MERCURY IS VERY ACTIVE
NASA
On 2008 Oct. 6 Messenger flew by Mercury for the second time, taking
more than 1,200 pictures of the planet. The fly-by has indicated that
the planet's atmosphere and magnetosphere, and its geological past,
display greater levels of activity than had been suspected. Mercury's
magnetosphere had changed a lot since the first fly-by in 2008
January. The magnetosphere is a region of space around Mercury
enveloped by the planet's magnetic field. Gusty solar winds buffeting
the global bubble of magnetism can potentially trigger magnetic storms
and other space-weather-related phenomena.
The part of Mercury's surface that was imaged for the first time in
October includes a large impact basin, now called Rembrandt, about 700
km in diameter; it was probably formed about 3.9 billion years ago,
near the end of the period of 'heavy bombardment' of the inner Solar
System. Half of Mercury's surface was unknown until a little more
than a year ago. Spacecraft images have since revealed 90% of the
planet's surface at high resolution. About 40% of it is covered
by smooth plains. Much of Mercury's crust may have formed through
repeated volcanic eruptions in a manner more similar to the crust of
Mars than to that of the Moon. A third Mercury fly-by is due to take
place on Sept. 29, and the probe is intended to be placed into orbit
around Mercury in 2011 March.
SATELLITE OF SATURN
IAU
The International Astronomical Union has declared that the recently
discovered 53rd satellite of Saturn, designated S/2008 S 1, has been
named Aegaeon.
PULSAR MISSING LINK
NRAO
Astronomers have discovered a double-star system that represents a
'missing link' stage in what they believe to be the birth process of
the most rapidly spinning stars -- millisecond pulsars. Pulsars are
super-dense neutron stars, the remnants left after massive stars have
exploded as supernovae. Their powerful magnetic fields generate
lighthouse-like beams of light and radio waves that sweep round as
the star rotates. Most rotate a few to tens of times a second,
slowing down over thousands of years. However, some rotate hundreds
of times a second. Astronomers believe that a companion star dumping
material onto the neutron star and spinning it up causes the fast
rotation. The material from the companion would form a flat, spinning
disc around the neutron star; while that was happening, radio waves
characteristic of a pulsar would not be seen. As the amount of matter
falling onto the neutron star decreased and stopped, the radio waves
could emerge, and the object would be recognized as a pulsar.
That sequence of events is apparently what happened with a binary-star
system containing a millisecond pulsar called J1023, which was
discovered in 2007 and is some 4,000 light-years away. The object had
previously been detected by the Very Large Array radio telescope
during a large sky survey in 1998, and the Sloan Digital Sky Survey
had observed it in visible light in 1999, revealing a Sun-like star.
When observed again in 2000, the object had changed dramatically,
showing evidence for a rotating disc of material, called an accretion
disc, surrounding the neutron star. In 2002 the evidence for the disc
had disappeared. Then in 2007 observations with the Green Bank radio
telescope showed the object as a millisecond pulsar, spinning 592
times per second.
No other millisecond pulsar has ever shown evidence for an accretion
disc. Another type of binary-star system, called a low-mass X-ray
binary (LMXB), also contains a fast-spinning neutron star and an
accretion disc, but does not emit radio waves. Astronomers thought
that LMXBs are in the process of being spun up, and will later emit
radio waves as pulsars. J1023 appears to be the 'missing link'
connecting the two types of systems. It appears the object has
changed from looking like an LMXB to looking like a pulsar, as it
experienced an episode during which material pulled from the companion
star formed an accretion disc around the neutron star. Later, that
mass transfer stopped, the disc disappeared, and the pulsar emerged.
The scientists have studied J1023 in detail with a number of radio
telescopes; their results indicate that the neutron star's companion
has less than half the Sun's mass, and orbits the neutron star once
every 4.75 hours.
COSMOLOGY'S 'STANDARD CANDLES'
Lawrence Berkeley Lab, Berkeley, California
An international group of researchers claims to have found a new
technique that estimates the intrinsic brightness of Type Ia
supernovae more accurately than before. Such exploding stars are the
best 'standard candles' for measuring cosmic distances. Astronomers
searching the spectra of 58 Type Ia supernovae found a key
spectroscopic ratio. They say that a supernova's distance can be
determined to better than 6% uncertainty simply by measuring the ratio
of the flux (visible power, or brightness) between two specific
regions in the spectrum observed on the same night. The new
brightness-ratio correlation appears to hold no matter what the
supernova's age or metallicity, its type of host galaxy, or how much
it is dimmed by intervening dust.
