ARSENIC AND SELENIUM FOUND IN ANCIENT STAR
Science Daily
The Big Bang produced lots of hydrogen and helium and a small amount
of lithium. All heavier elements found in the Periodic Table have
been produced by stars, but not all of them have been actually
observed in astronomical spectra. Now for the first time, astronomers
have detected arsenic and selenium, neighbouring elements near the
middle of the Periodic Table, in an ancient star in the stellar halo
that surrounds the Milky Way.
Stars like the Sun can make elements up to oxygen in the Periodic
Table. More-massive stars can synthesize heavier elements, those with
more protons in their nuclei, up to iron, by nuclear fusion -- the
process in which atomic nuclei fuse and release lots of energy. Most
of the elements heavier than iron are made by a process called
neutron-capture nucleosynthesis. Although neutrons have no charge,
they can decay into protons after they are in the nucleus, producing
elements with larger atomic numbers. One of the ways that that can
happen is by exposure to a burst of neutrons during the violent
supernova death of a star. That process is called the rapid process
(r-process). It can produce elements at the middle and bottom of the
periodic table -- from zinc to uranium. It cannot be replicated in an
laboratory since the conditions are so extreme, so our understanding
of the r-process relies on astronomical observations.
Astronomers who looked at an ultraviolet spectrum from the Hubble
telescope public archives identified arsenic and selenium in an old
star named HD 160617, to be found in the southern constellation
Scorpius. It has been known for a long time to have an interesting
chemical composition, with many r-process elements in its atmosphere.
The elements must have been formed in an even older star, which has
long since disappeared, and then incorporated into the star that we
see today. The astronomers also examined data for the star from the
public archives of several ground-based telescopes, and were able to
detect 45 different elements. In addition to arsenic and selenium,
they found rarely-seen cadmium, tellurium, and platinum, all of which
were produced by the r-process. This is the first time that those
elements have been detected together outside the Solar System. Now
that certain ultraviolet spectral lines have been identified as
arising from the newly observed elements, they can probably be
recognized in other stars as well.
WHITE-DWARF STARS CONSUMING PLANETS
RAS
Astrophysicists have used the Hubble telescope to survey the chemical
compositions of the atmospheres of more than 80 white dwarfs within a
few hundred light-years of the Sun. White dwarfs are the final stage
of evolution of stars like the Sun, the residual core of material left
behind after the fuel available for nuclear reactions has been
exhausted. The researchers found the atmospheres of four of them to
include elements presumed to have come from shattered planetary bodies
whose compositions were similar to that of the Earth. The elements
were oxygen, magnesium, iron and silicon -- the four elements that
between them make up roughly 93 per cent of the Earth. Another
significant observation was the relatively low proportion of carbon,
which matched closely that of the Earth and the other rocky planets
orbiting closest to our own Sun. The composition of the dust is seen
as evidence that those stars once had rocky planets which are now
destroyed.
The atmosphere of a white dwarf is made up of hydrogen and/or helium;
any heavy elements that come into the atmosphere are dragged downwards
to the core and out of sight very quickly by the dwarf's very high
gravity. The heavy elements seen in the atmospheres of the four
stars must therefore be constantly replenished. The Earth-like
material is seen as evidence that the white dwarfs once had rocky
planets which have now been disrupted, and we are now seeing the final
phase of their demise, as their material rains down on the stars at
rates of up to a thousand tons a second. (Put like that, it sounds a
lot, but it would take longer than the age of the Universe to
exhaust the Earth's mass at that rate.) One particular white dwarf,
PG0843+516, stood out from the rest owing to the relative over-
abundance of the elements iron, nickel and sulphur in its atmosphere.
Iron and nickel are found in the cores of terrestrial planets, as they
sink to the centre during planetary formation, and so does sulphur
thanks to its chemical affinity to iron. Therefore, researchers
believe they are observing PG0843+516 accreting material from the core
of a rocky planet that was large enough to undergo differentiation
(the process that separated the core and the mantle of the Earth).
It is suggested that the destructive processes which seem to be
causing planetary material to fall onto the white dwarfs may occur
eventually in our own Solar System. Stars like our Sun expand to
become red giants when the nuclear fuel in their cores is depleted.
When that happens in the Solar System, billions of years from now, the
Sun may expand enough to engulf Mercury. The Earth is not likely to
be swallowed up by the Sun in its red-giant phase, but its surface
will be roasted. During its transformation into a white dwarf, the
Sun will lose a large amount of mass, so all the planets will move
further out. That may destabilise their orbits and lead to collisions
between planets, as happened in the early days of the Solar System.
Entire planets may be shattered into asteroid-like bodies, some of
which would have chemical compositions similar to those of the
planetary cores. In our Solar System, Jupiter will survive the late
evolution of the Sun unscathed, and could scatter asteroids, old and
new, towards the white dwarf. It is possible that in PG0843+516 we
see the accretion of such fragments from the core material of what was
once a Earth-type planet.
