EVIDENCE FOR ICE ON THE MOON
NASA
India's Chandrayaan-1 spacecraft has detected ice deposits near the
Moon's north pole. During the past year, Chandrayaan-1's radar mapped
the Moon's permanently shadowed polar craters that are not visible
from the Earth. The radar uses the polarization properties of
reflected radio waves to characterize surface properties. It found
water ice in more than 40 small craters ranging in diameter from 2 to
15 kilometres. Although the total amount of ice depends on its
thickness in each crater, it is estimated that there could be at least
600 million tons of ice. Some scientists claim that the picture
emerging from recent missions indicates that water creation,
migration, deposition, and retention are occurring on the Moon.
MOST EXTREME WHITE-DWARF BINARY SYSTEM
University of Warwick
An international team of astronomers has shown that the two stars in
the binary HM Cancri definitely revolve around each other in a mere
5.4 minutes. This makes HM Cancri the binary star with by far the
shortest known orbital period. It is also the smallest known binary.
The binary separation is no more than a quarter of the distance from
the Earth to the Moon. The system, which is at a distance of 5000
parsecs (1 pc = 3.26 light-years), consists of two white dwarfs.
HM Cancri was first noticed in 1999 as an X-ray source showing a
5.4-minute periodicity, but it has not been clear whether that is the
actual orbital period of the system. It is so short that astronomers
were reluctant to accept that possibility. Now the Keck telescope has
been used to measure the radial velocities of the stars and to show
that they do indeed vary in the 5.4-minute period. The observations
of HM Cancri were difficult because the object is very faint
(magnitude 21) and its extremely short period meant that very short
exposures were needed to avoid excessive phase smearing.
AGE OF COMETARY MATERIAL
Science Daily
Though comets are thought to be some of the oldest and most primitive
bodies in the Solar System, new research on Comet Wild 2 indicates
that inner-Solar-System material was transported to the comet-forming
region at least 1.7 million years after the formation of the oldest
Solar-System solids. The research by Lawrence Livermore scientists
provides the first constraint on the age of cometary material from a
known comet. The Stardust mission, which was launched to Comet Wild 2
in 1999, was designed on the premise that comets preserve pristine
remnants of materials that helped to form the Solar System. It
extended a retractable device containing cells filled with a material
called aerogel, a porous substance designed to trap dust particles.
In 2006, Stardust returned with the first samples from a comet.
Though the mission was expected to provide a glimpse into the early
Solar System by returning a mix of Solar-System condensates, amorphous
grains from the interstellar medium and true stardust (crystalline
grains originating from distant stars), the initial results painted a
different picture. Instead, the comet materials consisted of
high-temperature materials including calcium-aluminium-rich inclusions
(CAIs), which are the oldest objects formed in the solar nebula and
are common in meteorites.
PROBE MAY ALSO HAVE RETURNED COSMIC DUST
BBC News
Scientists may also have identified two specks of interstellar dust in
the material collected by Stardust. The discovery was made by a
member of the public, using the Stardust@Home internet application,
which invited participants to search the aerogel collection medium for
tiny particles of the dust. The researchers have so far analysed
magnesium, aluminium, iron, chromium, manganese, nickel, copper and
gallium from the particles. Though prized by the Stardust team,
interstellar dust is a nuisance in optical astronomy, because it can
obscures objects they want to observe.
FIRST OF MISSING PRIMITIVE STARS DISCOVERED
Harvard-Smithsonian Center for Astrophysics.
Astronomers have discovered a relic from the early Universe -- a star
that may have been among the second generation of stars to form after
the Big Bang. Located in the Sculptor dwarf galaxy some 90,000
parsecs away, the star has a chemical make-up remarkably similar to
that of the Milky Way's oldest stars. There is a theory that our
Galaxy grew to its current size by swallowing dwarf galaxies, small
galaxies with just a few billion stars, compared to the hundreds of
billions in the Milky Way. If dwarf galaxies are indeed the building
blocks of larger ones, then the same kinds of stars should be found in
both, especially old, metal-poor stars. (To astronomers, 'metals'
means chemical elements heavier than hydrogen and helium.) Because
they are products of stellar evolution, metals were rare in the early
Universe, and so old stars tend to be metal-poor. Old stars in the
Milky Way's halo can be extremely metal-poor, with metal abundances
only a thousandth of those in the Sun, which is a typical younger,
metal-rich star.
