Topic: Late March Astronomy Bulletin

[Clive]

HUBBLE WITNESSES ASTEROID'S DISINTEGRATION
NASA

The Hubble telescope has observed the break-up of an asteroid into as
many as 10 pieces, something that has never been witnessed previously.
Fragile comets, composed of ice and dust, have been seen falling apart
as they approach the Sun, but nothing like that has been observed in
the asteroid belt.  The crumbling asteroid, designated P/2013 R3, was
first noticed as an unusual, fuzzy-looking object by the Catalina and
Pan-STARRS sky surveys on 2013 Sept. 15.  A follow-up observation on
October 1 at the Keck Observatory showed three bodies moving together
in an envelope of dust nearly the diameter of the Earth.  Hubble
observations showed the fragments drifting away from each other at a
leisurely 1 mph.  The asteroid began coming apart early last year, but
new pieces have continued to appear in the most recent images.  It is
unlikely that the asteroid is disintegrating because of a collision
with another one, which would have been instantaneous and violent by
comparison with what is observed.  Debris from a collision would also
be expected to travel much faster than observed.  Nor is the asteroid
coming unglued because of the pressure of interior ices warming and
vaporizing.

The most likely cause of the asteroid's disintegration is a subtle
effect of sunlight, which causes its rate of rotation gradually to
increase.  Gravity is very weak on an asteroid, whose whole structure
may not be solid but may consist of a lot of pieces loosely held
together by the weak gravity.  As it is slowly spun up by sunlight,
its component pieces succumb to centrifugal force and gently move
apart.  The possibility of disruption in that manner has been
discussed by scientists before, but never reliably observed.  If the
process has been correctly identified, P/2013 R3 must have a weak,
fractured interior -- probably as the result of non-destructive
collisions with other asteroids.  Most small asteroids are thought to
have been severely damaged in that way.  With the previous discovery
of an active asteroid, P/2013 P5, sporting six tails, astronomers are
finding more evidence that the action of sunlight may be the primary
cause of disintegration of small asteroids less than a mile across.
P/2013 R3's remnant debris, withe a probable mass of about 200,000
tons, will represent a rich source of meteoroids.  Most will
eventually plunge into the Sun, but a small fraction of the debris may
one day blaze across our skies as meteors.


WISE DOES NOT FIND 'PLANET X'
NASA/JPL

A survey made by the Wide-Field Infrared Survey Explorer (WISE) has
turned up no evidence of the existence of a hypothesized Solar-system
body sometimes dubbed 'Planet X'.  Pundits had theorized about the
existence of such a large but unseen celestial body somewhere beyond
the orbit of Pluto.  In addition to 'Planet X', the body had garnered
other nicknames, including 'Nemesis' and 'Tyche'.  The recent study,
which involved an examination of WISE data covering the entire sky in
infrared light, found that no object the size of Saturn or larger
exists out to a distance of 10,000 AU, and no object as large as
Jupiter exists out to 26,000 AU.  Speculation about the body has come
in part from geological studies that suggested a regular timing
associated with mass extinctions of forms of life on Earth.  The idea
was that a large planet or small star hidden in the farthest reaches
of the Solar System might periodically sweep through bands of outer
comets, sending some of them towards us.  The Planet-X-based mass-
extinction theories were largely ruled out even before the new WISE
study.  Other theories based on irregular comet orbits had also
postulated a Planet-X-type body.  The new WISE study now argues
against those theories as well.

But searches of the WISE catalogue have not all been negative.  A
second study, which concentrated on objects beyond the Solar System,
found 3,525 stars and brown dwarfs within 500 light-years of the Sun.
The WISE mission operated in 2010 and early 2011, and performed two
full scans of the sky, with a six-month gap between scans.  The survey
showed images of nearly 750 million celestial objects.  Astronomers
could compare the two full-sky surveys to look for moving objects.
In general, the more an object appears to have moved between the
scans, the closer it is likely to be.  Searches of the WISE catalogue
for moving objects are finding some of the closest stars.  The
discoveries include a star about 20 light-years away in the constell-
ation Norma, and (as reported last March) a pair of brown dwarfs only
6.5 light-years away -- the closest star system to be discovered for
nearly a century.


