SOLAR WIND LOSES POWER
NASA
The Ulysses spacecraft has found that the average pressure of the
solar wind has dropped more than 20% since the mid-1990s, to the
weakest that it has been since monitoring began almost 50 years ago.
Curiously, the speed of the million-mph solar wind has decreased
only 3%; the change in pressure has come mainly from reductions in
temperature and density. The solar wind is 13% less hot and 20% less
dense. Ulysses has also found that the Sun's underlying magnetic field
has weakened by more than 30% since the mid-1990s. Flagging solar
wind has repercussions across the entire Solar System, beginning with
the heliosphere. The heliosphere is a bubble of magnetism springing
from the Sun and inflated to colossal proportions --to beyond Pluto --
by the solar wind. The heliosphere shields the Solar System to some
extent from Galactic cosmic rays. The degree of shielding is less now
than it was, owing to the reductions in both the magnetic field
strength and in the solar wind. There is, however, no threat to
people on the Earth, as our thick atmosphere and planetary magnetic
field provide additional layers of protection that keep us safe.
But if the trend continues, astronauts on the Moon or en route to Mars
would get a higher dose of cosmic radiation, and space probes and
satellites in high Earth orbit are at an increased risk of instrument
malfunctions due to cosmic-ray strikes.
PHOENIX SEES DUST DEVILS
NASA
The Phoenix Mars lander has photographed several dust devils dancing
across the arctic plain on which it landed. Pictures taken on Sept. 8
show at least six different dust devils, ranging from about 2 to 5
metres in diameter. Images from spacecraft orbiting Mars had
previously indicated that dust devils occur in the region where
Phoenix landed. The ones that Phoenix has observed so far are much
smaller than those that the 'Spirit' rover has photographed much
closer to the equator. Phoenix monitors air pressure, and on the day
that the camera saw dust devils, the barometer recorded a sharper dip
than ever before. The change was still less than the daily change in
air pressure from daytime to nighttime, but over a much shorter time.
Also on the same day, Phoenix's anemometer indicated wind speeds
exceeding 5 metres per second.
DIFFUSION CAUSED JUPITER'S WHITE SPOT TO TURN RED
RAS
A study has suggested how Oval BA, an anticyclone on Jupiter, suddenly
turned from white to red in a period of just a few months. Oval BA
was formed in 2000 by the merger of smaller vortices called the White
Ovals, in a chain of collisions that started in 1998. The apparent
reddening was first reported by amateur astronomers in early 2006, but
it was not until April that professional astronomers were able to
image the impressive alteration of the oval, which is the
second-largest storm in the Solar System after the Great Red Spot
(GRS).
Using computer models, researchers tried to identify causes for the
colour change, including alterations to dynamical, photochemical and
diffusion processes. The most likely cause appears to be an upward
and inward diffusion of either a coloured compound or a substance
that may interact later with high-energy solar photons at the upper
levels of Oval BA. Comparing Oval BA with the GRS, the group found
that the GRS is still redder than BA, most likely because it is higher
in Jupiter's atmosphere, thicker and contains a higher concentration
of the unidentified chemical agents that give Jupiter its browny-red
colour.
The group was able to rule out the possibility that the reddening was
caused by any dynamical process. They found no change to the strength
of the 'hurricane' and, although some changes in the circulation
around the spot had taken place, the maximum wind speeds (up to 400
km/h or more) were consistent with measurements made before 2000 of
the Oval or its white predecessors. The group modelled the wind flow
in detail with high-resolution simulations, to try to understand why
the red material may be confined to the annulus region and how the
colour change happened on the observed time-scale. The model accounts
well for the temperature and wind structure inside Oval BA.
SATURN'S RINGS MAY BE MORE MASSIVE, OLDER
RAS
Saturn's rings may be more massive than previously thought, and
potentially much older. Astronomers at the University of Colorado
tried to model a simplified version of the ring system, by computing
the gravitational attraction and collisions between more than 100,000
particles, representing a sample of those in Saturn's rings. They
followed the orbit and history of each individual particle, and
calculated the amount of starlight that would pass through the ring.
Their results were compared to Cassini observations of starlight
blocked by the rings, a method that has long been used to estimate the
total amount of material in the ring system. In 1983 that method led
to am estimate that the rings contain as much material as Saturn's
moon Mimas, which is about 250 miles across. The new simulations
indicate that the ring particles aggregate into clumps, which would
lead to the previous estimate being low by a factor of 3 or more.
