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Monday, 8 July 2013

SPA ENB No. 356

                The SOCIETY for POPULAR ASTRONOMY

          Electronic News Bulletin No. 356  2013 July 7

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

BBC Online

This seemingly rather anachronistic item refers to the passage of the
'Sun-grazing' Comet Lovejoy through the solar corona on 2011 December
16.  The comet's tail was pulled about by magnetic fields.  It went
where satellites can't go because they would melt, and we can't see
that area well from here because there is not much light coming from
it.  Its passage through the corona (the outer part of the Sun's
atmosphere) was, however, watched by several spacecraft.  The images
show the comet getting increasingly bright as it enters the corona,
where the BBC item that forms (now rather distantly) the basis of this
report claims that it encountered temperatures of millions of degrees
Celsius (in fact it was a few thousand).  The tail did not follow the
comet's head perfectly, but was perturbed by the Sun's magnetic field.
That field drives the solar wind and is responsible for energetic
events that occur in the solar corona and can cause 'space weather' --
streams of particles which can damage satellites and telecommunications
infrastructure.  The observations of Comet Lovejoy may contribute to
our understanding of the magnetic field and its activities.

After the comet made its close approach, scientists were surprised to
see that it survived, re-emerging on the other side of the Sun.  Some
days later, however, it disintegrated.  There have been about 1,600
Sun-grazing comets observed, but most of them have been very small and
seen only by satellites.  Some of the brightest recorded comets have
been from the same family, evidently all derived from a major body that
disintegrated into pieces on a previous return many centuries ago.
The 'Great Comets' of 1843, 1880, 1882 and 1887, as well as others
from previous centuries, were among them.  The moderator of these
Bulletins actually made a flight in an RAF training aircraft in a
pre-dawn effort to see the tail of the Sun-grazing Comet Ikeya-Seki
in 1965 at the time that it rounded the Sun; the comet became so
bright that it could be seen later that day, very close to the Sun, at
high noon, by observers standing in the shadow of a chimney or other
obstruction that blocked the direct view of the Sun.

Another large Sun-grazing comet, Comet Ison, is approaching, and may
become a spectacular sight in November.

University of Oxford

Scientists have investigated the compositions of supposed Martian
meteorites found on Earth and data from the 'Spirit' rover that
examined surface rocks in Gusev crater on Mars.  The fact that the
surface rocks are five times richer in nickel than the meteorites was
puzzling and had cast doubt on whether the meteorites were typical
volcanic products of Mars.  The team showed that both meteorites and
surface volcanic rocks are consistent with similar origins in the deep
interior of Mars but that the surface rocks come from a more oxygen-
rich environment, probably caused by recycling of oxygen-rich
materials into the interior.  This result is surprising because, while
the meteorites are areologically 'young', around 180 million to 1400
million years old, the rover was analysing a very old part of the
Martian surface, more than 3700 million years old.

Whilst it is possible that the composition of Mars varies a lot from
one place to another, researchers believe that it is more likely that
the differences arise through a process known as subduction, in
which material is recycled into the interior.  They suggest that the
Martian surface was oxidised very early in the history of the planet
and that, through subduction, the oxygen-rich material was drawn into
the shallow interior and recycled back to the surface during eruptions
4000 million years ago.  The meteorites, by contrast, are much younger
volcanic rocks that emerged from deeper within the planet and so were
less influenced by that process.  The implication is that Mars had an
oxygen-rich atmosphere at a time, about 4000 million years ago, well
before the rise of atmospheric oxygen on the Earth around 2500 million
years ago.  As oxidation is what gives Mars its distinctive colour it
is likely that Mars was wet, warm and rusty long before the Earth's
atmosphere became oxygen-rich.

