MOON HAS EARTH-LIKE CORE
NASA
Some researchers have just got round to discussing data gathered from
four seismometers which were deployed during the Apollo Moon missions
between 1969 and 1972 and operated until late 1977. The team used
techniques that identified where seismic waves passed through, or were
reflected by, elements of the Moon's interior, signifying the
composition and state of layer interfaces at varying depths. Their
findings suggest that the Moon possesses a solid, iron-rich inner core
with a radius of nearly 240 km and a fluid, primarily liquid-iron,
outer core with a radius of roughly 330 km. It differs from the Earth
in having a partially molten layer around the core, estimated to have
a radius of nearly 480 km. There are indications that the core
contains a small percentage of light elements such as sulphur, echoing
new research that suggests the presence of light elements such as
sulphur and oxygen in a layer around the Earth's core. Researchers
had previously inferred the existence of a lunar core, on the basis of
indirect estimates of the Moon's interior properties, but there was
disagreement over its radius, state, and composition.
SUN-GRAZING COMET STORM
NASA
The Sun has just experienced a 'storm' of comets. The storm lasted
from Dec. 13 to 22, during which time the SOHO spacecraft detected 25
comets diving into the Sun. Such comets, known as 'sungrazers', are
frequent -- SOHO typically sees one every few days, plunging inward
and disintegrating as solar heat sublimes its volatile ices, but 25
comets in just ten days is unprecedented. The comets were 10-metre-
class objects, and as comets go they are considered small, but in the
past there have been major comets in orbits very similar to those of
the recent arrivals. The most recent major one was Comet Ikeya-Seki,
which appeared in 1965 and passed only 450,000 km from the Sun's
surface. Japanese and American observers saw it in broad daylight
right beside the Sun. Because Ikeya-Seki's nucleus was large, about
5 km, it survived the encounter; however, it broke into at least
three pieces before receding back into the remoter parts of the Solar
System. Similar but even greater sungrazing comets were observed in
1843 and 1882. Since SOHO was launched there has been a trend of
increasing numbers of sungrazers. SOHO detected 69 in 1997, compared
to 200 in 2010. The increase is significant and is not accounted for
by improvements in SOHO or the increasing skill of comet hunters.
KEPLER MISSION DISCOVERS ITS FIRST ROCKY PLANET
NASA
The Kepler mission claims to have discovered its first rocky planet,
named Kepler-10b. Measuring 1.4 times the size of the Earth, it is
the smallest planet discovered outside the Solar System. Kepler's
photometer measures the tiny decrease in a star's brightness that
occurs when a planet crosses in front of it. The size of the planet
can be derived from the periodic dips in brightness. The distance
between the planet and the star is estimated from the time between
successive dips as the planet orbits the star. The planet's orbital
period (its year) is only 0.84 days: Kepler-10b is more than 20 times
closer to its star than Mercury is to our Sun, so it is dreadfully
hot. Sensitive measurements, made with the Keck 10-m telescope in
Hawaii, of the Doppler shift in the spectrum of the star have
confirmed that the obscuring object is a planet. It is inferred to be
rocky, with a mass 4.6 times that of Earth and an average density of
8.8, similar to that of iron and considerably higher than the Earth's.
STANDARD CANDLE NOT SO STANDARD
ScienceDaily
Astronomers have found that some Cepheid variable stars, which are
used as 'standard candles' (astronomical objects that make up the
rungs of the so-called cosmic distance ladder, to estimate the
distances to farther and farther galaxies) lose mass, making them
not quite as standard as was once thought. Cepheids are pulsating
stars, whose period of pulsation is correlated with their absolute
magnitudes, and they feature largely in the first rung of the ladder.
By measuring the pulsation period astronomers can infer the star's
absolute magnitude and hence determine its distance.
