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Monday, 10 August 2009

SPA ENB No. 272

                 The SOCIETY for POPULAR ASTRONOMY
         Electronic News Bulletin No. 272     2009 August 9

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


A new bright spot has appeared in the clouds of Venus.  It was first
identified by an amateur astronomer on July 19 and was confirmed
retrospectively by the Venus Express spacecraft, whose data suggest
that the spot appeared at least as early as July 15.  The spot has
since started to expand, being spread by winds in Venus's thick
atmosphere.  Scientists do not know what caused the bright spot tens
of kilometres up.  A volcanic eruption is a possibility, but there is
no firm evidence for present-day vulcanism, and an eruption would have
to be extremely powerful to penetrate so far through the planet's
dense atmosphere.

University of Melbourne

Enceladus, discovered by Sir William Herschel in 1789, is the sixth-
largest of the satellites of Saturn and is unusually bright for its
size because its surface is covered in ice which reflects nearly all
the sunlight that falls upon it.  It had been presumed to be just an
ice ball until a water-vapour plume was seen erupting from its surface
in 2006.  The Cassini spacecraft has recently found Enceladus to be a
dynamic place, recording features such as geysers emerging from 'tiger
stripes' which are thought to be cracks caused by tectonic activity
near Enceladus' south pole.

Grappling with the problem of how a small moon with a completely
frozen interior could display such activity, scientists tried computer
simulations.  Ammonia has been indirectly observed to be present on
Enceladus and was postulated in the model, in which Enceladus
initially had a frozen shell composed of a mixture of ammonia and
water-ice surrounding a rocky core.  Over time, as Enceladus
interacted with other moons, a small amount of heat was generated
above the silicate core by tidal flexure, which according to the model
made the ice shell separate into chemically distinct layers.  An
ammonia-enriched liquid layer formed on top of the core while a thin
layer of pure water-ice formed above that.  The calculations indicated
that if a layer of pure water-ice formed near the core, it would have
enough buoyancy to rise, and such a redistribution of mass could
generate large tectonic stresses at the surface.  However, the pure
water-ice rising up was also slightly warmer and caused the separation
to occur again, this time forming an ammonia-enriched ocean just under
the surface.  The presence of ammonia, which acted as an anti-freeze,
then helped to keep the ocean in a liquid state.  Some of the
scientists are optimistic that their model does indeed simulate the
real Enceladus.


One of the problems posed by red supergiants is how they manage to
shed such tremendous quantities of material -- about the mass of the
Sun -- in only 10 000 years. Two teams of astronomers have used the
VLT to observe Betelgeuse.  The first team used the adaptive optics
instrument, and obtained images with resolutions down to 37 milli-
seconds of arc, almost reaching the theoretical limit for an 8-metre
telescope in the infrared.  The images reveal a plume of gas extending
into space at least six times the diameter of the star --
corresponding to the distance between the Sun and Neptune -- from the
surface of Betelgeuse.  That is a clear indication that the whole
outer shell of the star is not shedding matter evenly in all
directions.  [Not surprising! -- the solar corona is obviously
non-spherical.]  Such a plume could be generated above a convectively
rising current in the stellar photosphere, and could be somewhat
analogous to a solar prominence on a much larger scale; but whereas on
the Sun the convective granulation pattern is on a scale small in
comparison with the solar radius it has long been suspected that
supergiants have only a few convective cells each occupying a
significant part of the whole surface.  Interferometric observations
have confirmed the large size of the convective cells on Betelgeuse.
[This item seemed to set out to tell us about two teams of
astronomers, but shot off at a tangent after telling us about one and
never got to the second, unless by implication with regard to

Science Daily

Vega (Alpha Lyrae) is a well-known star, being the fifth-brightest
star in the sky and only 25 light years away.  It rotates very
rapidly, in less than a day, causing its equatorial regions to bulge
out and be more than 1000 degrees C less hot than the poles.
Astronomers using a new-generation stellar spectro-polarimeter at Pic
du Midi in southwest France have analyzed the polarization of light
emitted by Vega and detected a weak magnetic field at its surface.
That is really not a big surprise because it is known that the motions
of charged particles inside stars can generate magnetic fields --
indeed,that is how the solar and terrestrial magnetic fields are
produced.  However, theoretical models for stars more massive than the
Sun cannot predict the intensity and the structure of the magnetic
field, so the astronomers had no clue as to the strength of the signal
they were looking for. The strength of the Vega magnetic field is
about 50 micro-tesla, which is close to that of the mean field on the
Earth and the Sun.

Yale University

A team of astronomers has discovered a group of rare galaxies called
the 'Green Peas' with the help of lay enthusiasts working through an
on-line project called Galaxy Zoo.  The project was launched in 2007
by a team of astronomers in the UK and US.  Computer users volunteer
their spare time to help classify galaxies in an on-line image bank.
To date, 230,000 volunteers from all over the world have helped to
classify one million images of galaxies taken by the Sloan Digital Sky
Survey.  Some of the volunteers came across a number of objects that
stood out because of their small size and bright green colour.  They
dubbed them the 'Green Peas'.  With further help from the volunteers
to analyze the strange objects further, the astronomers discovered
that the Green Peas are small, compact galaxies which are among the
most active star-forming galaxies ever found.  Of the one million
classified galaxies, the team found only 250 Green Peas.  They are
between 1.5 billion and 5 billion light-years away, are 10 times
smaller than our own Milky Way galaxy and 100 times less massive.
But surprisingly, they are forming stars 10 times faster than the
Milky Way.  Such galaxies may have been normal in the early Universe,
but galaxies do not seem to be so active today.

Bulletin compiled by Clive Down

(c) 2009 the Society for Popular Astronomy

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