SURVEY COUNTS POTENTIALLY HAZARDOUS ASTEROIDS
NASA
Observations from the Wide-field Infrared Survey Explorer (WISE) have
led to a new assessment of potentially hazardous asteroids -- about
their total numbers, origins and the possible dangers they may pose.
Potentially hazardous asteroids are a subset of the larger group of
near-Earth asteroids; they are defined as those coming within 8
million km of the Earth and big enough to survive passing through
the atmosphere and then cause damage on a regional (or greater) scale.
It now appears the number of such asteroids, with diameters larger
than 100 metres, is about 4,700 +/- 1,500. So far, only an estimated
20-30% of those objects have been found.
VESTA IS 'LAST OF A KIND'
BBC News
Data from the Dawn probe which has been orbiting the second-largest
body in the asteroid belt for the past 10 months suggest that Vesta is
the only remaining example of the original objects that came together
to form the rocky planets, like the Earth and Mars, some 4.6 billion
years ago. They confirm that Vesta has a layered interior with a
metal-rich core, just as Mercury, Venus, the Earth and Mars do. Using
information about the shape of the asteroid and its gravity field,
the Dawn team calculates that the core is about 220 km across,
representing about 40% of the radius of Vesta, or roughly 18% of its
total mass. Dawn has studied in detail the pattern of minerals
exposed on Vesta's surface by innumerable impacts through the aeons.
It has also mapped the topography. The researchers believe that Vesta
formed within two million years of the first solids coming together in
the Solar System, before the planets as we know them today were
assembled. Short-lived radioactive materials would have generated
enough heat to melt Vesta's interior, producing a sub-surface ocean of
magma. The melting would have led to differentiation, to denser
materials like iron falling to the asteroid's centre. Other such
bodies in the early Solar System with magma oceans ended up becoming
parts of the Earth and the other planets. Somehow, Vesta did not --
it survived obliteration in the cascade of impacts that would have
marked those early times.
Another discovery is the definitive association that can now be made
between Vesta and the howardite-eucrite-diogenite, or HED, class of
meteorites. It had previously been suspected that such meteorites
originated from Vesta, but now the signatures of pyroxene -- a mineral
rich in iron and magnesium -- in those meteorites have been matched
precisely with the mineral signatures observed in Vesta's surface by
Dawn's instruments. The HED meteorites account for about 6% of all
the meteorites seen falling to Earth. Common they may be, but their
value to science is greatly increased now because researchers know
that they may offer insights into the earliest epoch of planetary
formation. Much of the HED material is likely to have come from two
huge impact basins at Vesta's south pole.
ULTRA-COOL COMPANION REVEALS GIANT PLANETS
RAS
An international team of astronomers has found a binary stellar
system, one of whose components is a brown dwarf that is more than 99%
hydrogen and helium. Described as ultra-cool, it has a temperature of
just 400°C and its discovery could be a step forward in helping
astronomers to distinguish between brown dwarfs and giant planets.
The problem is that compact brown dwarfs share many characteristics
with giant planets, so astronomers can be confused as to the nature of
what they detect. Brown dwarfs are star-like objects with
insufficient mass to ignite hydrogen fusion in their cores. Over time
they cool to temperatures of just a few hundred degrees. Formed like
stars from the collapse of a giant molecular cloud a few hundred
light-years across, brown dwarfs in binary systems are expected to
have the same atmospheric chemistry as their host stars. In contrast,
giant planets form with a more diverse chemistry. Those in our own
Solar System first formed as large solid cores, which then accreted
gas from the disc around them. That led to a different chemistry in
their outer layers. For example, when the Galileo spacecraft entered
Jupiter's atmosphere in 1995, it found the proportion of heavier
elements to be three times higher than in the Sun. Such differences
allow astronomers to discriminate between planets and brown dwarfs and
to theorise on their formation mechanisms.
The object, whose name is BD +1° 2920 B, was found by combining data
from ground- and space-based surveys. It is about 35 times more
massive than Jupiter and orbits its host star at a distance of 390
billion km or about 2600 times the average distance from the Earth to
the Sun.
HOW NATURE SHAPES THE BIRTH OF STARS
RAS
Using computer simulations, a team of astronomers has found evidence
that the way in which stars form depends on their birth environment.
