COMET WILD2: EVIDENCE OF 'SPACE WEATHERING'
RAS
The traditional picture of comets as cold, icy bodies, unchanging
throughout their history, is being reappraised in the light of
analyses of dust grains from Comet Wild2. Astronomers have detected
the presence of iron in a dust grain, evidence of space weathering
that could explain the rusty reddish colour of Wild2's outer surface.
The Wild2 grains were collected by the Stardust space mission and
returned to Earth in 2006. The fast-moving dust grains were collected
in arrays of aerogel, a silicon-based foam that is 99% empty space; it
slowed the particles with a deceleration of millions of g, from
velocities of 6 km/s to a halt over just a few millimetres. Since
then scientists have been analysing the samples and the carrot-shaped
tracks that they left in the aerogel. It has been possible to
determine the mineralogy and isotopes in microscopic samples dissected
from the grains. The analysis shows that the surface of Comet Wild2
has been bombarded by particles in the solar wind and micrometeorites
throughout its 4.5-billion-year history. That 'space weathering' has
deposited nanometre-size grains of iron metal and reddened the surface
of the comet. The mineralogical evidence for space weathering that
has now been identified in the Wild2 samples has been hinted at by
spectroscopic observations of other comets.
BILLIONS OF PLANETS AROUND RED DWARFS?
ESO
Astronomers using the HARPS planet finder at ESO say that rocky
planets not much bigger than the Earth are very common in the
'habitable zones' (where liquid water could exist on the surface of a
planet) around faint red-dwarf stars. They are so carried away with
their own discoveries, numbering just two, that they suggest that
there are tens of billions of such planets in our own Galaxy alone.
The HARPS team surveyed a sample of 102 red dwarf stars in the
southern skies over a six-year period. A total of nine super-Earths
(planets with masses between one and ten times that of the Earth) was
found, including two in the 'habitable zones' of Gliese 581 and Gliese
667 C respectively. The astronomers estimated how massive the planets
were and how far from their stars they orbited. By combining all the
data, including observations of stars that did not have planets, and
guessing the fraction of existing planets that could be discovered,
the team tried to work out how common different sorts of planets are
around red dwarfs. They asserted that the frequency of occurrence of
super-Earths in the 'habitable zone' is 41%, with a possible range
from 28% to 95%. [But in their actual survey they found only two such
objects out of a sample of 102 -- only 2% -- Ed.] As there are many
red-dwarf stars close to the Sun, the new estimate implies that there
might be about 100 super-Earth planets in the 'habitable zones' around
stars in the neighbourhood of the Sun at distances less than about 30
light- years. The zone where the temperature is suitable for liquid
water to exist is much closer to a red dwarf than the Earth is to the
Sun, and red dwarfs are subject to stellar eruptions or flares, which
would bathe any planet in X-rays or ultraviolet radiation and would
not encourage life. One of the planets discovered in the HARPS survey
of red dwarfs is Gliese 667 Cc. It is the second planet in a
triple-star system and seems to be situated close to the centre of the
'habitable zone'. Although the planet is more than four times the
mass of the Earth it is the closest twin to Earth found so far and
almost certainly has the right conditions for the existence of liquid
water on its surface.
'ORDINARY' BLACK HOLE DISCOVERED 12 MILLION LIGHT-YEARS AWAY
RAS
An international team of scientists has discovered an 'ordinary' black
hole in the galaxy Centaurus A, 12 million light-years away, the first
'normal'-mass black hole to have been detected away from the immediate
vicinity of our own Galaxy. Although exotic by everyday standards,
black holes are now thought by some astronomers to be everywhere. The
lowest-mass black holes are formed when very massive stars reach the
ends of their lives, ejecting most of their material into space in
supernova explosions and leaving compact cores that collapse into
black holes. There are suggested to be millions of such low-mass
black holes distributed throughout every galaxy. Despite their
ubiquity, they can be hard to detect as they do not emit light, so
their existence is inferred (if at all) only through their action on
objects around them, for example by dragging in material that is
heated up in the process and emits X-rays.
