Topic: Late June Astronomy Bulletin

[Clive]

RECENT FLYPAST ASTEROID HAS ITS OWN MOON
NASA

An asteroid that was first discovered in 1998 and named 1998 QE2 and
made an approach to the Earth in late May has a moon.  It was found in
radar images obtained when the asteroid was about 6 million kilometres
away.  The asteroid's moon is about 600 metres across, whereas the
asteroid itself, which has a rotation period of less than four hours,
is 2.7 kilometres in diameter.  In the near-Earth population, about
16% of asteroids that are 200 metres or larger have been found to be
binary or triple systems.  Also revealed in the radar imagery of 1998
QE2 are several dark surface features that suggest large concavities.
The closest approach of the asteroid occurred on May 31, at about 15
times the distance of our Moon; that is the closest approach the
asteroid will make for 200 years.

A 10-metre asteroid passed by at only a quarter the distance of the
Moon in early June, the latest in a parade of celestial visitors that
has raised awareness of potential impacts on the Earth.  Asteroid 2013
LR6 was discovered about a day before its closest approach.  On
February 15, a small asteroid exploded in the atmosphere over
Chelyabinsk, Russia, leaving more than 1,500 people injured by flying
glass and debris.  That same day, an unrelated asteroid passed just
17,200 miles away, closer than the ring of geostationary communication
satellites.  NASA claims now to have found about 95% of the asteroids
with diameters of a kilometre or more whose orbits can bring them
close to the Earth.  An object of that size impacted about 65 million
years ago in the Gulf of Mexico just off the Yucatan peninsula,
triggering a global climate change that is believed to have been
responsible for the demise of the dinosaurs and other life forms.


NEW ASTEROID FAMILIES DISCOVERED
NASA

Astronomers using data from the Wide-field Infrared Survey Explorer
(WISE) have been classifying asteroids into families.  An asteroid
family is formed when a collision breaks a large parent body into
fragments of various sizes.  Some collisions leave large craters --
for example, Vesta's southern hemisphere was excavated by two large
impacts.  Other collisions are catastrophic, shattering an object into
numerous fragments, as in the case of the Eos asteroid family.  The
pieces move together in packs, travelling on similar paths round the
Sun, but over time the pieces become more and more spread out.  The
WISE team considered about 120,000 main-belt asteroids, out of the
approximately 600,000 known.  They found that about 38,000 of them
could be assigned to 76 families, 28 of which are new.  The findings
are a step towards understanding the origins of the families and the
collisions thought to have been responsible for them.


RARE ALIGNMENT OFFERS OPPORTUNITY TO HUNT FOR PLANETS
NASA

The Hubble telescope will have two opportunities in the next few years
to hunt for Earth-sized planets around the red dwarf Proxima Centauri.
The opportunities will occur in 2014 October and 2016 February when
Proxima Centauri, the star nearest to our Sun, passes nearly in front
of two other stars.  Red dwarfs are the most common class of stars in
our Milky Way galaxy.  Any such star ever born is still shining today,
and there are about 10 red dwarfs for every star like our Sun.  Red
dwarfs are less massive than other stars, and because lower-mass stars
tend to have smaller planets red dwarfs are good places to hunt for
Earth-sized planets.  Previous attempts to detect planets around
Proxima Centauri have not been successful, but it may be possible to
detect smaller terrestrial planets, if they exist, by looking for
microlensing effects during the two rare stellar alignments.
Microlensing occurs when a foreground star passes close to our line of
sight to a more distant background star.  The image of the background
star may be distorted, brightened and/or multiplied, depending on the
alignment between the foreground lens and the background source.  The
events should enable astronomers to measure precisely Proxima's mass,
by seeing how far the background stars are offset from their real
positions in the sky.  The offsets would result from Proxima's
gravitational field warping space.  If there are any planets, their
gravitational fields will produce small extra positional shifts.
Because Proxima is so close, the area of sky warped by its gravity
is larger than for more distant stars, making it easier to look for
shifts in apparent stellar positions.  However, the positional shifts
will be too small to be perceived by any but the most sensitive
telescopes.  The proposed Gaia space telescope and the VLT in Chile
may be able to make measurements comparable with Hubble's.