Stars are thought to form in interstellar space from dark clouds of
gas and dust. Their properties are expected to depend on the
conditions there, in the same way that the temperature and
constitution of clouds on the Earth determines whether we experience
drizzly weather, rain with large or small droplets, or a hail shower.
In contrast, until now stars have been supposed to form in the same
manner everywhere. The scientists now have evidence that the mass
distribution of stars does in fact depend on the environment in which
they form. Surprisingly, the evidence does not come from regions of
ongoing star formation, but from a very old class of objects, globular
star clusters. The number of observed stars less massive than our Sun
in globular clusters is at odds with their structure.
Globular clusters are massive congregations of thousands of stars
surrounding our Galaxy, the Milky Way. Star-formation in such
clusters ceased billions of years ago. Nevertheless, using
simulations astronomers found that the connection between star
formation and birth environment can be understood when invoking a
process that occurs very early in the life of any cluster, called
residual-gas expulsion. Once a star completes its formation it starts
to shine and eventually the radiation coming from the cluster of
freshly-hatched stars quickly drives out the remains of the gas from
which they formed. The region of star birth is thus destroyed,
leaving behind stars of different masses. The process leads to
expansion of the whole aggregate of stars, with the accompanying
stripping of some of the stars from the cluster by the gravitational
attraction of the rest of the Galaxy. The faster the gas is blown out
the stronger is the expansion and the more stars are removed. The
imprint of that process is still visible in the present-day mass
distribution, so observations of present-day stellar populations in
globular clusters may allow their initial star content to be
reconstructed.
The astronomers find that globular clusters must have formed with many
more massive stars than are counted in individual star-forming regions
today, otherwise the star-birth region from which the globular cluster
formed would not be destroyed quickly enough and the subsequent
expansion would be too weak to remove enough stars from the cluster.
If that had happened the distribution of masses of the stars we see
today would be quite different. The deduced differences in the number
of massive stars having formed in globular clusters depending on the
cloud conditions is indeed in agreement with theoretical expectation.
According to their results, differences in the initial star content
appear only when conditions in the star-birth regions are very extreme
compared to those we see today, but may have well been frequent when
globular clusters were born around 12 billion years ago. The work
leads to an expectation that stars form in the same way, with the same
range of masses, in different sites in the present-day Milky Way.
ONE SUPERNOVA TYPE, TWO DIFFERENT SOURCES
Harvard-Smithsonian Center for Astrophysics
The exploding stars known as Type-Ia supernovae serve an important
role in measuring the Universe. They are bright enough to be seen
from far away, and similar enough to act as 'standard candles' --
objects of known luminosity. The 2011 Nobel Prize in Physics was
awarded for the discovery by means of Type-Ia supernovae of
acceleration in the expansion of the Universe. However, an
embarrassing fact is that astronomers still do not know what star
systems make Type-Ia supernovae.
Two very different models explain the possible origin of such
supernovae. New evidence suggests that both models are correct -- some
of the supernovae are created one way and some the other. Previous
studies have produced conflicting results, but the conflict disappears
if both types of explosion are happening. Type-Ia supernovae are
known to originate from white dwarfs -- the dense cores of white
dwarfs are also called degenerate stars because they are supported by
something called quantum degeneracy pressure. In the
single-degenerate model for a supernova, a white dwarf gathers
material from a companion star until it reaches a tipping point where
a runaway nuclear reaction begins and the star explodes. In the
double-degenerate model, two white dwarfs merge and explode.
Single-degenerate systems should have gas from the companion star
around the supernova, while the double-degenerate systems will lack
that gas.
Scientists studied 23 Type-Ia supernovae to look for signatures of gas
around the supernovae, and found that the more powerful explosions
tended to come from systems with outflows of gas. However, only a
fraction of supernovae showed evidence of outflows. That finding has
important implications, because if two different mechanisms are at
work then the two types of supernovae ought to be considered
separately in the calculation of cosmic distances and expansion rates.
But it does seem surprising, if there are two different mechanisms,
that the explosions can look so similar and be of such apparently
similar brightness.
