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« Last post by Clive on November 18, 2018, 09:54 »
COMET TAILS BLOWING IN SOLAR WIND
NASA/Goddard Space Flight Center
In 2007 January, scientists saw the first data from the STEREO (Solar and
Terrestrial Relations Observatory) spacecraft. Instead of the star field
they expected, a pearly white, feathery smear filled the frame. That bright
object was not a defect: it was the first satellite image of Comet McNaught
C/2006 P1), named for astronomer Robert McNaught, who discovered it in 2006
August. It was one of the brightest comets of the past 50 years. Through-
out 2007 January, the comet fanned across the Southern Hemisphere's sky, so
bright that it was visible to the naked eye even during the day. McNaught
belongs to a rare group of comets, dubbed the Great Comets and known for
their exceptional brightness. Setting McNaught apart further still from
its peers, however, was its highly structured tail, composed of many
distinct dust bands, called striae or striations, that stretched more than
100 million miles behind the nucleus. One month later, a spacecraft called
Ulysses encountered the comet's long tail. McNaught was a huge deal when it
came, because it was so ridiculously bright and beautiful in the sky. It
had striae -- dusty fingers that extended across a great expanse of the sky.
It was one of the most beautiful comets seen for decades.
How exactly the tail broke up, scientists do not know. It called to mind
reports of another storied comet from long ago -- the Great Comet of 1744,
which was said to have fanned out dramatically in six tails over the
horizon, a phenomenon that astronomers then could not explain. By
untangling the mystery of McNaught's tail, scientists hoped to learn
something new about the nature of comets -- and solve two cosmic mysteries
in one. A key difference between studying comets in 1744 and 2007 is, of
course, our ability now to do so from space. In addition to STEREO's
serendipitous sighting, another mission, SOHO -- the Solar and Heliospheric
Observatory -- made regular observations as McNaught flew by the Sun.
Researchers hoped that those images might contain their answers.
Comets are cosmic crumbs of frozen gas, rock and dust left over from the
formation of the Solar System 4.6 billion years ago -- so they may contain
important clues about Solar System's early history. Those clues may be
unlocked, as if from a time capsule, every time a comet's orbit brings it
close to the Sun. Intense heat vaporizes the frozen gases and releases the
dust within, which streams behind the comet, forming two distinct tails: an
ion tail carried by the solar wind -- the constant flow of charged particles
from the Sun -- and a dust tail. Understanding how dust behaves in the tail
-- how it fragments and clumps together -- can teach scientists a great deal
about similar processes that formed dust into asteroids, moons and even
planets all those billions of years ago. Appearing as one of the biggest
and most structurally complex comets in recent history, McNaught was a
particularly good subject for that type of study. Its brightness and high
dust production made it much easier to resolve the evolution of fine
structures in its dust tail. Astronomers noticed weird goings-on in the
images of those striations, a disruption in the otherwise clean lines. The
rift seemed to be located at the heliospheric current sheet, a boundary
where the magnetic orientation, or polarity, of the electrified solar wind
changes direction. That puzzled scientists, because while they have long
known that a comet's ion tail is affected by the solar wind, they had never
seen the solar wind affect dust tails before. Dust in McNaught's tail --
roughly the size of cigarette smoke -- is too heavy, the scientists thought,
for the solar wind to push it around. On the other hand, an ion tail's
miniscule, electrically charged ions and electrons easily sail along the
solar wind. But it was difficult to tell exactly what was going on with
McNaught's dust, and where, because at roughly 60 miles per second, the
comet was rapidly travelling in and out of STEREO and SOHO's view. In
looking for a way to bring it all together to get a complete picture of
what's happening in the tail they used an image-processing technique that
compiles all the data from different spacecraft using a simulation of the
tail, where the location of each tiny speck of dust is mapped by solar
conditions and physical characteristics like its size and age, or how long
it had been since it had flown off the head, or coma, of the comet. The end
result is a map, which layers information from all the images taken at any
given moment, allowing the dust's movements to be followed. The maps made
it easier to explain the strange effect that drew attention to the data in
the first place. Indeed, the current sheet was the culprit behind the
disruptions in the dust tail, breaking up each striation's smooth, distinct
lines. For the two days it took the full length of the comet to traverse
the current sheet, whenever dust encountered the changing magnetic condi-
tions there, it was jolted out of position, as if crossing some cosmic speed
bump. That is strong evidence that the dust is electrically charged, and
that the solar wind is affecting the motion of that dust. Scientists have
long known that the solar wind affects charged dust; missions like Galileo,
Cassini and Ulysses watched it move electrically-charged dust through the
space near Jupiter and Saturn. But it was a surprise for them to see the
solar wind affect larger dust grains like those in McNaught's tail -- about
100 times bigger than the dust seen ejected from around Jupiter and Saturn
-- because they are that much heavier for the solar wind to push. The work
sheds light on the nature of striated comet tails from the past and provides
a lens for studying other comets in the future. But it also opens a new
line of questioning: what roles did the Sun have in the Solar System's
formation and early history?