BBC Online

The recently discovered fourth and fifth moons of Pluto now have
official names, Kerberos and Styx.  The International Astronomical
Union (IAU), which gives the official designations, stipulates in its
rules that names derive from mythology.  The names -- referring to a
three-headed dog and a river separating the living from the dead,
ranked second and third in an international public vote.  The winning
submission, Vulcan, was vetoed by the IAU on the grounds that it is
used elsewhere in astronomy, and not sufficiently associated in
mythology with Pluto, the ruler of the underworld.  The two moons,
formerly known simply as P4 and P5, were discovered in 2011 July and
2012 July, respectively.  Pluto was discovered by Clyde Tombaugh in
1930.  It is now regarded not so much as a planet as one of the many
objects in the Kuiper Belt, a disc-shaped volume of icy objects beyond


Astronomers using the new Atacama Large Millimetre Array (ALMA) have
imaged a region around a young star where dust particles can grow by
clumping together.  This is the first time that such a dust trap has
been clearly observed and modelled.  It illustrates how dust particles
in discs grow to larger sizes and can eventually form comets, planets
and other rocky bodies.  Computer models suggest that dust grains grow
when they collide and stick together.  However, when those bigger
grains collide at high speed they are often smashed to pieces again.
Even when that does not happen, the models indicate that the larger
grains would quickly move inwards because of friction between the dust
and gas and fall onto their parent star, leaving no chance that they
could grow further.  Somehow the dust needs a safe haven where the
particles can continue growing until they are big enough to survive on
their own.  Such 'dust traps' have been proposed, but there has been
no observational proof of their existence up to now.

Astronomers used ALMA to study the disc in a system called Oph-IRS 48.
They found that the star was surrounded by a ring of gas with a
central hole that was probably created by an unseen planet or
companion star.  Earlier observations by the VLT had already shown
that the small dust particles also formed a similar ring structure.
But the new ALMA view of where the larger millimetre-sized dust
particles were found was very different.  What had been discovered was
a region where bigger dust grains were trapped and could grow much
larger by colliding and sticking together.  It was a dust trap -- just
what the theorists were looking for.  The trap forms as bigger dust
particles move in the direction of regions of higher pressure.
Computer modelling has shown that such a high-pressure region can
originate from the motions of the gas at the edge of a hole just like
the one found in that disc.


Astronomers using the Swiss 1.2-m Euler telescope at the La Silla
Observatory in Chile have found a new type of variable star.  They
made regular measurements of the brightnesses of more than 3000 stars
in the open star cluster NGC 3766 over a period of seven years.  The
brightness of 36 of the stars exhibited tiny regular variations at the
level of 0.1% of the stars' normal brightness, with periods between
about 2 and 20 hours.  The stars are somewhat hotter and brighter than
the Sun, but otherwise apparently unremarkable.  The new class of
variable stars is yet to be given a name.  Many stars are known as
variable or pulsating stars, because their apparent brightness changes
over time.  How their brightness changes often depends in complex ways
on the properties of their interiors.  The phenomenon has allowed the
development of a whole branch of astrophysics called asteroseismology,
where astronomers follow stellar vibrations and can infer some of the
physical properties of the stellar interiors.  Although the cause of
the newly discovered type of variability remains unknown, there is a
tantalising clue: some of the stars seem to be fast rotators.  They
spin at speeds that are more than half of their critical velocity,
which is the threshold at which stars become unstable and throw off
material into space.


Nearly a decade ago, the Chandra X-ray Observatory saw signs of what
appeared to be a black hole accreting gas at the middle of the nearby
Sculptor galaxy (NGC 253).  Now, the 'Nuclear Spectroscopic Telescope
Array' (NuSTAR), which sees higher-energy X-ray light, has found the
black hole to be dormant.  The black hole is about 5 million times the
mass of our Sun and lies at the centre of NGC 253, a so-called
starburst galaxy actively giving birth to new stars.  At 13 million
light-years away, it is one of the closest starbursts to our own
galaxy, the Milky Way.  The Milky Way is much quieter than the
Sculptor galaxy.  It makes far fewer new stars, and its black hole,
about 4 million times the mass of the Sun, is also sleeping.  Black
holes feed off surrounding accretion discs of material, and when they
run out of that fuel, they go dormant.  NGC 253 is somewhat unusual
because the black hole is asleep in the midst of tremendous star-
forming activity all around it.  The findings are teaching astronomers
how galaxies grow over time.  Nearly all galaxies are suspected to
harbour super-massive black holes at their hearts.  In the most
massive ones, the black holes are thought to grow at the same rate
that new stars form, until radiation blasting from the black holes
ultimately shuts down star formation.  In the case of the Sculptor
galaxy, astronomers do not know whether star formation is winding down
or ramping up.