MORE PROBLEMS FOR DARK MATTER
RAS
Astronomers from the University of Bonn have discovered a structure of
satellite galaxies and clusters of stars surrounding our Galaxy. The
work challenges the existence of dark matter, part of a model for the
evolution of the Universe. The Milky Way galaxy consists of around
300 thousand million stars as well as large amounts of gas and dust
arranged in arms that wind out from a central bar in a flat disc.
The diameter of the main part of the Milky Way is about 100,000
light-years, and a number of smaller satellite galaxies and globular
star clusters orbit at various distances from the main Galaxy. Some
models for the origin and evolution of the Universe are based on the
presence of a lot of 'dark matter' that has never been detected
directly; the models want the Milky Way to have far more satellite
galaxies than are actually seen. Now, a lot of fainter satellite
galaxies and younger globular clusters has been found. The objects
seem to be distributed in a plane at right angles to the galactic
disc. The newly-discovered objects extend as far as a million
light-years from the centre of the Galaxy. The fact that they lie in
a plane suggests that the satellite galaxies and clusters must have
formed together in one major event, a collision of galaxies. Such
collisions are quite common and lead to large chunks of the galaxies
being torn away, forming tails that are the birthplaces of new objects
like star clusters and dwarf galaxies. It seems likely that the Milky
Way collided with another galaxy in the remote past. The other galaxy
lost part of its material, material that then formed the Milky Way's
satellite galaxies and the younger globular clusters and the bulge at
the galactic centre. The model appears to discredit the idea the
there is a lot of dark matter in the Universe, threatening a central
pillar of much current cosmological theory.
OLD GALAXIES HAVE MANY MORE STARS
University of Oxford
Some of the oldest galaxies in the Universe seem to have three times
more stellar mass, and so many more stars, than current models of
galactic evolution would like. The finding comes from an
international team, led from Oxford University, who pointed out that
the facile assumption that the brightness of a galaxy could serve as
an indicator of its mass was not realistic. Galaxies can contain huge
numbers of small stars, planets or black holes that have lots of mass
but give out very little light or no light at all. Models have
assumed that stellar light could be used to infer the stellar masses,
and if there were evidence that the mass of a galaxy was greater than
that, the discrepancy could be attributed to 'dark matter'. The team
reckons it has shown that the relationship between observable light
and stellar mass is not universal, but varies between different types
of galaxies, with some older galaxies having three times the mass
suggested by the light they give off.
ESA SELECTS 'JUICE' PROBE TO JUPITER
BBC News
The European Space Agency (ESA) is to mount a billion-euro mission
named JUICE to Jupiter and its icy moons. It should be launched in
2022, although it would be a further eight years before it reached the
Jovian system. The mission has been selected after a five-year
competition for the next 'large-class' space venture by Europe.
Juice stands for JUpiter ICy moon Explorer. The proposal is for a
spacecraft weighing nearly five tons, to go to Jupiter and investigate
three of its principal satellites. The spacecraft would use the
gravity of Jupiter to initiate a series of close fly-bys of Callisto
and Europa, and finally to put itself into a settled orbit around
Ganymede. Europa is suspected to have a water ocean beneath its
icy surface, and there is interest in whether it could support
microbial life.
AMATEUR ASTRONOMY IN KABUL
BBC News
There is a plan to take astronomy to schools, orphanages and refugee
camps throughout Afghanistan. Amateur astronomers, government
officials and teachers of science are behind the project, which will
hand out astronomy kits first around Kabul. The project will
establish the country's first astronomy curriculum for young children.
During the so-called 'dark ages' in Europe, Islamic civilisation
championed both astronomy and physics, shaping our modern science.
In more recent times there has been a reversal, in which it has been
taught that the skies are the realm of Allah, and astronomical study
and investigation were un-Islamic and forbidden. Now, the joint
initiative of the international organisation Astronomers Without
Borders and the Afghanistan Astronomy Association, with support from
Afghanistan's Ministry of Education, will send out the astronomy kits
which include The Little Book of Stars, a specially written
introductory text for young Afghan children. It will cover
astronomical topics from A to Z accompanied by cartoon illustrations,
in English and Pashto, the language spoken by Afghanistan's largest
ethnic group. As many schools lack even the most basic classroom
materials, the kits will also include pencils, pens and paper. Solar
viewing glasses, star charts and simple learning exercises will teach
children the movements of the Sun and planets. The kits are designed
to familiarise children with the Universe at a basic level. The
intention is that, once established around Kabul, the programme will
expand to cover the rest of Afghanistan. The eventual aim is to reach
other parts of Asia, the Middle East and Africa.
Topic: Mid Mayl Astronomy Bulletin (Read 925 times)