MARS IS SMALL
Science Daily
Certain American astronomers say that Mars developed in as little as
two to four million years after the birth of the Solar System, far
more quickly than the Earth, and that that helps to explain why it is
so small. They claimed that they could refine the chronology of the
formation of Mars by using the radioactive decay of hafnium to
tungsten in meteorites as a chronometer, but their (or their
apologists from 'ScienceDaily's) explanation of the procedure is
incomprehensible and there is no point in reproducing it here.
NEW LOOK AT PLANET GLIESE 581 d
Astropublishing
The planetary system around the red-dwarf star Gliese 581, just 20
light-years away, has caught the attention of astronomers in search of
planets that could support life. In 2007 two planets, called c and d,
were detected around Gliese 581; they are located at the inner and outer
edges of the 'habitable zone', which means merely the range of orbital
radii where a hypothetical rocky planet would be able to retain liquid
water on its surface. Of the two, d was thought to be too cold, so
the atmosphere on the night side would freeze and be precipitated out
onto the ground, making the surface uninhabitable. Later simulations
of c made that planet, too, seem less than hospitable owing to an
extremely strong greenhouse effect, like that of Venus, that would
boil any water.
Attention was shifted from both c and d when, at the end of 2010,
another small planet, g, was allegedly detected (like the others, by
the gravitational perturbations that the planets exert on the star),
with dimensions similar to the Earth's and located in the centre of the
'habitable zone'. But follow-up work indicates that g may not exist,
the original 'discovery' being a mistake. But now a new study has
appeared that suggests that d is somewhat similar to the Earth,
although with a mass seven times greater and a surface gravity double
the terrestrial value it is by no means a 'twin'. In view of the past
mistakes involving the Gliese 581 system, however, it would be prudent
not to over-do the optimism. The trouble with planets is that they are
very topical, and some of the people who are trying to find them feel a
need to publish premature and possibly erroneous conclusions for fear
that others will do so first.
'FREE-FLOATING' PLANETS
BBC News
Japanese astronomers claim to have found free-floating 'planets' which
do not seem to orbit a star. The objects revealed themselves by
bending the light of more distant stars, an effect called
'gravitational microlensing'. The mass of an object bends light, as
Einstein predicted. If a massive object passes in front of a more
distant star, it can act as a lens, bending and distorting the light
of that star so it may appear to brighten significantly. The
researchers examined data collected from microlensing surveys of the
central area of our own Milky Way and found evidence of ten Jupiter-
sized objects for which no parent star could be seen within 10
Astronomical Units (Sun-Earth distances) of them. Wildly extrapolat-
ing from the number of such bodies in the area surveyed, the
astronomers said that such objects could be extremely common.
According to astronomical convention, planets orbit a star or stellar
remnant, so if those objects have not got a host star, then they are
not within the definition of planets, even if they may have formed in
the same way as what we call planets, which is open to question.
SN 1987A CONTINUES TO BRIGHTEN
Nature
Astronomers from the University of Stockholm report that the remains
of supernova 1987A, that exploded in the Large Magellanic Cloud 24
years ago, are continually increasing in brightness. After the
explosion the remnant slowly faded, but about 2001 that tendency
reversed, and the structure continued to brighten in successive years,
until today, when the characteristic ring (about a light-year across)
which surrounds the site of the explosion is as bright as it has ever
been. The reason seems to be that, thousands of years before
exploding, the progenitor star of the supernova became unstable and
threw off a lot of surface material. After the star exploded, the
material that was expelled so violently is catching up with the slower
ejecta from the previous phase, compressing them and causing shock
waves where X-rays are emitted. The interaction of the X-rays with
gas in the remnant results in the emission of ultraviolet and visible
photons.
SOLITARY SUPERSTAR DISCOVERED IN NEARBY GALAXY
ESO
An international team of astronomers has used the Very Large Telescope
to study the star VFTS 682 in the Large Magellanic Cloud. They have
found spectroscopically that it is about 150 times the mass of the
Sun. Such stars have previously been found only in the crowded
centres of star clusters, but VFTS 682 exists on its own. It was
observed earlier in a survey of the most brilliant stars in and around
the Tarantula Nebula in the Large Magellanic Cloud, which has a lot of
gas, dust and young stars and is a very active star-forming region.
At first VFTS 682 was thought to be hot, young and bright, but
unremarkable, but the new study indicates that much of the star's
energy is being absorbed and scattered by dust clouds before it can
reach us, so the star is actually more luminous than previously
thought and is among the brightest stars known in terms of candle-
power.
Red and infrared light emitted by the star can get through the dust,
but the shorter-wavelength blue and green light is scattered more and
lost. As a result the star appears reddish, although if the view were
unobstructed it would shine a brilliant blue-white. As well as being
very bright, VFTS 682 is also very hot, with a surface temperature of
about 50 000 C. Although VFTS 682 seems now to be alone, it is not
very far away from the very rich star cluster R 136, which contains
several similar objects. There is no agreement as to whether such a
massive star could have been formed in the cluster and then been
ejected, although a layman might imagine that a measurement of its
motion would be of more value than speculation.