A SUPERNOVA COCOON BREAKTHROUGH
Chandra X-ray Center
Observations with the Chandra X-ray Observatory have provided the
first X-ray evidence of a supernova shock-wave breaking through a
cocoon of gas surrounding the star that exploded. The discovery may
help astronomers understand why some supernovas are much more powerful
than others. On 2010 November 3 a supernova was discovered in the
galaxy UGC 5189A, located about 160 million light-years away. Using
data taken earlier by the All-Sky Automated Survey telescope in
Hawaii, astronomers found that that supernova was seen to explode in
early October in 2010. A team of researchers used Chandra to observe
the supernova in 2010 December and 2011 October. The supernova was
one of the most luminous that has ever been detected in X-rays. In
optical light, the object, called SN 2010jl, was about ten times more
luminous than a typical supernova resulting from the collapse of a
massive star, adding to the class of very luminous supernovae that
have been discovered recently by optical surveys. Different
explanations have been proposed to explain such super-energetic
supernovae, including (1) the interaction of the supernova's blast
wave with a dense shell of matter around the pre-supernova star, (2)
radioactivity resulting from a pair-instability supernova (triggered
by the conversion of gamma-rays into particle and anti-particle
pairs), and (3) emission powered by a neutron star with an unusually
powerful magnetic field.
In the first Chandra observation of SN 2010jl, the X-rays from the
explosion's blast wave were strongly absorbed by a cocoon of dense gas
around the supernova. The cocoon was formed by gas blown away from
the massive star before it exploded. In the second observation taken
almost a year later, there was much less absorption of X-ray emission,
indicating that the blast wave from the explosion has broken out of
the surrounding cocoon. The Chandra data show that the gas emitting
the X-rays is at a very high temperature -- above 100 million degrees
-- strong evidence that it has been heated by the supernova blast
wave. The energy distribution of SN 2010jl in optical light reveals
features that the researchers think are explained by matter around the
supernova having been heated and ionized (electrons stripped from the
atoms) by X-rays generated when the blast wave ploughed through it.
While that type of interaction has been proposed before, the new
observations seem to show, for the first time, that it is really
happening. The discovery is therefore consonant with the idea that
the unusual luminosities of some supernovae are caused by their blast
waves ramming into surrounding material.
BLACK HOLES HEAT UNIVERSE
Heidelberg Institute for Theoretical Studies
Astrophysicists think that they have recognised a heating source that
they had not thought of previously. They have been supposing that
super-massive black holes could influence only their immediate
surroundings. They now believe that diffuse gas in the Universe can
absorb gamma-ray emission from black holes and heat up strongly.
That has important implications. Black holes, such as are supposed to
exist in the centres of many galaxies, can emit high-energy gamma-rays
and are then called blazars. Whereas other radiation such as visible
light and radio waves traverses the Universe without undue hindrance,
gamma-rays interact with optical light, creating electrons and
positrons. Initially, those elementary particles move almost at the
speed of light, but as they are slowed down by the ambient diffuse
gas, their energy is converted into heat, just as in other braking
processes, and the heat goes into the gas. Scrabbling round for some
support for their idea, the scientists concerned suggest that the fact
that there are fewer dwarf galaxies near the Milky Way than some
people think there ought to be may be because the gas out of which
they might have formed was too hot to collapse together.
'SPACE LAUNCH SYSTEM'
Science Daily
NASA's space launch system (SLS) is on track to produce the rocket
will need to send astronauts further afield than ever before,
according to the program's manager. He said that an uncrewed test
flight of the Orion spacecraft in 2014, SLS mission in 2017 and a 10-
to 14-day mission with astronauts going to the Moon in 2021 will
demonstrate an ability to go anywhere in the Solar System that people
want to go, in particular to Mars. It is hoped that a test version of
the Orion capsule will be launched in a couple of months without any
astronauts aboard, to see that it works and to test its heat shield.
Many elements of the SLS itself are already being testing, including
the engines and solid-rocket boosters that will give the rocket about
3600 tons' thrust at launch, 10% more than the Saturn V.
NASA intends to cannibalise space-shuttle main engines to power the
core stage. The SLS will also use solid rocket boosters like the
shuttle, but the SLS versions will be five segments long instead of
four. The core stage, which will hold the fuel tanks for the main
engines, is early in its design but still is on schedule; it is about
15 feet longer than the shuttle's external tank, and it will be
shipped to Cape Canaveral on the Pegasus barge, another element shared
with the shuttle. Using shuttle components where possible is saving a
lot of design and development costs.
Topic: Late May Astronomy Bulletin (Read 1303 times)