AMATEUR ASTRONOMERS FIND A BRIGHT NEW COMET
Spaceweather.com
There's a new comet in the morning sky. Discovered just last week by three
amateur astronomers -- one in Arizona and two in Japan -- Comet Machholz-
Fujikawa-Iwamoto (C/2018 V1) has quadrupled in brightness over the past few
days and is now glowing like a fuzzy 8th-magnitude star in the constellation
Virgo. The discovery of a comet by amateur astronomers is a rare event
nowadays because robotic Near-Earth-Object search programmes usually catch
them first. Comet Machholz-Fujikawa-Iwamoto appears to be a first-time
visitor to the inner Solar System. It is plunging toward the Sun on a
nearly-parabolic orbit that will take it just inside the orbit of Mercury.
Closest approach to the Sun (0.38 AU) is on Dec. 3-4; closest approach to
Earth (0.67 AU) is Nov. 27. Fresh comets like this one are notoriously
unpredictable. They can surge in brightness, seeming to promise a
spectacular display, but suddenly fizzle out as their deposits of ice are
exhausted by solar heat. So it is uncertain whether the new comet will even
become a naked-eye object. At the moment it is an easy target for amateur
telescopes.
AGEING FLOCK OF STARS IN WILD DUCK CLUSTER
University of Arizona
Astronomers have long believed that many open clusters consist of a single
generation of stars because, once stars have formed, their radiation blows
away nearby material needed to make new stars. But in M11 (the 'Wild Duck
Cluster') -- stars of the same brightness appear in different colours,
suggesting that they are of different ages. Unless scientists had missed
important clues about stellar evolution, there had to be another explanation
for the spread of colours in that cluster of about 2,900 stars. Open
clusters contain thousands of stars that astronomers hypothesize formed
from the same giant clouds of gas. Those stars come in all sizes, from
short-lived, giant blue stars dozens of times more massive than our Sun, to
long-lived low-mass dwarfs that will burn for 10 billion years or longer.
The brightness and colour of each star change as it grows older, allowing
scientists to determine its age. Astronomers plot stars' brightness and
colour in a diagonal line called the main sequence in the Herttzsprung--
Russell Diagram. The turning point -- the point at which a star ages and
veers off the main sequence -- is used to estimate the ages of clusters on
the basis of the known life expectancy of each star. If the stars leave the
main sequence at the same point, then they must all be the same age. In
M11, however, the stars veer off the diagonal at different points. The team
observed M11 with the MMT to examine the colour spectrum of the stars. They
used the 'Hectochelle', which can capture detailed spectra of many stars at
once.
Rotation of a star causes its spectral lines to be broadened. The spectra
of stars in M11 show that they are spinning at different rates. A rapidly
rotating star can remain in the main-sequence stage longer than a slowly
rotating one. A wide range of rotational velocities of stars in a cluster
results in differences of lifetimes among the stars. Rotational speed is
like a fountain of youth to a star: the faster it spins, the better it mixes
hydrogen -- the star's fuel -- into its core. The more hydrogen the core
receives, the longer the star lives, causing it to appear redder than
younger siblings. Stars in the cluster appear of different colours because
the cloud in which they were born set them in motions that would extend the
lifetimes of some of them.
