Topic: Late October Astronomy Bulletin

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WATER SIMILAR TO THAT IN EARTH'S OCEANS FOUND IN COMET
The Register

Ice found in a comet is consonant with a theory that the Earth's
oceans were delivered here by comets.  (Of course it would take an
awful LOT of comets to fill an ocean!)  Be that as it may, it has been
proposed that the oceans formed about 8 million years after the Earth
itself.  An instrument on the Herschel space observatory is able to
determine the isotopic composition of hydrogen, and has found that ice
in Comet Hartley 2 has the same isotopic composition, i.e. the same
proportion of deuterium to ordinary hydrogen, as the water found in
our oceans.  Herschel has observed six other comets, but only the ice
in Hartley 2 matches the Earth's water.  Hartley 2 comes from the
Kuiper belt, not too far beyond Pluto, whereas the other five come
from the hypothesised far-off Oort Cloud.  The Kuiper belt is about 30
times further from the Sun than the Earth is; the Oort Cloud is
supposed to be more than 5,000 times further out than the Earth.

Meanwhile, analysis of data from the Deep Impact spacecraft, which
passed close to Hartley 2 on 2010 November 4, shows that that comet
throws off much more material in relation to its own size than Tempel
1, which was encountered by Deep Impact in 2005, or Wild 2, which was
observed by the Stardust mission.  Halley, which was observed by the
Giotto mission, lies somewhere in the middle of the spectrum of
activity.  Hartley 2 also shows surprising diversity -- ice on the
comet's sunlit surface is found in patches that are isolated from
areas of dust.  In addition, one lobe of the dog-bone-shaped comet
may have lost much more of its primordial material than the other,
suggesting that Hartley 2 may originally have been two comets that
came together in a gentle collision.


THE CAUSE OF ASTEROID SCHEILA'S OUTBURST
RAS

On 2010 December 12 a 110-km asteroid named Scheila changed its
appearance and looked more like a comet, with a bright tail.  Comets
tend to have highly elliptical orbits that keep them most of the time
in the cold outer parts of the Solar System, and it is when they come
briefly close to the Sun that the heat causes icy material in the
comet to vaporize and stream out, forming the characteristic tails.
Asteroids, however, are supposed to be just rocky bodies that
primarily circle the Sun between the orbits of Mars and Jupiter.
That an asteroid acquired a comet-like tail took some explaining.
Astronomers measured the brightness of Scheila's tail, noting how it
declined over the course of several weeks.  They think that the tail
was caused by another object colliding with Scheila and causing debris
to be thrown off.  The best estimates are that the collision occurred
within three days either side of 2010 November 27, and that the
impactor had a diameter in the range 60--180 m.


ABOUT URANUS AND ITS SIDEWAYS SPIN
RAS

Uranus is unusual in that its spin axis is inclined by 98° to its
orbital plane around the Sun.  That is quite different from the axial
inclinations of the other planets (Mercury O°, Venus 177°, Jupiter
3°, or the Earth, Mars, Saturn and Neptune, all around 25°),
although Pluto, at 120°, is not so very different.  Uranus is, in
effect, spinning on its side.

It has been suggested that in the past a body a few times more massive
than the Earth collided with Uranus, knocking the planet on its side.
There is, however, a significant flaw in that notion: the moons of
Uranus should have been left orbiting in their original plane, but
they too lie at almost exactly 98 degrees.  Scientists at Observatoire
de la Cote d'Azur in Nice have now realised that if Uranus had been
hit when still surrounded by a protoplanetary disc (the material from
which the moons would form) then the disc would have re-formed in the
new, highly-tilted equatorial plane, and would then go onto form the
moons in the positions in which we see them today.  The computer
models, however, threw up an unexpected result: the moons displayed
retrograde motion -- they orbited in the opposite direction to that
which we observe. The researchers discovered that if Uranus were not
tilted all in one go, but rather was bumped in at least two smaller
collisions, then there is a much higher probability that the moons
would orbit in the direction we observe.  The received theory includes
an assumption that the outer planets formed by accreting only small
objects in their protoplanetary discs; the indication now that Uranus
may have suffered at least two major impacts may warrant a revision of
that assumption.


UNUSUAL MULTI-PLANET SYSTEM DISCOVERED
NASA

The Kepler spacecraft is constantly measuring, very accurately, the
magnitudes of a large number of stars in a field in Cygnus, looking
for small dips in brightness caused by planets transiting across the
faces of the stars.  It has recently discovered an unusual planetary
system containing a super-Earth and two Neptune-sized planets orbiting
in resonance with each other around Kepler-18, a star that is just 10%
larger than the Sun and contains 97% of the Sun's mass.  The planets
are designated b, c, and d; they all orbit much closer to Kepler-18
than Mercury does to the Sun.  The closest, b, has a 3.5-day period,
is about 6.9 times the Earth's mass, and twice Earth's size; it is
what has been dubbed a 'super-Earth'.  Planet c has a mass of about
17 Earths, is about 5.5 times the Earth's size, and orbits in 7.6
days.  Planet d has about 16 Earth masses, 7 times Earth's size, and
a 14.9-day orbit.  The masses and sizes of c and d qualify them as
low-density 'Neptune-class' planets.