Now, new observations from the Spitzer infrared space telescope show
that keeping the ladder secure requires even more careful attention to
Cepheids. Observations of one particular star, Delta Cephei itself,
the 'type star' of the class, show that it is losing mass. Many
stars (the Sun included) lose mass in the form of a 'stellar wind',
but in the Sun's case the rate of mass loss is not enough to have a
significant effect on the Sun as a whole. Cepheid are much larger,
and could have winds that would blow off significant amounts of gas
and dust, forming a dusty cocoon around the star that would affect how
bright it appears and therefore the distance assigned to it. Spitzer
has indeed observed dust around Delta Cephei. That star is
travelling through space at a high speed, pushing interstellar gas and
dust into a bow shock ahead of it. A nearby companion star happens to
be lighting the area, making the bow shock easier to see. By studying
the size and structure of the shock, the astronomers were able to show
that a strong, massive wind from the star is pushing against the
interstellar gas and dust. The wind is up to a million times stronger
than the wind blown off by our Sun, and its existence implies that
Delta Cephei must be shrinking slightly. Follow-up observations of
other Cepheids with Spitzer have shown that up to 25% of those
observed are also losing significant mass.
REMNANTS OF 'PRISTINE STAR'
BBC News
Astronomers have found the remnants of a star that exploded more than
13 billion years ago -- one of the very first stars to shine in the
Universe. All that is left is the gas cloud it threw out when it blew
itself apart; it was identified only because it is illuminated by
light coming from the surroundings of a distant black hole. The
cloud's atoms occur in abundances that are quite unlike those found in
the 'nearby' cosmos today, and are more what experts expect from stars
that were originally made only of hydrogen and helium. The study is
said to provide insight into events in the early stages of the
Universe; in particular it bears on the ending of the so-called 'Dark
Ages', the interval before the first stars formed. That is a time
about which we know very little, but the Universe was then a rather
uniform place, just filled with hydrogen and helium gas and not much
else; there was no light -- that is why it is called the Dark Ages.
Working on the Keck telescope and the VLT, the team investigated the
composition of distant clouds of gas that are lit up by quasars. One
particular cloud had a unique chemical signature -- the ratio of
carbon to iron atoms was 35 times greater than in the Sun. The group
inferred that the gas was released by a star that was originally 25
times the mass of the Sun and originally consisted only of hydrogen
and helium -- exactly the type of stars expected to have ended the
Dark Ages. Astronomers think that they all lived very short and
furious lives. They are all dead now, and we cannot observe them
directly even with the most powerful telescopes. But what has been
found appears to be the remnants of one of those first stars to form,
which not only seeded the cosmos with the heavier elements, but also
ionized the neutral gas around them with their intense ultraviolet
radiation, transforming it into the diffuse plasma that still fills
the spaces between galaxies.
PLANCK TELESCOPE OBSERVES SUPER-CLUSTERS OF GALAXIES
BBC News
The Planck space telescope, observing at microwave wavelengths, has
identified many clusters of galaxies, which are the largest structures
in the Universe. Astronomers are interested in such observations
because they say something about the way the Universe is built on the
grandest scales -- how matter is organised into vast filaments and
sheets and separated by great voids. Planck made the discoveries
during its on-going survey of the Cosmic Microwave Background (CMB) --
the relict radiation from the Big Bang, which fills the entire sky in
the microwave portion of the electromagnetic spectrum. In an effort
to get a clear view of the background, scientists must first try to
subtract the light emitted by other sources shining in the same
frequencies. Although regarded as 'noise' in the context of Planck's
main mission, the additional radiation is of interest to astronomers
mapping galaxy clusters.
