PLANETS
By Andrew Robertson, Planetary Section Director
Mercury reaches greatest western elongation of 22° on December 23, so
it will be a morning object visible in the SE before sunrise. At the
start of civil twilight (07:19, about 41 minutes before sunrise) on
December 18 and 19 it will be at 9° altitude. By the 23rd it will be
at 8° altitude and by the 27th it will be down to 7° altitude, and
thereafter it descends quickly. So it is an opportunity for early
risers with a good horizon to the SE to view that elusive planet.
Twilight times are for my location at 1°.5 east of Greenwich; they
will be four minutes later for each degree further west you are.
Latitude may make a difference too.
IS VESTA THE 'SMALLEST TERRESTRIAL PLANET?'
NASA
The 'Dawn' spacecraft has spent the last four years on its way to the
asteroid Vesta, which was discovered over 200 years ago but until now
has been seen only as little more than an unresolved starlike point
and considered simply as a large rocky asteroid. Now the spacecraft's
instruments are revealing its true complexity. Astronomers are seeing
enormous mountains, cliffs, craters of all sizes, and plains. It has
a rich chemical history and an iron core. Its surface features
indicate that it is 'differentiated' like the 'terrestrial' planets
Mercury, Venus, the Earth and Mars. Differentiation is what happens
when the interior of a planet gets hot enough to melt, separating its
materials into layers: the light material floats to the surface while
the heavy elements, such as iron and nickel, sink to the centre.
The history begins about 4.57 billion years ago, when the planets of
the Solar System started forming from the primordial solar nebula.
As Jupiter gathered itself together, its powerful gravity stirred up
the material in the asteroid belt so objects there could no longer
coalesce. Vesta was in the process of growing into a planet when
Jupiter interrupted the process. Although Vesta's growth was stunted,
it is still differentiated like a true planet. Astronomers believe
that the Solar System received radioactive aluminium and iron from a
nearby supernova explosion at the time when Vesta was forming. Those
materials decayed and gave off heat. As the asteroid was gathering
material into a big ball of rock, it was also trapping the heat inside
itself. As Vesta's core melted, lighter materials rose to the
surface. Vesta may have had volcanoes and flowing lava at one time,
although none has yet been found. Vesta has so much in common with
the terrestrial planets that there may be a case to have it re-
classified as a 'dwarf planet'. Dawn will leave Vesta towards the
end of 2012 for Ceres, where it will arrive in 2015.
PLANET IN 'HABITABLE ZONE' OF SUN-LIKE STAR
NASA
A newly confirmed planet, Kepler-22b, is about 2.4 times the radius of
the Earth and is said to be in the 'habitable zone' of its star
(Kepler 22), about 600 light-years away. Although it is much larger
than the Earth, its 290-day orbit is much like our own, and its star is
a G-type one like the Sun, but slightly smaller and cooler. Scientists
do not know anything about Kepler-22b's composition, but its discovery
is claimed to be a step closer to finding Earth-like planets.
Kepler stares continuously at a particular area of sky in Cygnus and
Lyra; it measures the brightness of more than 150,000 stars, looking
for dips that may be caused by planet candidates that transit in front
of the stars. It requires at least three dips equally spaced in time
to verify a signal as a transit. Since its first list was published
last February, the number of planet candidates identified by Kepler
has nearly doubled, to over 2000, but now there are only 48 candidates
deemed to be in their respective stars' 'habitable zones', an actual
decrease from the 54 reported in February, largely because the Kepler
team has adopted a more realistic definition of what constitutes the
zone. Candidates require follow-up observations to see whether they
are actual planets. The Kepler team uses ground-based telescopes and
the Spitzer space telescope to review planet candidates found by the
spacecraft. The matter is seen a bit more in perspective when it is
recognised out that, out of the 54 'habitable-zone' planet candidates
reported in February, Kepler-22b is the *only one* that has been
confirmed as a planet.
[(Editorial aside:) The 'habitable zone' of a planetary system merely
refers to the band of orbits where the mean temperature of a body in
thermal equilibrium between the heat arriving from the star and that
conjectured to be lost by the planet's own radiation into space should
allow liquid water to exist on the surface. The anthropomorphic
reference to 'habitability', when all that is meant is that the
surface temperature promises to allow water to be a liquid rather than
ice or steam, appears to be a ruse to persuade people in general, and
perhaps funding agencies in particular, to believe that the discovery
of such a body brings very near the discovery of 'little green men'.
More realistic souls have drawn attention to the fact that there is
much more to habitability than mere equilibrium temperature. If the
Kepler team lived on Alpha Centauri it would probably consider Venus,
the Moon and Mars to be in the Sun's 'habitable zone', but living as
we do nearer at hand we can see that neither Venus nor the Moon would
be likely to represent pleasant abodes even for little green men.
