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PostPosted: Tue Sep 09, 2014 6:10 pm 

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Electronic News Bulletin No. 382 2014 September 7

Here is the latest round-up of news from the Society for Popular
Astronomy. The SPA is Britain's liveliest astronomical society, with
members all over the world. We accept subscription payments online at
our secure site and can take credit and debit cards. You can join or
renew via a secure server or just see how much we have to offer by


The Pluto-bound New Horizons spacecraft has crossed the orbit of
Neptune. That is its last major crossing en route to becoming the
first probe to make a close encounter with Pluto, which is expected to
occur on 2015 July 14. The piano-sized spacecraft, which was launched
in 2006, reached Neptune's orbit -- nearly 2750 million miles from the
Earth -- in a record eight years and eight months. New Horizons'
milestone matches precisely the 25th anniversary of the historic
encounter of the Voyager 2 spacecraft with Neptune in 1989. Voyager's
visit to the Neptune system revealed previously unseen features of
Neptune itself, such as the Great Dark Spot, a massive storm similar
to, but not as long-lived as, Jupiter's Great Red Spot. Voyager also,
for the first time, obtained clear images of Neptune's ring system.
Some researchers feel that the 1989 Neptune fly-by may have offered a
preview of what is to come next year. Some scientists suggest that
Neptune's major satellite Triton, with its icy surface, bright poles,
varied landforms and cryo-volcanoes, is a Pluto-like object that
Neptune captured into orbit. Scientists recently re-worked Voyager's
observations of Triton and used them to construct an improved global
colour map -- further whetting appetites for a Pluto close-up. There
is a lot of speculation over whether Pluto will look like Triton.
Like Voyagers 1 and 2, New Horizons is also on a path toward potential
discoveries in the Kuiper Belt, which is a disc-shaped region of icy
objects past the orbit of Neptune, and further parts of the outer
Solar System.

Arizona State University

A larger catalogue of stellar compositions than any previous ones has
been produced. It is called after Hypatia, who was one of the first
female astronomers and lived about 350 AD in Alexandria. The
catalogue is a compilation of spectroscopic abundance data from 84
literature sources for 50 elements in 3,058 roughly solar-type (F, G
and K) stars within 160 parsecs. Vizier, a data base provided by the
Stellar Data Centre in Strasbourg, was a good starting point, but is
not quite exhaustive. The Hypatia catalogue has a number of
applications. It tends to show that the compositions of nearby stars
are not as uniform as was thought. It has been known that stars with
Jupiter-like planets tend to have high metal abundances, but in
another example of its application the new catalogue makes it easier
to study other elements systematically to see if there are relation-
ships between the presence of a planet (gaseous or terrestrial) and
the various elemental abundances.


About once every 50 years, a massive star explodes somewhere in the
Milky Way. The resulting blast puts out more energy in a split second
than the Sun emits in a million years. At its peak, a supernova can
outshine the entire Milky Way, and it seems obvious that a supernova
explosion too near the Earth would be undesirable. Yet there is some
evidence that, about 10 million years ago, a 'nearby' cluster of
supernovae went off. We know that, because the explosions blew an
enormous bubble in the interstellar medium, and we are inside it.
Astronomers call it the 'Local Bubble'. It is peanut-shaped, about
100 parsecs long, and filled with almost nothing. Gas inside the
bubble is very thin (0.001 atoms per cubic centimetre) and very hot
(roughly a million degrees), a sharp departure from ordinary
interstellar material. The Local Bubble was discovered gradually in
the 1970s and 1980s. Optical and radio astronomers looked for
interstellar gas in our part of the Galaxy, but did not find much.
Meanwhile, X-ray astronomers were getting their first look at the sky
from sounding rockets and orbiting satellites, which revealed a
million-degree X-ray glow coming from all directions. It seemed to
add up to the Earth being inside a bubble of hot gas blown by
exploding stars.

Within the last decade, however, some scientists have disagreed with
the supernova interpretation, suggesting that much or all of the
soft-X-ray diffuse background is instead a result of 'charge
exchange'. Charge exchange happens when the electrically-charged
solar wind comes into contact with neutral gas. The solar wind can
steal electrons from the gas, resulting in an X-ray glow that looks a
lot like the glow from an old supernova. Charge exchange has been
observed many times in comets. So, is the X-ray glow that fills the
sky a sign of peaceful charge exchange in the Solar System or evidence
of great explosions in the past? To find out, researchers developed
an X-ray detector that could distinguish between the two
possibilities. The device was named DXL, for Diffuse X-ray emission
from the Local Galaxy. DXL was launched on a sounding rocket from
White Sands in New Mexico, reaching a peak altitude of 160 miles and
spending five minutes above the atmosphere. That was all it needed to
measure the amount of charge-exchange X-rays inside the Solar System.
The results indicated that only about 40% of the soft-X-ray background
originates within the Solar System. The rest must come from a Local
Bubble of hot gas, the relic of ancient supernovae outside the Solar
System. Obviously, the supernovae were not close enough to
exterminate life on Earth, but they were close enough to wrap the
Solar System in a bubble of hot gas that persists millions of years

National Radio Astronomy Observatory

Astronomers have used a worldwide network of radio telescopes to
resolve a controversy over the distance to the Pleiades -- a
controversy that posed a potential challenge to scientists' basic
understanding of how stars form and evolve. The new work shows that
the measurement made by the Hipparcos satellite was inaccurate. The
Pleiades include hundreds of young, hot stars formed about 100 million
years ago. As a nearby example of such young clusters, the Pleiades
have served as a key for refining scientists' understanding of how
such clusters form. Moreover, astronomers have used the measured
physical characteristics of Pleiades stars as a tool for estimating
the distances of other, more distant, clusters. Until the 1990s, the
consensus was that the Pleiades are about 430 light-years away.
However, the satellite Hipparcos, which was launched in 1989 and over
four years of operation measured distances to 118,000 stars, found a
distance of 'only' about 390 light-years. That may not seem like a
huge percentage difference, but it did not fit the physical character-
istics of the Pleiades stars according to our general understanding
of how stars form and evolve.