Surveys over the past decade have failed to turn up any such extremely
metal-poor stars in dwarf galaxies, however. The Milky Way seemed to
have stars that were much more primitive than any in the dwarf
galaxies. If dwarf galaxies were the original components of the Milky
Way, then they ought to have similar stars. The team suspected that
the methods used to find metal-poor stars in dwarf galaxies were
biased in a way that caused the surveys to miss the most metal-poor
stars. They developed a method to estimate the metal abundances of
large numbers of stars at a time, making it possible to cut corners in
the search for the most metal-poor stars in dwarf galaxies. Among
stars they found in the Sculptor dwarf galaxy was an 18th-magnitude
one which spectroscopy with the Magellan telescope in Chile showed to
have a metal abundance 6,000 times lower than that of the Sun, five
times less than in any other star found so far in a dwarf galaxy. The
researchers expect that further searches will discover additional
metal-poor stars in dwarf galaxies, although the distance and
faintness of the stars pose a challenge for current optical
telescopes. The next generation of extremely large telescopes should
open up a new window for studying the growth of galaxies through the
chemistries of their stars.
ASTRONOMERS DISCOVER THAT ANCIENT GALAXIES ARE MERGING
Penn State University
Astronomers using the Hubble telescope have discovered that a
collection of small, ancient galaxies, called the Hickson Compact
Group 31 (HCG 31), about 50 Mpc away (Megaparsecs; 1 Mpc = 1 million
parsecs = 3.26 million light-years) is finally coming together into
one larger galaxy after 10 billion years. Most other small galaxies
in the Universe came together into larger galaxies billions of years
ago, but the galaxies within HCG 31 have been interacting for only a
few hundred million years. The new observations have allowed the team
to estimate when the encounter began and to predict a future merger.
The galaxies have been distorted by their gravitational effects on
each other. The HCG 31 group of galaxies is also aglow with a
firestorm of star birth, which is triggered when hydrogen gas is
compressed by the close encounters between galaxies and then collapses
to form stars. Everywhere in HCG 31 there are batches of young star
clusters and regions brimming with star birth. The observations
indicate that the brightest clusters, groups of at least 100,000
stars, are less than 10 million years old. The stars are formed from
an abundance of hydrogen gas, but a measurement of the gas content
shows that very little has been used up -- proof that the
star-formation is a recent event. This is a clear example of a group
of galaxies on its way towards a merger. The galaxies are relatively
small, comparable in size to the Large Magellanic Cloud, a satellite
galaxy of our Milky Way. Their relative velocities, measured in
previous studies, are only about 60 km/s, and it seems likely that the
system will merge into a single elliptical galaxy in another billion
years.
GALACTIC LENSES MEASURE AGE AND SIZE OF UNIVERSE
DOE/SLAC National Accelerator Laboratory
Researchers have been trying to measure the size and age of the
Universe, and how rapidly it is expanding, by the use of gravitational
lensing which enables them to measure the distances light travelled
from a bright, active galaxy to the Earth along different paths.
By understanding the time it took to travel along each path and the
effective speeds involved, they could infer not just how far away the
galaxy lies but also the overall scale of the Universe and some
details of its expansion. It is often difficult for scientists to
distinguish between a bright source far away and a dimmer one much
closer. A gravitational lens circumvents that problem by providing
multiple clues as to the distance light travels. That extra
information allows them to determine the size of the Universe, often
expressed by astrophysicists in terms of a quantity called the Hubble
constant. Scientists have known for a long time that lensing is
capable of making a physical measurement of the Hubble constant, but
gravitational lensing has never before been used in such a precise
way. The measurement is now claimed to provide as precise a
measurement of the constant as established methods such as observation
of supernovae and the cosmic microwave background. There are several
factors scientists still need to account for in determining distances
with lenses. For example, dust in the lens can skew the results.
The Hubble telescope has infra-red filters useful for reducing dust
effects. The images also contain information about the number of
galaxies lying around the line of vision; they contribute to the
lensing effect at a level that needs to be taken into account.
Topic: Mid March Astronomy Bulletin (Read 590 times)