Calculations suggest that meteorites slowly grind and shatter the
particles in the ring. Gradually, a layer of dust and fragments
builds up and covers each particle. The layer includes both ice (from
the particle) and meteoritic dust. As time passes, the ring system
becomes increasingly polluted and darkened by meteoritic dust.
Because the rings appear so clean and bright, it has been argued that
they are much younger than Saturn, which is some 4.5 billion years
old; in fact it was concluded from Voyager measurements that the rings
are only about 100 million years old. The new calculations show that
if the rings are more massive, they appear less polluted, and thus
could be proportionately older. Recycling of ring material extends
their lifetime and reduces the expected darkening.
5TH DWARF PLANET NAMED HAUMEA
The Register
The International Astronomical Union (IAU) has decreed that the object
formerly known as 2003 EL61 will henceforth be known as Haumea, and
joins Ceres, Eris, Makemake and Pluto in the Solar System's league of
dwarf planets. The IAU describes trans-Neptunian Haumea as a
fast-spinning "bizarre object with a shape resembling a plump cigar",
with a diameter "approximately the same as that of the dwarf planet
Pluto". The name is taken from the goddess of childbirth and
fertility in Hawaiian mythology, which the IAU considers particularly
appropriate since "Haumea also represents the element of stone, and
observations of Haumea hint that, unusually, it is almost entirely
composed of rock with a crust of pure ice".
The IAU notes that Haumea is joined in its orbit by two satellites
thought to have been created by debris knocked off its surface by
historic impacts. The goddess Haumea's children "sprang from
different parts of her body", according to Hawaiian lore. The larger
satellite has been christened Hi'iaka in honour of Hawaii's patron
goddess, who was born from the mouth of Haumea. The second is
Namaka, a water spirit who emerged from Haumea's body.
SUN COULD BE FAR FROM WHERE IT STARTED
University of Washington
It has long been believed that stars tend to remain in the same
general part of a galaxy where they originally formed. Some astro-
physicists have recently questioned whether that is true, and now
new simulations show that, at least in galaxies similar to our own
Milky Way, stars such as the Sun can migrate great distances. Using
ever such a lot of computer time, the scientists ran a simulation of
the formation and evolution of a galactic disc. The simulations began
with conditions about 9 billion years ago, after material for the disc
of our Galaxy had largely come together but the actual disc formation
had not yet started. The scientists set basic parameters to mimic the
development of the Milky Way to that point, but then let the simulated
galaxy evolve on its own.
It has previously been supposed that if a star, in an approximately
circular orbit around the centre of a galaxy, is intercepted by a
spiral arm, its orbit would become more eccentric. However, in the
new simulations the orbits of some stars get larger or smaller but
still remain very circular after passing through the massive spiral
wave. Our Sun has a nearly circular orbit, so the findings mean that
when it formed, more than 4 billion years ago, it could have been
either nearer to or farther from the centre of the galaxy, rather than
where it is now. Migrating stars also help to explain a long-standing
problem in the chemical mix of stars in our neighbourhood, which has
been regarded as more mixed and diluted than would be expected if
stars spent their entire lives where they were born. The researchers
claim that by containing stars from very different initial locations
the Sun's neighbourhood can be viewed as a more diverse and
interesting place.
1843 ERUPTION OF ETA CARINAE MAY BE NEW TYPE OF STELLAR EXPLOSION
University of California at Berkeley
The 1843 giant outburst of Eta Carinae, one of the brightest and most
massive stars in our Galaxy, now appears to have been driven by a
hitherto unrecognized type of stellar explosion that is not as
comprehensive as that of a typical supernova and does not destroy the
star. It was probably related to a class of faint stellar explosions
seen in other galaxies in recent years by telescopes searching for
supernovae.
Eta Carinae is a massive, hot, variable star far down in the Southern
Hemisphere, and is located about 7,500 light-years from us in a
star-forming region called the Carina Nebula. It was observed to
brighten immensely in 1843, becoming for a time second only to Sirius.