LARGEST YELLOW HYPERGIANT STAR OBSERVED
ESO

Astronomers using the Very Large Telescope Interferometer (VLTI) have
found that the yellow hypergiant star HR 5171 A is 1300 times the
diameter of the Sun, much bigger than was expected and making it the
largest yellow star known.  It is also in the top ten of the largest
stars known -- 50% larger than Betelgeuse and about a million times
brighter than the Sun.  The new observations also showed that the star
has a very close binary partner; the two stars are so close together
that they are touching.  Yellow hypergiants are very rare, with only a
dozen or so being known in our galaxy, the best-known example being
Rho Cassiopeiae.  They are at a stage in their evolution when they are
unstable and changing rapidly, and they expel material, forming a
large, extended atmosphere around the star.  Despite its great
distance of nearly 12000 light-years, HR 5171 can just about be seen
with the naked eye by the keen-sighted.

The team looked at previous observations of the star spanning more
than 60 years.  It has been getting bigger over the last 40 years,
cooling as it grows, and its evolution has been caught in action.
Only a few stars are caught in that very brief phase, where they
undergo a dramatic change in temperature as they rapidly evolve.
Photometric observations show the object to be an eclipsing binary
system, with a period of 1300 days.  The smaller companion has a
surface temperature slightly hotter than HR 5171 A's 5000°C.  The
companion is very significant, as it can have an influence on the
evolution of HR 5171 A, for example by stripping off its outer layers.


A LENS BIGGER THAN A GALAXY
NASA

Researchers have been observing a particularly distant patch of sky
where objects are brightened up by a 'lens' that is actually a massive
cluster of galaxies known as Abell 2744.  As predicted by Einstein's
theory of General Relativity, the mass of the cluster warps the fabric
of space around it.  Starlight passing by is deflected, somewhat as by
an ordinary lens.  Lately, the Hubble telescope, along with the
Spitzer space telescope and the Chandra X-ray observatory, has been
looking through the gravitational lens as part of a programme called
'Frontier Fields' which is an effort to explore the first billion
years of the Universe's history.  An international team looking at the
observations of Abell 2744 has discovered one of the most distant
galaxies ever seen -- a star system 30 times smaller yet 10 times more
active than the Milky Way.  Bursting with new-born stars, the object is
giving astronomers a rare glimpse of a galaxy born not long after the
Big Bang itself.  Overall, the Hubble exposure of Abell 2744 has
revealed almost 3,000 distant galaxies magnified and brightened as
much as 10 to 20 times.  Without the boost of gravitational lensing,
almost all of them would be invisible.  'Frontier Fields' is using six
clusters of galaxies as gravitational lenses.


GALAXIES IN EARLY UNIVERSE MATURE RAPIDLY
Swinburne University of Technology

An international team of researchers, including astronomers from
Swinburne University of Technology, has discovered the most distant
examples of galaxies in the early Universe that were already mature
and massive.  The mature galaxies were found at a distance of 12
billion light years, seen now as they were when their light started
out when the Universe was just 1.6 billion years old.  Astronomers
used deep images at near-infrared wavelengths to search for galaxies
with red colours in the early Universe (red colours indicate the
presence of old stars and a lack of active star formation).  Using
special filters to produce images in narrow slices of the near-
infrared spectrum, they were able to estimate distances to thousands
of distant galaxies at a time, providing a 3-D map of the early
Universe.  Surprisingly, they located 15 galaxies at an average
distance of 12 billion light years.  The galaxies are barely
detectable at visual wavelengths, but in near-infrared light they are
easily observed, from which it may be inferred that they already
contained as many as 100 billion stars on average per galaxy.  The
mature galaxies probably have masses similar to that of the Milky Way,
but had already finished star-formation when the Universe was only 12%
of its current age.  While the Milky Way still forms new stars at a
slow rate today, the galaxies referred to here must have formed in a
relatively 'short' time -- roughly one billion years -- with rates of
star-formation several hundred times higher than in the Milky Way
today.  This is the best evidence to date that some galaxies grew up
'quickly'.  The finding raises new questions about how the galaxies
formed so rapidly and why they stopped forming stars so early.