Using classical methods that are based on a supernova's colour and the
shape of its light-curve, distances to Type Ia supernovae can be
measured with a typical uncertainty of 8-10%. But obtaining a light-
curve takes up to 2 months of high-precision observations. The new
method provides better accuracy with a single night's full spectrum.
The most accurate standardization factor the team found was the ratio
between wavelengths 6420 and 4430 Angstroms, in the red and blue parts
of the spectrum, respectively. In the analysis, no assumptions are
needed about the possible physical significance of the spectral
features.
GIANT GALAXY MESSIER 87 FINALLY MEASURED
ESO
At a distance of approximately 50 million light-years, the Virgo
Cluster is the nearest cluster of galaxies. It contains many hundreds
of galaxies, including giant and massive elliptical galaxies. In
particular, it contains the giant elliptical galaxy M87. Astronomers
using FLAMES, a spectrograph at the Very Large Telescope in Chile,
made measurements of a lot of planetary nebulae in the outskirts of
M87 and in the intergalactic space around it within the Virgo Cluster
of galaxies. FLAMES can take spectra of many sources, spread over an
area of the sky about the size of the Moon, simultaneously. It
appears that M87's outer halo has been cut short, with a diameter of
about a million light-years, significantly smaller than expected,
despite being about three times the extent of the halo surrounding our
Milky Way. Beyond that zone only a few intergalactic stars are seen.
Numerical models seem to want the halo around M87 to be several times
larger than the observations show. Among the speculative explanations
of the discrepancy is the possibility that another major galaxy in the
cluster, M84, came much closer to M87 and dramatically perturbed it
about a billion years ago.
COSMIC DISTANCE RECORD BROKEN
NASA
Gamma-ray bursts are the latest candidates for being the most luminous
explosions in the Universe. It is thought that most of them occur
when massive stars run out of nuclear fuel. As their cores collapse
into black holes or neutron stars, jets of matter punch through the
star and blast into space. There, they strike gas previously shed by
the star and heat it, generating short-lived afterglows at many
wavelengths.
Astronomers using the 'Swift' satellite have found a gamma-ray burst
from a star that blew up when the Universe was only 630 million years
old -- less than 5% of its present age. The event, dubbed GRB 090423,
is the most distant cosmic explosion so far recognized. The burst
occurred on April 22. Swift quickly pinpointed the explosion,
allowing telescopes on the ground to observe it before its afterglow
faded away. Astronomers working in Chile and the Canary Islands
independently measured its redshift to be 8.2, well in excess of the
previous record of 6.7 set by an explosion in 2008 September.
KEPLER'S PLANET-HUNT BEGINS
JPL
The Kepler spacecraft, launched on 2009 March 6, has begun its search
for other Earth-like planets. It will spend the next 3.5 years
staring at more than 100,000 stars for telltale signs of planets.
Kepler could in principle find planets as small as the Earth that
orbit Sun-like stars at distances where temperatures are right for
water to exist in liquid form. Kepler will hunt for planets by
looking for periodic dips in the brightnesses of stars when planets
cross in front of them and partially block their light. The first
finds are expected to be large gas planets situated close to their
stars.
SPITZER BEGINS WARM MISSION
JPL
After more than 5 years of observing cool objects, the Spitzer space
telescope has run out of the coolant that chilled its infrared
instruments. The telescope will warm up slightly, but two of its
infrared detector arrays should still operate.
SPECTROGRAPH CAN RECORD ENTIRE SPECTRUM
ESO
The Very Large Telescope has been equipped with the first of its
second-generation instruments, 'X-shooter'. It can record the entire
spectrum of an object in one go from the ultraviolet to the near-
infrared. The name of the instrument is said to have been chosen (but
it must have been done by a crossword-puzzle guru) to stress its
capacity to observe a source whose nature and energy distribution are
not known in advance of the observation. Until now, some initial
information was desirable to indicate which instrument to use for a
detailed study. The instrument was installed at the telescope at the
end of 2008 and the first observations in its full configuration were
made in March and demonstrated that the instrument works efficiently
over the full spectral range with satisfactory resolution and quality.
X-shooter has already obtained complete spectra of low-metallicity
stars, X-ray binaries, distant quasars and galaxies, and the nebulae
associated with Eta Carinae and the supernova 1987A, as well as a
distant gamma-ray burst that coincidently occurred at the time of the
commissioning run.
Topic: Late May Astronomy Bulletin (Read 873 times)