ULTRA-HOT GAS AROUND WHITE DWARFS
RAS
Solving a decades-old mystery, astronomers have discovered an extremely hot
magnetosphere around a white dwarf, a remnant of a star like our Sun. White
dwarfs are the final stage in the lives of stars like our Sun. At the end
of their lives, those stars eject their outer atmospheres, leaving behind a
hot, compact and dense core that cools over billions of years. The
temperature on their surfaces is typically around 100,000 degrees C. Some
white dwarfs, though, challenge scientists, as they show evidence for highly
ionized metals. In astronomy, 'metals' means every element heavier than
helium, and high ionization here means that all but one of the outer
electrons usually in their atoms have been stripped away. That process
needs a temperature of 1 million degrees C, far hotter than the surfaces of
even the hottest white-dwarf stars. The team used the 3.5-m Calar Alto
telescope in Spain to discover and observe a white dwarf in the direction of
the constellation of Triangulum, catalogued as GALEXJ014636.8+323615,
located 1200 light-years away. Spectra of the white dwarf revealed the
signatures of highly-ionized metals. Intriguingly, they varied over a
period of six hours -- the same time it takes for the white dwarf to rotate.
The team concluded that the magnetic field around the star -- the magneto-
sphere -- traps material flowing from its surface. Shocks within the
magnetosphere heat the material dramatically, stripping almost all the
electrons from the metal atoms. The axis of the magnetic field of the white
dwarf is tilted with respect to the rotational axis. That means that the
amount of shock-heated material we see varies as the star rotates. More and
more such stars have been found, without there being any clue as to where
the highly-ionized metals come from, but now the shock-heated magnetosphere
model finally explains their origin. Magnetospheres are found around other
types of stars, but this is the first report of one around a white dwarf.
The discovery might have far-reaching consequences. Ignoring their
magnetospheres could mean measurements of other basic properties of white
dwarfs are wrong, such as their temperatures and masses. The team now plans
to model them in detail and to extend the research by studying more of them.
MATERIAL ORBITING CLOSE TO BLACK HOLE
ESO
ESO's GRAVITY instrument has added further evidence to the long-standing
assumption that a supermassive black hole lurks in the centre of the Milky
Way. New observations show clumps of gas swirling around at about 30% of
the speed of light on a circular orbit just outside its event horizon -- the
first time material has been observed orbiting close to the point of no
return, and the most detailed observations yet of material orbiting so close
to a black hole. The GRAVITY instrument on the Very Large Telescope (VLT)
Interferometer has been used by scientists to observe flares of infrared
radiation coming from the accretion disc around Sagittarius A*, the massive
object at the heart of the Milky Way. The observed flares provide long-
awaited confirmation that the object in the centre of our galaxy is, as has
long been assumed, a supermassive black hole. The flares originate from
material orbiting very close to the black hole's event horizon, making these
observations the most detailed ones yet of material orbiting so close to a
black hole. While some matter in the accretion disc -- the belt of gas
orbiting Sagittarius A* at relativistic speeds -- can orbit the black hole
safely, anything that gets too close is doomed to be pulled beyond the event
horizon. The closest point to a black hole that material can orbit without
being irresistibly drawn inwards by the immense mass is known as the
innermost stable orbit, and it is from there that the observed flares
originate. Those measurements were only possible thanks to international
collaboration and state-of-the-art instrumentation. The GRAVITY instrument
which made this work possible combines the light from four telescopes of
the VLT to create a virtual super-telescope 130 metres in diameter, and
has already been used to probe the nature of Sagittarius A*.
Earlier this year, GRAVITY and SINFONI, another instrument on the VLT,
allowed the same team accurately to measure the close fly-by of the star S2
as it passed through the extreme gravitational field near Sagittarius A*,
and for the first time revealed the effects predicted by Einstein's general
relativity in such an extreme environment. During S2's close fly-by, strong
infrared emission was also observed. That emission, from highly energetic
electrons very close to the black hole, was visible as three prominent
bright flares, and exactly matches theoretical predictions for hot spots
orbiting close to a black hole of four million solar masses. The flares are
thought to originate from magnetic interactions in the very hot gas orbiting
very close to Sagittarius A*.