Chandra first observed signs of what appeared to be a feeding super-
massive black hole at the heart of the Sculptor galaxy in 2003.  As
material spirals into a black hole, it heats up to tens of millions of
degrees and glows in X-ray light that telescopes like Chandra and
NuSTAR can see.  Then, in September and November of 2012, Chandra and
NuSTAR observed the same region simultaneously.  The NuSTAR
observations -- the first to detect focused, high-energy X-ray light
from the region -- allowed the researchers to say conclusively that
the black hole is not accreting material.  Another possibility is that
the black hole was not actually awake 10 years ago, and Chandra
observed a different source of X-rays.  Future observations with both
telescopes may solve the puzzle.  The observations also revealed a
smaller, flaring object that the researchers were able to identify as
an 'ultra-luminous X-ray source', or ULX.  ULXs are black holes
feeding off material from a partner star.  They shine more brightly
than typical stellar-mass black holes generated from dying stars, but
are fainter and more randomly distributed than the super-massive holes
at the centres of massive galaxies.


Using more than 150 Chandra observations, spread over 13 years,
researchers have identified 26 black-hole candidates in the Andromeda
galaxy.  The candidates are in the stellar-mass category, meaning that
they formed at the demise of very massive stars and typically have
masses 5 to 10 times that of the Sun.  Astronomers can detect such
otherwise invisible objects as material is pulled from a companion
star and heated up to produce radiation before it disappears into the
black hole.  The first step in identifying the black holes was to make
sure that they were stellar-mass systems in the Andromeda galaxy
itself, rather than super-massive black holes at the hearts of more
distant galaxies.  To do that, the researchers used information about
the brightness and variability of the X-ray sources in the Chandra
data: stellar-mass systems vary much more quickly than super-massive
ones.  To classify the Andromeda systems as black holes, astronomers
observed that the X-ray sources had special characteristics: they were
brighter than a certain high level of X-rays, and also had a
particular X-ray 'colour'.  Sources containing neutron stars, the
dense cores of dead stars that would be the alternative explanation
for the observations, do not show both of those features simultaneous-
ly, but sources containing black holes do.  The XMM-Newton X-ray
observatory supported this work by providing X-ray spectra for some of
the candidates.

The research group previously identified nine black-hole candidates
within the region covered by the Chandra data, and the new results add
another 26.  They may be just the 'tip of the iceberg', since most
black holes won't have close companions and will be invisible to us.
Eight of the 26 are associated with globular clusters; that
differentiates Andromeda from the Milky Way, as astronomers have yet
to find a similar black hole in one of the Milky Way's globular
clusters.  Seven of the candidates are within 1000 light-years of the
Andromeda galaxy's centre -- more than the number of black-hole
candidates with similar properties near the centre of our own galaxy.
That is not necessarily a surprise, because the bulge of stars in the
middle of Andromeda is bigger.

University of Colorado at Boulder

New observations with the Hubble telescope's 'Cosmic Origins
Spectrograph' (COS) show that normal spiral galaxies are surrounded by
haloes of gas that can be over 1 million light-years in diameter.  The
current estimate of the diameter of the Milky Way, for comparison, is
about 100,000 light-years.  The gas in the haloes was ejected from the
galaxies by supernovae, and is stored and then recycled through an
extended galactic halo, falling back into the galaxies to invigorate
a new generation of star formation.  Building on earlier studies
identifying oxygen-rich gas clouds around spiral galaxies, scientists
think that such clouds may contain almost as much mass as all the
stars in their respective galaxies.  In addition, they found
reservoirs of hot gas enshrouding the spiral galaxies and haloes under
study.  The haloes of the spiral galaxies were relatively cool by
comparison.  Previous theoretical studies suggested that spiral
galaxies should possess about five times more gas than was detected;
the new COS observations are much more in line with theory.  The team
used distant quasars as 'torches' to track ultraviolet light as it
passed through the extended gaseous haloes of foreground galaxies.
Spectra yielded estimates of temperatures, densities, velocities,
distances and chemical compositions of the gas clouds.

Bulletin compiled by Clive Down

(c) 2013 the Society for Popular Astronomy

Good Clear Skies
Colin James Watling
Various Voluntary work-Litter Picking for Parish Council (Daytime) and also a friend of Kessingland Beach (Watchman)
Real Astronomer and head of the Comet section for LYRA (Lowestoft and Great Yarmouth Regional Astronomers) also head of K.A.G (Kessingland Astronomy Group) and Navigator (Astrogator) of the Stars (Fieldwork)
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