In recent years, researchers have made some progress in finding
ordinary black holes in binary systems, by looking for the X-ray
emission produced when material from their companion stars falls into
them. So far the objects have been relatively close by, either in our
own Milky Way Galaxy or in 'nearby' galaxies in the Local Group (the
cluster of galaxies that includes the Milky Way and Andromeda). The
team used the orbiting Chandra X-ray observatory to make day-long
exposures of Centaurus A, detecting an object with 50,000 times the
X-ray brightness of our Sun. A month later, it had dimmed by more
than a factor of 10, and then later by a factor of more than 100 and
had became undetectable. Such behaviour is characteristic of a
low-mass black hole in a binary system during the final stages of an
outburst, and is typical of similar black holes in the Milky Way.
LOTS OF GAS AND DUST AROUND BLACK HOLES IN EARLY UNIVERSE
RAS
The Institut de Radioastronomie Millimetrique (IRAM) array is made up
of six 15-m telescopes, sited on the 2550-m-high Plateau de Bure in
the French Alps, that detect emission at millimetre wavelengths. The
IRAM telescopes work together as an interferometer to simulate a
single much larger telescope that can study objects in fine detail.
They have recently been used to observe a galaxy called J1120+0641
that surrounds the most distant super-massive black hole known. Its
light has taken so long to reach us that the galaxy is seen as it was
'only' 740 million years after the Big Bang, when the Universe was
1/18th of its current age. IRAM has shown that it contains a lot of
gas and dust that includes significant quantities of carbon. That is
quite unexpected, as carbon is created by the nuclear fusion of helium
in the centres of massive stars and is subsequently ejected into
interstellar space when those stars end their lives as supernovae.
It is interesting that such a lot of carbon-enriched gas could have
formed at such early times. The presence of so much carbon indicates
that the formation of massive stars must have occurred in the
relatively 'short' period between the Big Bang and the time at which
we now see the galaxy. The emission from the dust shows that the
galaxy was still forming stars at a rate 100 times greater than the
present rate in our Milky Way. The galaxy concerned, in the
constellation Leo, is at a low enough declination to be accessible to
the Atacama Large Millimetre Array (ALMA), currently under
construction in Chile. ALMA should enable a detailed study to be made
of the structure of that galaxy, including the way the gas and dust
moves within it.
MONOLITH AT GARDOM'S EDGE IS ASTRONOMICALLY ALIGNED
RAS
Researchers at Nottingham Trent University have gathered new evidence
that a 4000-year-old monolith was aligned to be an astronomical
marker. The monolith is located at Gardom's Edge, a striking
Millstone-Grit ridge midway between Nottingham and Manchester
in the Peak District National Park. The 2.2-metre-high monument
has a striking, right-angled-triangular shape that slants up towards
the south. The orientation and inclination of the slope is aligned to
the altitude of the Sun at mid-summer. The researchers have carried
out a survey of the surface surrounding the monolith. Owing to the
sensitivity of the site, they are not allowed to dig it up, but their
survey indicates the presence of packing stones around the base of the
monolith, with a higher density of stones on one side, supporting the
case that the stone was orientated intentionally. The landscape
surrounding the stone harbours many ancient monuments such as
Bronze-Age roundhouses, a late Neolithic enclosure, and other traces
of a long-lasting human occupation. The researchers believe that the
stone is also late Neolithic, set up around 2000 BC.
Presumably because the Sun itself does not actually shine very often in
the Peak District, they have also carried out 3-D modelling of the
solar illumination of the stone through the seasons. The modelling
shows that during the winter half-year, the slanted side of the stone
remains in permanent shadow; during most of the summer half-year it
would be illuminated only during the morning and afternoon; close to
midsummer it would be illuminated all day [as distinct from morning
and afternoon? -- Ed.]. The researchers are now hoping to confirm
their modelling work by actual photography of the stone.
The stone might have been a marker for a social arena for seasonal
gatherings. It is not a sundial in the sense that people would have
used it to determine an exact time. Scientists think that it was set
in position to give a symbolic meaning to its location, in the same
way as some religious buildings are aligned in a specific direction
for symbolic reasons. The use of shadow-casting in monuments of its
period is quite rare in the British Isles, but there are some examples
including New Grange, Ireland, and some Clava cairns in the north-east
of Scotland that have been proposed to include the intentional use of
shadows. Both are associated with burial sites using the symbolism of
a cyclic light-and-shadow display to represent eternity. Given the
proximity of the Neolithic enclosure and possible ritual importance of
the site, the Gardom's Edge monolith could be another such example.