Planet c orbits the star twice for every one orbit that d makes.  But
the times that each of the planets transit the face of Kepler-18 are
not keeping exactly on that orbital period.  One is slightly early
when the other one is slightly late, then both are on time at the same
time, and then vice-versa.  They are in what is called an orbital
resonance, in which they interact with one another and periodically
exchange energy.


SEVEN SUPERNOVAE IN ONE GALAXY
ScienceDaily

It is hardly surprising that galaxies that are prolifically forming
stars are also prolifically letting off supernovae.  Now seven
previously unknown supernovae have been detected in a galaxy called
Arp 220, 250 million light-years away.  Astronomers using a worldwide
network of radio telescopes obtained extremely sharp images of Arp
220. They observed about 40 radio sources in the centre of the galaxy;
the sources are invisible in ordinary telescopes, hidden behind thick
layers of dust and gas.  The variation of radio brightness with wave-
length and time convinced the astronomers that seven of the sources
are supernovae that appear to have exploded within the last 60 years.
In Arp 220, we see far more supernovae than in our own Galaxy, the
Milky Way, in which there is said to be only one supernova per
century.  (But the last one was seen in 1604!)  The radio measurements
have also offered some insight into how radio waves are generated in
supernovae and their remnants.


CRAB PULSAR BEAMS UNEXPECTEDLY ENERGETIC GAMMA RAYS
University of California

The Crab pulsar is a rapidly spinning neutron star, the collapsed core
of a massive star that exploded in a spectacular supernova in the year
1054, leaving behind the brilliant Crab Nebula, with the pulsar at its
heart. It is one of the most intensively studied objects in the sky.
Rotating about 30 times a second, the pulsar has an intense,
co-rotating magnetic field from which it emits beams of radiation.
The beams sweep around like a lighthouse beacon because they are not
aligned along the star's rotation axis, so although the beams are
steady, they are detected on Earth as rapid pulses of radiation.
Scientists have long agreed on a general picture of what causes pulsar
emission -- electromagnetic forces created by the star's rapidly
rotating magnetic field accelerate charged particles to near the speed
of light, producing radiation over a broad spectrum -- but the details
remain obscure.  The picture led to an expectation of an exponential
decay of the emission spectrum above about 10*10 electron volts (10
GeV), so it has come as a surprise that the Whipple Observatory in
Arizona has now found pulsed gamma-ray emission at energies above 100
GeV -- far beyond what current theoretical models of pulsars can
explain.


THE TIME OF RE-IONISATION
ESO

It is thought that, early in the history of the Universe, there was a
time when the initial universal fog of neutral hydrogen gas was
clearing, allowing ultraviolet light to pass unhindered for the first
time.  It is called the 'epoch of re-ionisation', and occurred about
13 billion years ago.  ('Neutral' hydrogen atoms consist of a proton
and an electron; they can be 'ionised' -- the electron can be removed
-- by energetic (ultraviolet-light) photons.  Such photons trying to
pass through a lot of neutral hydrogen gas get absorbed and their
energy goes into ionising the  gas; when the gas is already ionised
there is nothing left for them to do, so then they can pass freely.)

Scientists have been using the Very Large Telescope to observe some
very distant galaxies, in an effort to establish a calendar for
re-ionisation.  It seems that that phase must have happened more
quickly than was previously thought.  One of the strongest ultraviolet
emission lines is the Lyman-alpha line of hydrogen, which is bright
and recognisable enough to be seen even in observations of very faint
galaxies.  Observations of the Lyman-alpha line in the spectra of five
very distant galaxies allowed the team to do two things.  First the
red-shift of the line enabled them to place them in chronological
order.  Secondly, they were able to see the extent to which the
Lyman-alpha emission from within the galaxies was absorbed by the
neutral-hydrogen fog in inter-galactic space at different points in
time.

The scientists saw a great difference in the amount of ultraviolet
light that was blocked between the earliest and latest galaxies in
their sample.  When the Universe was 780 million years old the neutral
hydrogen was quite abundant, filling from 10 to 50% of the Universe's
volume.  But 'only' 200 million years later the amount of neutral
hydrogen had dropped to a very low level, similar to its present one.
As well as indicating the rate at which the primordial fog cleared,
the observations also hint that the source of the ultraviolet light
which ionised the hydrogen was the first generation of stars, which
must have included many very massive stars which would burn up very
quickly and emit a great deal of ultraviolet.