In an effort to solve the problem, astronomers used a global network
of radio telescopes to make a new determination of the parallax of the
Pleiades stars. They obtained a distance of 443 light-years, accurate,
the astronomers said, to 1%. That is close enough to the pre-Hipparcos
distance that the standard scientific models of star formation
accurately represent the stars in the Pleiades. The question now is
what happened to Hipparcos? The cause of its error in measuring the
distance to the Pleiades is unknown.

National Radio Astronomy Observatory

Rocky planets like the Earth start out as microscopic bits of dust, so
theories suggest. Astronomers using the Green Bank radio telescope
(GBT) have discovered that filaments of star-forming gas near the
Orion Nebula may contain lots of pebble-size particles -- planetary
building blocks 100 to 1000 times larger than the dust grains
typically found around proto-stars. If confirmed, such rocky material
may well represent a new, mid-size class of interstellar particles
that could help to initiate planet formation. The new observations
extend across the northern portion of the Orion Molecular Cloud
complex, a star-forming region that includes the Orion Nebula. The
star-forming material in the section studied by the GBT, called
OMC-2/3, has condensed into long, dust-rich filaments. The filaments
are dotted with many dense knots known as cores. Some of the cores
are just starting to coalesce while others have begun to form
proto-stars -- the first early concentrations of dust and gas along
the path to star formation. Astronomers speculate that in the next
100,000 to 1 million years, that region will probably evolve into a
new star cluster. The OMC-2/3 region is located approximately 1,500
light-years from us and is roughly 10 light-years long.

From earlier maps of that region made with the IRAM 30-m radio
telescope in Spain, the astronomers expected to find a certain
brightness to the dust emission when they observed the filaments at
slightly longer wavelengths with the GBT. Instead, they discovered
that the area was shining much more brightly than expected at mm
wavelengths. That implies that the material there has properties
differing from those expected for normal interstellar dust. In
particular, since the particles are more efficient than expected at
emitting at millimetre wavelengths, the grains are very likely to be
at least a millimetre, possibly as large as a centimetre, across.
Though tiny compared to even the most modest of asteroids, dust grains
on the order of a few millimetres to a centimetre are extraordinarily
large for such young star-forming regions. Owing to the unique
environment in the Orion Molecular Cloud, the researchers propose two
intriguing theories for their origin. The first is that the filaments
themselves helped the dust grains grow to such unusual proportions.
Those regions, compared to molecular clouds in general, have lower
temperatures, higher densities, and lower velocities -- all of which
would encourage grain growth. The second idea is that the rocky
particles originally grew inside a previous generation of cores or
perhaps even proto-planetary discs. The material could then have
escaped back into the surrounding molecular cloud rather than becoming
part of the original newly forming star system.

Moscow Institute of Physics and Technology

A group of astrophysicists has detected the formation of radioactive
cobalt during a supernova explosion, lending credence to a theory of
such explosions. The team reported an analysis of data collected with
the INTEGRAL gamma-ray orbital telescope, which they used to detect
the radioactive isotope cobalt-56. Cobalt-56 has a half-life of just
77 days, so it does not exist in normal conditions. However, during a
supernova explosion, that short-lived isotope is produced in large
quantities. Radiating cobalt was recorded in the supernova 2014J,
11 million light-years away. Astrophysicists had not obtained similar
spectra before; the reason was the rarity of explosions at such a
distance -- 11 million light-years is a long way on the Galactic scale
but on an intergalactic scale it is a relatively short distance.
There are several hundred galaxies within a radius of ten million
light-years; supernovae produce explosions of the relevant type (type
Ia) once every few centuries in a galaxy. For example, a type-Ia
supernova last exploded in the Milky Way in 1606. [Disclaimer: it is
not the fault of the compiler or editor if that does not agree with
the frequency of one every 50 years asserted in the item provided by
NASA, the third item in this Bulletin, above.]

SN 2014J was discovered in the galaxy M82 last January by astronomers
from University College London. Several observatories, including
INTEGRAL, started observations immediately. Russian researchers spent
a million seconds of their quota for the use of the INTEGRAL telescope
to study the supernova. In addition to the spectra, they obtained
data on how its brightness changed over time. According to a theory
that was developed earlier, during an explosion of the Ia type the
remnants of the star barely radiate in the gamma-ray range in the
initial weeks. The star's shell is opaque in that region of the
spectrum; a supernova begins to produce gamma radiation only after the
outer layer becomes sufficiently rarefied. By that time, radioactive
nickel-56, with a half-life of 10 days, synthesized during the
explosion, transforms into radioactive cobalt-56, whose spectral lines
at energies of 847 and 1237 keV were detected by INTEGRAL. The data
also allowed the researchers to assess how much radioactive cobalt was
produced during the explosion -- the equivalent of about 60% of the
Sun's mass. Over time, cobalt-56 turns into the most common isotope
of iron, iron-56. That is the most common isotope because it can be
obtained from nickel formed during supernova explosions (nickel turns
into cobalt, and cobalt turns into iron). Thus, the new results back
up simulations of supernova explosions and also suggest that our
planet includes matter that has gone through thermonuclear explosions
of an astronomical scale.

Bulletin compiled by Clive Down

(c) 2014 the Society for Popular Astronomy

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