ELUSIVE STAR HAS ORIGINS CLOSE TO BIG BANG
Johns Hopkins University
Astronomers have found what could be one of the Universe's oldest stars, a
body almost entirely made of materials spewed from the Big Bang. The
discovery of this approximately 13.5-billion-year-old tiny star means that
more stars with very low mass and very low metal content are likely to be
out there -- perhaps even some of the Universe's very first stars. The star
is unusual because unlike other stars with very low metal content, it is
part of the Milky Way's 'thin disc' -- the part of the Galaxy in which our
own Sun resides. And because this star is so old, researchers say it is
possible that our galactic neighbourhood is at least 3 billion years older
than previously thought. The Universe's first stars would have consisted
entirely of elements like hydrogen, helium, and small amounts of lithium.
Those stars then produced elements heavier than helium in their cores and
seeded the Universe with them when they exploded as supernovae. The next
generation of stars formed from clouds of material laced with those metals,
incorporating them into their makeup. The metal content, or metallicity,
of stars in the Universe increased as the cycle of star birth and death
continued. The newly discovered star's extremely low metallicity indicates
that, in a cosmic family tree, it could be as little as one generation
removed from the Big Bang. Indeed, it is the new record holder for the star
with the smallest complement of heavy elements -- it has about the same
heavy-element content as the planet Mercury. In contrast, our Sun is many
generations down the line and has a mass of heavy elements equal to 14
Jupiters. Astronomers have found around 30 ancient 'ultra-metal-poor'
stars with the approximate mass of the Sun. The star the team found,
however, is only 14 percent the mass of the Sun.
The star is part of a two-star system orbiting around a common point. The
team found the tiny, almost invisibly faint, secondary star after another
group of astronomers discovered the much brighter primary star. That team
measured the primary's composition by studying a high-resolution optical
spectrum, and found it to have extremely low metallicity. The existence of
the companion star turned out to be the big discovery. The team was able to
infer its mass by studying the slight variation that it induces in the
radial velocity of the primary star. As recently as the late 1990s,
researchers believed that only massive stars could have formed in the
earliest stages of the Universe -- and that they could never be observed
because they burn through their fuel and die so quickly. But as astro-
nomical simulations became more sophisticated, they began to hint that, in
certain situations, a star from that time period but with particularly low
mass could still exist. Unlike huge stars, low-mass ones can live for
exceedingly long times. Red dwarf stars, for instance, with a fraction of
the mass of the Sun, are thought to live for billions of years. The
discovery of the new ultra-metal-poor star, named 2MASS J18082002-5104378 B, opens the possibility of observing even older stars.
MILKY WAY MERGED WITH LARGE GALAXY
University of Groningen
Some ten billion years ago, the Milky Way merged with a large galaxy. The
stars from that partner, named Gaia-Enceladus, make up most of the Milky
Way's halo and also shaped its thick disc, giving it its inflated form.
Large galaxies like our Milky Way are the result of mergers of smaller
galaxies. An outstanding question is whether a galaxy like the Milky Way is
the product of many small mergers or of a few large ones. Researchers have
looked for 'fossils' in our Milky Way which might offer some hints as to its
evolution. The research uses the chemical composition, the position and the
trajectory of stars in the halo to deduce their history and thereby to
identify the mergers which created the early Milky Way. The second data
release from the Gaia satellite mission last April provided data on around
1.7 billion stars, and they has been used to look for traces of mergers in
the halo. Astronomers expected stars from fused satellites in the halo.
What they did not expect to find was that most halo stars actually have a
shared origin in one very large merger. The chemical signature of many halo
stars was clearly different from that of the 'native' Milky Way stars. They
are a fairly homogeneous group, which indicates that they share a common
origin. In plots of both trajectory and chemical signature, the 'invaders'
stood out clearly.