JUST 4% OF GALAXIES ARE SIMILAR TO MILKY WAY
National Science Foundation.
Researchers at Stanford University compared the Milky Way to similar
galaxies and found that just 4% are like ours. They were interested
in how the Milky Way fits into the broader context of the Universe,
and whether our Galaxy is typical or not. They compared the Milky Way
to similar galaxies in terms of luminosity, and distance to other
bright galaxies. They found galaxies that have two satellites that
are as bright and close by as the Milky Way's two closest satellites,
the Large and Small Magellanic Clouds, are rare. The findings are
based on analyses of data collected by the Sloan Digital Sky Survey.
BLACK HOLES SPIN FASTER
RAS
Two UK astronomers using radio, optical and X-ray data say that black
holes in the centres of galaxies are on average spinning faster than
they did long ago. There is evidence that many galaxies have black
holes in their centres. The holes have masses from a million to a
billion (10*9) Suns and are called 'supermassive'. They cannot be
seen directly, but material swirls around a hole in a so-called
accretion disc before it falls in and is lost. That material can
become very hot and emit radiation, including X-rays that can be
detected by space-based telescopes, whilst associated radio emission
can be detected by telescopes on the ground. As well as radiation,
twin jets are often associated with the accretion discs. There are
many factors that might cause jets to be produced, but the spin of the
supermassive black hole is believed to be important. However, there
are conflicting predictions about how the spins of the black holes
should be evolving, and their evolution has not been understood.
The astronomers compared theoretical models of spinning black holes
with radio, optical and X-ray observations, and said that the theories
can explain the population of supermassive black holes with jets.
Using the radio observations, they were able to sample the population
of black holes, deducing the spread of the power of the jets. By
estimating how they acquire material (the accretion process) the
scientists tried to infer how quickly the holes are spinning. The
observations might also give information on how the spins of the holes
have evolved. It seemed that, on average, supermassive black holes
are spinning faster now than they used to do. It is suggested that
those holes that grow by swallowing matter will barely spin, while
those that merge with other holes will be left spinning rapidly; so
the idea that in 'recent' times some of the most massive holes have
spun up may indicate that they have merged with other holes of
comparable mass.
SPIRIT ROVER COMPLETES MISSION ON MARS
NASA
The Mars rover 'Spirit' last communicated on 2010 March 22, as Martian
winter approached and the rover's solar-energy supply declined. The
rover operated for more than six years after landing in 2004 January
for what was planned as a three-month mission. NASA checked
frequently in recent months for possible reawakening of Spirit as
solar energy available to the rover increased, but got no reply.
NASA bosses are now concentrating on Spirit's still-active twin,
'Opportunity'.
Spirit drove 7.73 kilometres, more than 12 times the goal set for the
mission. The rover crossed a plain to reach a distant range of hills
that appeared as mere bumps on the horizon from the landing site,
climbed slopes up to 30 degrees, and covered nearly a kilometre after
one wheel seized in 2006. It sent back more than 124,000 pictures,
ground the surfaces off 15 rocks, and scoured 92 places with a brush to
prepare them for inspection with spectrometers and a microscope. One
major finding came, ironically, from dragging the seized wheel as the
rover was driving backwards in 2007. The wheel ploughed up bright
white soil, and Spirit's instruments identified the bright material as
nearly pure silica.
ROSETTA GOES INTO HIBERNATION
BBC News
Europe's Rosetta spacecraft, which is heading for a rendezvous with a
comet in 2014, has been put into hibernation by its controllers.
Nothing will be heard from the spacecraft for the next 2.5 years --
not even a reassuring beep. Rosetta should wake up on 2014 January
20. The spacecraft will then be just a few months away from its
appointment with Comet Churyumov-Gerasimenko out near Jupiter.
The plan is for it to orbit that 4-km ball of ice and dust and put a
small lander on its surface. Rosetta has actually seen the comet
already, even though it is still more than 150 million kilometres
away. The probe's camera system, managed by scientists from the
Max-Planck-Institute for Solar-System Research in Germany, recently
succeeded in identifying the comet against a dense field of stars in
Scorpius.
The probe's instruments had already been powered down in recent
months. Rosetta was turned so that its solar wings faced the Sun, and
was put in a gentle spin so that it no longer needed to use thrusters
to maintain a stable configuration. The onboard computer has been
left running to monitor two functions -- the clock system that will
eventually wake it up, and a pair of heaters, which will come on
occasionally to ensure that the entire satellite does not freeze up.