Such structures are found by the 'Sunyaev-Zel'dovich' (SZ) effect in
the Planck data. Clusters of galaxies are surrounded by fantastically
hot gas -- at many millions of degrees. [The idea of 'hot' in such
situations does not equate to terrestrial experience. If you were
able to put a thermometer there, the mercury would freeze. What
passes for temperature is a measure of the rate at which the very few
individual atoms or molecules are moving, which is very high and in
terms of physics corresponds to a very high temperature.] In those
conditions, electrons become detached from atomic nuclei and move
around at great speed. About 1% of the photons of CMB light moving
through the structures will interact with the hot electrons. That
has the effect -- the SZ effect -- of distorting the CMB in a
characteristic way: it becomes depleted at lower frequencies and
boosted at higher frequencies. Researchers look for spots on the sky
that are less bright than average at low frequencies and brighter than
average at high frequencies. The SZ distortions in the Planck data
have to be followed up with observations by X-ray telescopes such as
XMM-Newton, which can see the emission coming from the hot electrons
themselves. Information on all the Planck clusters has been made
public as part of the Planck Early Release Compact Source Catalogue
(ERCSC), a list of some 15,000 items seen so far by Planck.
'DARK' GAMMA-RAY BURSTS
ESO
Gamma-ray bursts (GRBs) are among the most energetic events in the
Universe, but some appear curiously faint in visible light. This
bulletin tries to explain how a study of such 'dark' gamma-ray bursts,
made with an instrument called GROND on the 2.2-m German telescope at
La Silla, has found that their faintness is explained by a combination
of causes, the most important being light-absorption by dust between
the explosions and the Earth.
Gamma-ray bursts are fleeting events that last from less than a second
to several minutes, detected by orbiting observatories that pick up
their high-energy (gamma) radiation. They do also emit a longer-
lasting stream of less-energetic radiation, called the 'afterglow',
which can last for weeks or even years after the initial explosion.
While all gamma-ray bursts have afterglows in X-rays, only about half
of them show up in visible light. Although sensationalists naturally
favoured exotic explanations, a more mundane and probable explanation
lay in the existence of obscuring dust between the burst and us.
The Swift satellite, launched in 2004, detects gamma-ray bursts and
immediately relays their positions to other observatories so that the
afterglows can be studied. In the new study, astronomers combine
Swift data with observations made with GROND, a dedicated gamma-ray-
burst follow-up instrument. Favoured by a special scheduling-override
protocol, GROND can observe a burst within minutes of an alert coming
from Swift. It observes simultaneously through seven colour filters
covering the visible and near-infrared parts of the spectrum, enabling
an estimate to be made of the amount of obscuring dust that the light
passed through en route to Earth. The team found that a significant
proportion of bursts are dimmed to about 60-80% of the original
intensity by obscuring dust, but the effect is exaggerated for very
distant bursts, leaving only 30-50 percent of the light. The
astronomers conclude that most dark gamma-ray bursts are simply ones
that have had the small amount of their visible light almost
extinguished by dust on its way towards us.
DETAILED COLOUR IMAGE OF THE SKY
Sloan Digital Sky Survey
The Sloan Digital Sky Survey has produced the largest digital colour
image of the sky ever made, and it is free to all. The image has been
put together over the past decade from millions of images. It is
accompanied by a catalogue of the objects that it shows, including
asteroids, stars, galaxies and quasars. It will be a useful source of
information for a long time to come, in the same way as the Palomar
Sky Survey of the 1950s is still being used today.
The image, which covers a third of the whole sky, was started in
1998 with a 138-megapixel imaging detector on the dedicated 2.5-m
telescope at Apache Point in New Mexico; the detector is now being
de-commissioned and will go to the museum of the Smithsonian
Institution. The SDSS telescope is being turned over to spectroscopy,
and is working on a number of new projects. One of them, called BOSS,
hopes to provide the red-shifts of a million galaxies and thereby
offer a detailed three-dimensional map of part of the Universe.
Another (SEGUE) studies the properties and motions of hundreds of
thousands of stars in the outer parts of the Milky Way.
MARVELS is intended to obtain spectra repeatedly for approximately
8,500 nearby stars similar to the Sun, looking for variations that
might be caused in their radial velocities by large Jupiter-like
planets orbiting them. Finally, APOGEE is intending to take infrared
spectra of stars in all parts of our galaxy -- even ones on the far
side beyond the central bulge, whose visible light is obscured by
large amounts of dust in the galactic disc.
Topic: Late January Astronomy Bulletin (Read 708 times)