The youngest stars from Gaia-Enceladus are actually younger than the native
Milky Way stars in what is now the thick-disc region. That means that the
progenitor of the thick disc was already present when the fusion happened,
and Gaia-Enceladus, because of its large size, shook it and puffed it up.
In a previous paper, the team had already described a huge 'blob' of stars
sharing a common origin. Now, it shows that stars from the blob in the halo
are the debris from the merging of the Milky Way with a galaxy which was
slightly more massive than the Small Magellanic Cloud, some ten billion
years ago. That galaxy is called Gaia-Enceladus, after the giant Enceladus
who in Greek mythology was born of Gaia (the Earth goddess) and Uranus (the
Sky god).
KEPLER SPACE TELESCOPE RETIRED
NASA
After nine years in deep space collecting data that indicate our sky to be
filled with billions of hidden planets -- more planets even than stars --
the Kepler space telescope has run out of fuel needed for further science
operations. NASA has decided to retire the spacecraft within its current,
safe orbit, away from the Earth. Kepler leaves a legacy of more than 2,600
planet discoveries from outside the Solar System, many of which could be
promising places for life. Kepler has opened our eyes to the diversity of
planets that exist in our Galaxy. The most recent analysis of Kepler's
discoveries concludes that 20 to 50 per cent of the stars visible in the
night sky are likely to have small, possibly rocky, planets similar in size
to the Earth and located within the habitable zones of their parent stars.
That that means they are located at distances from their parent stars where
liquid water -- a vital ingredient to life as we know it -- might pool on
the planetary surface. The most common size of planet Kepler found does not
exist in the Solar System -- between the sizes of the Earth and Neptune --
and we have much to learn about such planets. Kepler also found that Nature
often produces jam-packed planetary systems, in some cases with so many
planets orbiting close to their parent stars that our own Solar System
looks sparse by comparison. Launched on 2009 March 6, the Kepler space
telescope combined cutting-edge techniques in measuring stellar brightness
with the largest digital camera outfitted for outer-space observations at
that time. Originally positioned to stare continuously at 150,000 stars in
one star-studded patch of the sky in the constellation Cygnus, Kepler took
the first survey of planets in our galaxy and became the agency's first
mission to detect Earth-size planets in the habitable zones of their stars.
Four years into the mission, after the primary mission objectives had been
met, mechanical failures temporarily halted observations. The mission team
was able to devise a fix, switching the spacecraft's field of view roughly
every three months. That enabled an extended mission for the spacecraft,
dubbed K2, which lasted as long as the first mission and raised Kepler's
count of surveyed stars up to more than 500,000. The observation of so many
stars has allowed scientists to understand better their behaviours and
properties, which is critical information in studying the planets that orbit
them. New research into stars with Kepler data also is furthering other
areas of astronomy, such as the history of our Milky Way Galaxy and the
beginning stages of supernovae. The data from the extended mission were
also made available to the public and scientific community immediately,
allowing discoveries to be made at an incredible pace and setting a high bar
for other missions. Scientists are expected to spend a decade or more in
search of new discoveries in the treasure trove of data Kepler is providing.
DAWN MISSION TO ASTEROID BELT COMES TO END
NASA
The Dawn spacecraft has gone silent, ending a historic mission that studied
time capsules from the Solar System's earliest chapter. Dawn has missed
scheduled communications sessions with NASA's Deep Space Network, and
mission managers concluded that the spacecraft finally ran out of hydrazine,
the fuel that enables the spacecraft to control its pointing. Dawn can no
longer keep its antennae trained on the Earth to communicate with mission
control or turn its solar panels to the Sun to recharge. The spacecraft was
launched 11 years ago to visit the two largest objects in the main asteroid
belt. Currently, it is in orbit around the dwarf planet Ceres, where it
will remain for decades. Propelled by ion engines, the spacecraft achieved
many firsts along the way. In 2011, when Dawn arrived at Vesta, the second
largest asteroid in the main belt, the spacecraft became the first to orbit
a body in the region between Mars and Jupiter.
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