Guide, Philip's Map
of the Solar System and Philip's Solar System Phenomena poster --
£12.99

For more information on these and other Philip's titles please visit
www.philips-maps.co.uk or call 020 7644 6935 for a catalogue.

Bulletin compiled by Clive Down

(c) 2006 the Society for Popular Astronomy


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