Obviously there has to be, around *any* star, a range of radii that
satisfies the basic temperature demand of 'habitability': at the
surface of the star it is too hot and in the far distance (outer
space) it is too cold, so somewhere in between it must be 'just
right'. There is no sense in which an Earth-like temperature is un-
usual or anomalous -- it can not avoid occurring over some appreciable
range of radii around every star, although in an individual case (that
of Venus is notable) the hypothetical temperature calculation is
falsified by conditions related to the characteristics of the planet
itself.]
FASTEST ROTATING STAR
ESO
An international team of astronomers has been using the Very Large
Telescope at the Paranal Observatory in Chile to make a survey of the
most massive and luminous stars in the Tarantula Nebula, in the Large
Magellanic Cloud. Among the many brilliant stars there, the team has
discovered one, called VFTS 102, that has an equatorial rotational
velocity of more than 600 km/s -- more than 300 times faster than the
Sun and very close to the point at which it would be torn apart by
centrifugal forces. VFTS 102 is the fastest-rotating star known to
date. The astronomers also found that the star, which is around 25
times the mass of the Sun and about 100,000 times brighter, is moving
through space at a significantly different speed from its neighbours.
That could imply that it has been ejected from a double-star system
after its companion exploded as a supernova, an idea that is supported
by the existence of a pulsar and an associated supernova remnant in
its vicinity. Its high rotational velocity might have arisen from its
accretion of a stream of gas lost from the hypothetical companion star
before the latter exploded as a supernova.
DOUBLE STAR OBSERVED WITH VLT INTERFEROMETER
ESO
In a considerable technical achievement, four telescopes at ESO's
Paranal Observatory in Chile have been used together as an
interferometer with a 130-m baseline, giving the resolution equivalent
to that of a telescope of that aperture. The system was used to
observe the double star SS Leporis, whose components orbit one another
in 260 days. The stars are separated by only a little more than the
distance between the Sun and the Earth, while the larger and cooler of
the two stars is a red giant that extends about a quarter of the way
to its hot companion, a distance corresponding roughly to the orbit of
Mercury. The hot component appears to have accreted about half of the
mass that was originally in the giant. The new observations show that
the giant star is smaller than was previously thought, making it much
more difficult to explain how it has lost matter to its companion.
The astronomers now think that, instead of the matter pouring off the
giant through the gravitational effect of the hot star, a process
called 'Roche-lobe overflow', it must be being expelled from the giant
star as a stellar wind, like the 'solar wind' but a lot denser, and
captured by the hot companion.
CLUSTER HAS MANY MASSIVE STARS
University of Toronto.
Astronomers have found the most numerous batch of young, massive stars
yet observed in our Galaxy, in a cluster of hundreds of thousands of
stars that include several hundred massive blue stars dozens of times
the mass of our Sun. The light of all the newborn massive stars is so
intense that it has expelled and heated the gas that remained in the
cluster after their birth, creating a glowing hollow shell about a
hundred light-years across. Comparable clusters of massive stars have
been noticed in other galaxies, but they are so far away that all the
stars tend to appear blurred together. The cluster now observed is
located nearly halfway across our galaxy, 10,000 parsecs (30,000
light-years) away, and the line of sight is badly obscured by dust.
The researchers used the New Technology Telescope at ESO; they could
measure only the brightest stars in the cluster, but from the
uppermost portion of the colour-magnitude diagram they deduced that at
least a dozen stars were of the most massive types, some possibly a
hundred times as massive as the Sun. In fact, before observing the
cluster from the ground, the team had first noticed the glow from the
large shell of heated gas in images from the WMAP satellite, which is
sensitive to microwaves (between radio waves and visible light). To
make an image of the gas shell being blown away and heated up, they
used the Spitzer satellite, which works with infrared light. Most of
the light emitted by the heated gas is infrared which is less obscured
than visible light by gas or dust. The giant stars in the cluster
emit mostly in the ultraviolet, which is very badly blocked by dust
and thus not visible in the image. Such stars burn their nuclear fuel
very quickly in astronomical terms; they will last only a few million
years, even though they contain dozens of times more fuel than the
Sun.
GALACTIC TAILS
RAS
An international group of astronomers has discovered in two groups of
galaxies extraordinarily long one-sided gaseous tails that are amongst
the longest structures ever observed in such environments. They
emanate from CGCG 097-026 and FGC1287, two spiral galaxies in small
groups in the outskirts of the cluster known as Abell 1367 in the
constellation of Leo, at a distance of 100 Mpc (Megaparsecs). The
astronomers used the Expanded Very Large Array radio telescope in the
USA to study Abell 1367. The projected lengths of the gaseous tails
are about 9 and 10 times the sizes of the respective parent galaxies,
i.e. about 160 and 250 kiloparsecs. In both cases the amount of cold
hydrogen gas in the tails is approximately the same as that remaining
in the galaxies themselves. In other words, the galaxies have lost
half their fuel for star formation. There is an idea that galaxies in
large clusters lose their gas through its being swept out by the
movement of the galaxies through the intergalactic gas already present
there, but the galaxies concerned here are only on the outskirts of
the cluster and ought not to have lost their gas in that way. The
origin of the extraordinary tails remains a puzzle for the scientists.