WHEN DARK ENERGY TURNED ON
RAS
The Sloan Digital Sky Survey (SDSS-III) has announced the most
accurate measurements yet of the distances to galaxies in remote parts
of the Universe. The results represent more than two years' work by
the team behind the Baryon Oscillation Spectroscopic Survey (BOSS),
one of the SDSS-III's four component surveys. A proposal that was
recognized with the 2011 Nobel Prize in Physics was that not only is
our Universe expanding, but that its expansion is accelerating. The
leading contender for the cause of the supposed acceleration is an
alleged property of space dubbed 'dark energy'. Another explanation,
considered less likely, is that at very large distances the force of
gravity deviates from Einstein's General Theory of Relativity and
becomes repulsive. Whether the answer to the puzzle of the
accelerating Universe is dark energy or modified gravity, the first
step to finding that answer is to measure accurate distances to as
many galaxies as possible. From those measurements, astronomers can
trace out the history of the Universe's expansion.
BOSS is producing the most detailed map of the Universe so far made,
with a spectrograph on the SDSS 2.5-m telescope at Apache Point
in New Mexico. It is intending to measure spectra of more
than a million galaxies over six years. The maps so far analysed are
based on data from the first year and a half of observations, and
contain more than 250,000 galaxies. Some of the galaxies are so
distant that their light has travelled more than six billion years to
reach the Earth -- nearly half the age of the Universe. Maps of the
Universe like BOSS's show that galaxies and clusters of galaxies are
clumped together into walls and filaments, with voids between them.
The structures grew out of subtle variations in density in the early
Universe, which bore the imprint of 'baryon acoustic oscillations' --
pressure-driven (acoustic) waves. Because of the regularity of those
ancient waves, there is a slightly increased probability that any two
galaxies today will be separated by about 500 million light-years,
rather than 400 million or 600 million. In a graph of the number of
galaxy pairs versus separation distance, that number of 500 million
light-years shows up as a peak, so some astronomers speak of the
'peak separation' between galaxies. The distance that corresponds to
the peak depends on the amount of 'dark energy' in the Universe. But
measuring the peak separation between galaxies depends critically on
having the right distances to the galaxies in the first place.
In addition to providing accurate distance measurements, the BOSS data
also enable a stringent new test to be made of General Relativity.
Since gravity attracts, galaxies at the edges of galaxy clusters fall
in toward the centres of the clusters. General Relativity predicts
just how fast they should be falling. If our understanding of General
Relativity is incomplete, we should be able to tell from the shapes we
see in BOSS's maps near known galaxy clusters. After accounting for
effects of 'dark energy', the team found that the rate at which
galaxies fall into clusters is consistent with Einstein's predictions.
We already knew that the predictions of General Relativity are
extremely accurate for distances within the Solar System, and now we
can say that they are accurate for distances of 100 million light-
years. We're looking a billion times further away than Einstein
looked when he tested his theory, but it still seems to work.
NASA EXTENDS KEPLER, SPITZER, PLANCK MISSIONS
Science Daily
Kepler has been approved for extension until 2016 September. The
extension provides four additional years to find Earth-size planets in
the 'habitable zone'. Spitzer, launched in 2003, has continued to
obtain infrared images since it ran out of coolant, as expected, in
2009; during its current 'warm mission', it is trying to observe the
atmospheres of extra-solar planets and investigating some of the most
distant galaxies known. As requested by the project, Spitzer received
two additional years' funding. NASA will fund one additional year of
U.S. participation in ESA's Planck mission, for the U.S. Planck data
centre and for operations of the low-frequency instrument. Planck,
launched in 2009, is gathering data from the very early Universe; its
observations may yield insight into the origin, evolution and fate of
the Universe.
EXPEDITION TO THE OBSERVE THE TRANSIT OF VENUS
By Dr Rhodri Evans
There is a once-in-a-lifetime (literally, for those who missed the
event in 2004) chance to see the Transit of Venus in June, on an
expedition to the Gobi Desert. I am acting as astronomy consultant on
the expedition, which will also have a NASA astronaut in the team.
http://isset.org/astronomy_meets_adventure/transit_of_venus.phpThe cost is a factor of 3 or 4 cheaper than the trips Sky & Telescope
Magazine are running to Hawaii, so represents outstanding value for
money.
Topic: April Astronomy Bulletin (Read 1610 times)