The 31 months of sleep will see Rosetta fly an arc from some 660
million km from the Sun out to 790 million km and back. The probe is
the most distant spacecraft to operate on solar power, but it is so
far away now that the solar panels are providing very little energy.
Putting the probe to sleep will draw minimum power. Rosetta was
launched in 2004 and has taken a rather circuitous route, making a
number of flybys of the inner planets and using their gravity to pick
up sufficient speed for the eventual comet encounter. It has already
delivered some fascinating science, particularly at the close passes
it made to two asteroids -- Steins in 2008 and Lutetia in 2010.
COSMIC DISTANCE RECORD BROKEN
BBC News
A cataclysmic stellar explosion may be the most distant single object
yet observed with a telescope. Scientists believe that the blast,
detected by the Swift space observatory, occurred 520 million years
after the Big Bang. That means that its light has taken 13 billion
years to reach Earth. The event, which was picked up by Swift in 2009
April, has been designated GRB 090429B. GRB stands for 'gamma-ray
burst' -- a sudden pulse of very-high-energy radiation usually
associated with extremely violent processes, such as the final
collapse of giant stars. The star would have been perhaps 30 times
the mass of our Sun. Swift, as its name implies, has to act quickly
to catch gamma-ray flashes because they often last only a few minutes.
Fortunately, an afterglow at longer wavelengths will persist sometimes
for days, which allows follow-up observations by other telescopes that
can then determine distance.
There are other competing candidates for the title of 'most distant
object'. Hubble, for example, was given much more powerful
instruments during its servicing mission in 2009, and teams working on
new images from the famous space telescope have seen galaxies that
look not far short of GRB 090429B -- and some potentially even further
away. Scientists are keen to observe at great distances because
they might learn how the early Universe evolved. They are particularly
keen to trace the very first populations of stars -- hot blue giants
that would have formed out of the cold neutral gas that pervaded the
young cosmos. Such stars would have had brilliant but brief lives,
producing the very first heavy elements.
VOYAGERS FIND 'MAGNETIC BUBBLES'
BBC News
The Voyager probes, which were launched in 1977, are now approaching
the very edge of the Sun's influence, more than 14 billion km away.
Sustained by their radioactive power packs, the probes' instruments
continue to function well and return data. They are so far away that a
radio message now has a travel time of about 16 hours.
Their information has allowed scientists to build a better picture of
what conditions are like in the zone where matter blown out by the Sun
pushes up against interstellar space. Computer modelling based on the
Voyager insights suggests that the edge of our Solar System is a froth
of activity, like an 'agitated jacuzzi'. Magnetic-field lines carried
in the wind of material coming from the Sun are breaking and
reconnecting, a process that is sculpting the wind into discrete
bubbles that are many tens of millions of kilometres across. It is
likely that the mass of individual magnetic structures actually makes
the Solar System more porous to cosmic rays. Researchers say that it
is more like a membrane that is permeable to the galactic cosmic rays,
so they expect those rays to enter and wander through the sea of
magnetic bubbles until they can access field lines that connect back
to the Sun and quickly escape. The observations are of interest not
just to physicists but also to astronauts, who must be protected
from the dangers of cosmic rays, and to spacecraft engineers who have
to 'harden' the electronic circuitry in satellites against the impacts
from high-energy particles. Astronomers confess to being surprised;
they thought that the outskirts of our solar neighbourhood would be
more sedate -- that the Sun's field lines would simply turn round and
reconnect with the Sun.
CHEMISTRY OF CHONDRITES
Science
According to some scientists, the carbon-containing chemicals found in
meteorites played a fundamental role in the appearance of life on
Earth. Given the presence of certain chemicals in meteorites, there
is little reason to suppose that they are unique to our own planet,
and a logical conclusion would be that similar compounds are found
throughout the Solar System. That hypothesis is supported by such
evidence as the similarity between certain carbonaceous chondrites
(rich in silicates and carbon compounds originating from the birth of
the Solar System) and the tail of comet Wild 2, Antarctic micro-
meteorites and interplanetary dust.
Despite the similarities, there are large chemical variations within
the carbonaceous-chondrite group, thought to be due to processes that
acted on their parent bodies after their formation. A study in that
field, led by the University of Alberta (Canada), shows that the
variations have their origin in hydrodynamic activity that occurred
when our Solar System was in its infancy, and the meteorites were part
of much larger bodies, such as asteroids. Among the samples studied
are pieces of a meteorite that fragmented during its fall onto the
frozen Tagish Lake in Canada, in 2000 January. After being recovered
they have been kept continuously below freezing temperature to
minimise possible terrestrial contamination. The variations in the
chemical make-up within the samples supports the hypothesis of
chemical and hydrothermal alteration within a larger parent body.
It would seem, then, that some of the compounds essential for life
were widely distributed throughout the primordial cloud from which the
Solar System formed.
Topic: Mid June Astronomy Bulletin (Read 1216 times)