MASSIVE BLACK HOLES DISCOVERED
University of California - Berkeley.
The most massive black holes are found in elliptical galaxies, which
are thought to result from mergers of two or more galaxies. To date,
approximately 63 super-massive black holes have been found in the
cores of nearby galaxies. The most massive one known until now was
one reputedly of 6.3 billion solar masses in the centre of the
'nearby' galaxy M87. Astronomers at the Gemini, Keck and McDonald
observatories have been obtaining spectra of the diffuse starlight at
the centres of several massive elliptical galaxies, each the brightest
galaxy in its cluster. In that way they can estimate a velocity
dispersion for the unresolved stars, and try to interpret it to
indicate the mass of the central object. Finding huge masses
contained within a volume only a few hundred light-years across, the
astronomers conclude that the masses must be massive black holes.
They have found two that appear to have masses of about 10 billion
Suns, though the uncertainties in the actual values are substantial.
The holes are at the centres of two elliptical galaxies, both about
100 Mpc away. One is in the elliptical galaxy NGC 3842, the brightest
galaxy in the Leo cluster; the other is in NGC 4889, the brightest in
the Coma Berenices cluster. Physical laws show that holes of such
masses must have event horizons (distances from within which nothing,
even light, can escape) at radii of about 200 astronomical units (five
times the size of the orbit of Pluto). The black hole at the centre
of the Milky Way is less massive by a factor of about 2500, and its
event horizon is at a distance about one-fifth the radius of the orbit
of Mercury.
TWO SPECULATIONS ABOUT THE CHRISTMAS GAMMA-RAY BURST
NASA
Two competing theories have been put forward, in separate papers
published in 'Nature', to explain a gamma-ray burst (GRB 101225A) that
was detected in Andromeda by the orbiting 'Swift' observatory on
Christmas Day 2010. Follow-up observations by the Hubble telescope
and ground-based ones failed to determine its distance. The gamma-ray
emission lasted at least 28 minutes, which is unusually long.
Astronomers from the Institute of Astrophysics of Andalusia in Spain
suggest that the burst occurred in a binary system where a neutron
star orbited a normal star that had just entered its red-giant phase,
enormously expanding its outer atmosphere. The expansion engulfed the
neutron star, they say, causing the giant's atmosphere to be ejected
and the neutron star's orbit to be made smaller. The neutron star may
then have merged with the giant's core, creating a black hole and
oppositely-directed jets of particles, producing gamma rays, followed
by a weak supernova. The team says that the event would have to have
taken place about 2000 Mpc away. Astronomers at Brera Observatory in
Italy, however, suggest an entirely different model. It involves the
tidal disruption of a large comet-like object and the ensuing crash of
debris onto a neutron star located 'only' about 3000 parsecs away.
Gamma-ray emission would have occurred when debris fell onto the
neutron star; X-ray variations lasting several hours, detected by
Swift's X-Ray telescope, might have been caused by late-arriving
clumps of matter striking the neutron star.
REDDEST GALAXIES IN THE UNIVERSE
Astropublishing
A group from the Harvard-Smithsonian Center for Astrophysics has
discovered four extremely distant galaxies that are amongst the
reddest ever detected. Located at a distance of around 4000 Mpc, they
must have formed during the first billion years of existence of the
Universe. The four galaxies appear to be physically related to each
other; one also shows evidence of an active nucleus, presumably owing
to the presence of a super-massive black hole. They were observed
with the Spitzer space telescope at infrared wavelengths, where the
galaxies are much brighter than they are in visible light. Redness in
distant galaxies can arise in three ways -- a large population of very
red stars, dust in their interstellar medium, or a great distance
resulting in a large red-shift caused by the Doppler effect.
According to the researchers involved in this study, it is likely that
all three effects are important in the galaxies that they observed.
CLOSEST SPACECRAFT TO APPROACH PLUTO
Johns Hopkins University
The 'New Horizons' mission has reached a milestone on its way to
Pluto, now being closer to Pluto than any previous spacecraft has
been. It has taken New Horizons 2,143 days of high-speed flight --
covering more than a million kilometres a day for nearly six years --
to break the closest-approach mark of 1.58 billion kilometres set by
Voyager 1 in 1986. New Horizons is healthy and on course for its
closest approach of 12,500 kilometres to Pluto on 2015 July 14. At
its current distance from Pluto (about as far as from here to Saturn)
Pluto remains just a faint point of light. But when it rushes through
the Pluto system in 2015, its cameras will be able briefly to observe
fine details on the former planet and it moons. New Horizons is
currently in hibernation, with all but its most essential systems
turned off; operators will waken the spacecraft in January for a
month of testing and maintenance activities.
Topic: Mid December Astronomy Bulletin (Read 922 times)