SODIUM ABUNDANCES SEEM TO BE RELATED TO OLD AGE FOR STARS
ESO

Astronomers expect that stars like the Sun will blow off much of their
atmospheres into space near the ends of their lives.  But new
observations made with the VLT of a globular star cluster have shown
-- against all expectations -- that a majority of the stars studied
simply did not get to that stage at all.  The international team found
that the amount of sodium in the stars was a good predictor of how
they ended their lives.  The way in which stars evolve and expire was
for many years considered to be well understood.  Computer models
indicated that stars of a mass similar to the Sun's would go through a
late stage of evolution called the Asymptotic Giant Branch, or AGB,
when they would undergo a final burst of nuclear burning and throw off
a lot of their mass in the form of gas and dust.

The expelled material goes on to form the next generations of stars,
and this cycle of mass loss and rebirth is vital to explain the
evolving chemistry of the Universe.  But when scientists of the Monash
University Centre for Astrophysics, Melbourne, scoured old papers they
found tantalising suggestions that some stars may somehow not follow
the rules and might skip the AGB phase entirely.  The team used the
VLT to study the globular star cluster NGC 6752 in the southern
constellation of Pavo.  That ball of ancient stars contains both a
first generation of stars and a second that formed somewhat later.
The two generations can be distinguished by the amount of sodium they
contain -- something that the VLT data can be used to measure.
FLAMES, the multi-object spectrograph on the VLT, allowed the
astronomers to get data for 130 stars at a time, which let them
observe a large part of the globular cluster in one go.  The results
were a surprise: all of the AGB stars in the study were first-
generation stars with low levels of sodium and none of the higher-
sodium second-generation stars had become AGB stars at all.  As many
as 70% of the stars seemed not to be undergoing the final nuclear
burning and mass-loss phase.  AGB stars are the brightest stars in
globular clusters, so it looks as if there will be 70% fewer of the
brightest stars than previously received theory would predict.  There
is an obvious need to check that result for other star clusters.


LIGHTEST EXOPLANET IMAGED SO FAR?
ESO

Astronomers using the Very Large Telescope (VLT) with its adaptive-
optics instrument 'NACO' has imaged a faint object moving near a
bright star.  The observations were made in infrared light and used a
technique called differential imaging, which improves the contrast
between the planet and its dazzling host star.  The object may be the
least-massive planet to be directly observed outside the Solar System;
if it is indeed a planet, its brightness suggests that it may have a
mass of four or five times that of Jupiter.  Although nearly a
thousand exoplanets have been detected indirectly -- most by the
radial-velocity or transit methods -- and many more candidates await
confirmation, only a dozen exoplanets have been imaged directly.
In the initial observation, the supposed planet appeared as a faint
but clear dot close to the young star HD 95086, about 100 parsecs away
from us.  A later one showed that it shared the star's 'proper motion'
across the sky, so it has been called HD 95086 b.  It probably orbits
at a distance of around 56 astronomical units from the star, twice the
Sun-Neptune distance.  The star itself is a little more massive than
the Sun and is surrounded by a debris disc.


LOCAL ARM OF GALAXY MORE SIGNIFICANT
National Radio Astronomy Observatory

We reside in a structure called the Local Arm, between two major
spiral arms of our Galaxy, the Sagittarius Arm and the Perseus Arm.
The Sagittarius Arm is closer to the Galactic centre and the Perseus
Arm is farther out in the Galaxy.  New research with the Very Long
Baseline Array (VLBA) indicates that the Local Arm, previously thought
to be only a relatively small spur, is actually much more like the
adjacent major arms, and may be a significant branch of one of them.
Determining the structure of our own Galaxy is difficult because we
are inside it.  While astronomers agree that our Galaxy has a spiral
structure, there are disagreements as to how many arms it has and on
their specific locations.  The VLBA allows the measurement of
parallaxes more accurately than other methods, so it can determine
distances further out than has previously been possible.  It has
measured the distances to some objects called masers (like lasers, but
occurring naturally in the Galaxy and operating at radio instead of
optical wavelengths) and also tracked their motion through space, and
it is that information that has led to the reappraisal of the status
of the Local Arm.