Astronomers now see the resulting cloud of gas and dust, known as the
Homunculus Nebula, wafting away from the star. A faint shell of
debris from an earlier explosion is also visible, probably dating from
around 1,000 years ago. Recent observations indicate that extremely
fast filaments of gas, moving five times faster than the debris in the
Homunculus Nebula, were propelled away from Eta Carinae in the 1843
event. The amount of mass in the relatively slow-moving Homunculus
was already at the edge of plausibility in terms of what an extreme
stellar wind could do physically. The much faster material now
discovered poses even harsher difficulties for current theories.
Instead, the speeds and energies involved are reminiscent of material
accelerated by the fast blast wave of a supernova explosion.
The high speeds in the blast wave could roughly double earlier
estimates of the energy released in the 1843 eruption of Eta Carinae,
an event that astronomers argue was not just a surface eruption
associated with the stellar wind but an actual explosion deep within
the star. In fact, the fast-moving blast wave is now colliding with
the slow-moving cloud from the 1,000-year-old eruption and generating
X-rays that have been observed by the orbiting Chandra Observatory.
Eta Carinae probably once had a mass 150 times that of the Sun. Such
large stars burn very brightly but only for only a few million years,
all the while shedding mass as their own intense radiation pushes the
outer layers of the star away in a stellar wind. After 2 to 3 million
years of wind loss, and having shed about 10 solar masses in its most
recent 1843 eruption alone, Eta Carinae now probably has a mass of 90
to 100 solar masses.
LEAST LUMINOUS GALAXY
Yale University
A team of astronomers has discovered the least luminous galaxy yet
known to exist. The galaxy, called Segue 1, is one of about two dozen
small satellite galaxies orbiting our own Milky Way galaxy. It is a
billionth as bright as the Milky Way, according to the team's results.
But despite its small number of visible stars, Segue 1 is nearly a
thousand times more massive than it would be if it were made of
objects with the same ratio of mass to light as the Sun. Segue 1 is
the most extreme example of a galaxy that contains only a few hundred
stars, yet has a relatively large mass.
The team has observed about half of the dwarf satellite galaxies that
orbit the Milky Way. Those objects are so faint and contain so few
stars that at first they were thought to be globular clusters. But by
spectroscopy with the Keck telescope in Hawaii, the astronomers showed
that the objects are actually galaxies themselves, albeit very dim
ones. Finding ultra-faint galaxies like Segue 1 may provide clues as
to how galaxies form and evolve, especially at the smallest scales.
It is only recently that it has become apparent just how prevalent
such dwarf satellite galaxies are, thanks to projects like the Sloan
Digital Sky Survey, which imaged large areas of the sky in greater
detail than before. In the past two years alone, the number of known
dwarf galaxies orbiting the Milky Way has doubled from the dozen or so
brightest that were discovered during the first half of the twentieth
century.
FARTHEST GAMMA-RAY BURST
NASA
The Swift satellite has observed the most distant gamma-ray burst so
far detected. The event, designated GRB 080913, arose from an
exploding star 12.8 billion light-years away -- less than a billion
years after the Universe began. Gamma-rays from the explosion
triggered Swift's burst-alert telescope on Sept. 13. The spacecraft
established the event's location in the constellation Eridanus and
quickly turned to examine the spot. Less than two minutes after the
alert, Swift's X-ray telescope began observing the position, where it
found a fading, previously unknown X-ray source. Astronomers on the
ground captured the burst's fading afterglow with the 2.2-m telescope
at ESO in Chile. Analysis of the spectrum obtained with the VLT at
Paranal established the redshift at 6.7 -- among the most distant
objects known.
NEW MARS MISSION WILL STUDY PLANET'S ATMOSPHERE
NASA
NASA is to undertake a Mars mission that will provide information
about the planet's atmosphere and climate history in greater detail
than previous missions. Called the Mars Atmosphere and Volatile
EvolutioN (MAVEN) spacecraft, the $485 million mission is scheduled
for launch in late 2013. Among 20 proposals submitted in 2006, Maven
was selected as having the best scientific value and lowest
implementation risk.
Mars once had a denser atmosphere than it has now, one that allowed
liquid water to exist on the surface. It seems that, as part of a
dramatic climate change, most of the atmosphere was lost. MAVEN will
measure present-day atmospheric loss, which may offer clues about the
planet's history.
Topic: Late September Astronomy News (Read 1575 times)