ASTEROID BUZZES EARTH, HEADS FOR MOON
Spaceweather.com
Newly discovered asteroid 2016 RB1 flew past the Earth on Sept. 7,
only 25,000 miles above the South Pole. Because of the asteroid's
southern trajectory, it did not pass within the orbits of any
communication or weather satellites -- but it was close. After it
passed by the Earth, the space rock turned and headed for the Moon,
executing a wider fly-by at 179,000 miles on Sept. 8. Researchers
say that 2016 RB1 is about 50 feet in diameter. Astronomers used a
remotely-controlled telescope at Siding Spring, in Australia, to
photograph the asteroid as it was approaching the Earth on Sept. 7.
It looked like a 12th-magnitude star. The asteroid spends all of its
time in the inner Solar System. In 2017 October it will fly by Venus,
and will do so again in 2020 March before it returns towards the Earth
in June of that year. None of those encounters is expected to result
in an impact.
Asteroid 2016 RB1 was discovered on Sept. 5 by astronomers using the
60-inch Cassegrain reflector of the Catalina Sky Survey, on Mount
Lemmon in the Catalina Mountains north of Tucson, Arizona.
ASTRONOMERS OBSERVE DISINTEGRATING COMET
University of California, Los Angeles
Astronomers using the Hubble telescope have obtained detailed
observations of a comet breaking apart 67 million miles from the
Earth. In a series of images taken over three days in 2016 January,
Hubble showed 25 fragments consisting of a mixture of ice and dust
that are drifting away from the comet at a rate similar to walking
speed. The images suggest that the roughly 4.5-billion-year-old
comet, named 332P/Ikeya-Murakami, or comet 332P, may be spinning so
fast that material is ejected from its surface. The resulting debris
are now scattered along a 3,000-mile-long trail. The observations
provide insight into the volatile behaviour of comets as they approach
the Sun and begin to vaporize. The three-day observations show that
the comet shards brighten and dim as icy patches on their surfaces
rotate into and out of sunlight. Their shapes change, too, as they
break apart. The icy relics comprise about 4% of the parent comet and
range in size from about 20 to 60 metres. The Hubble images show that
the parent comet changes brightness frequently, completing a rotation
every two to four hours. The comet is much smaller than astronomers
originally thought, measuring only about 500 m across. Comet 332P was
discovered in 2010 November by two Japanese amateur astronomers, after
it surged in brightness. From the Hubble data, the research team
suggests that sunlight heated the surface of the comet, causing it to
expel jets of dust and gas. The jets act like rocket engines,
spinning up the comet's rotation. The faster spin rate loosened
chunks of material, which are drifting off into space. The research
team calculated that the comet probably shed material over a period of
months, between October and December last year. The researchers
estimate that comet 332P has enough mass for 25 more outbursts. If
the comet has an episode every six years, the equivalent of one orbit
around the Sun, then it will be gone in 150 years.
PLUTO 'PAINTS' ITS LARGEST MOON RED
NASA
In 2015 June, when the cameras on the approaching New Horizons
spacecraft first observed the large reddish polar region on Pluto's
largest moon, Charon, mission scientists had never seen anything like
it elsewhere in the Solar System, and they couldn't wait to get the
story behind it. Over the past year, after analyzing the images and
other data that New Horizons has sent back from its historic 2015 July
flight through the Pluto system, the scientists think they understand
why the polar region on Pluto's largest moon, Charon, is red. The
colouring comes from Pluto itself -- as methane gas that escapes from
Pluto's atmosphere and becomes 'trapped' by the moon's gravity and
freezes to the cold, icy surface at Charon's pole. That is followed
by chemical processing by ultraviolet light from the Sun that
transforms the methane into heavier hydrocarbons and eventually into
reddish organic materials called tholins. The team combined analyses
from detailed Charon images obtained by New Horizons with computer
models of how ice evolves at Charon's poles. Mission scientists had
previously speculated that methane from Pluto's atmosphere was trapped
at Charon's north pole and slowly converted into the reddish material,
but had no models to support that theory. The New Horizons team tried
to determine whether conditions on the Charon (which has a diameter of
1,212 km) could allow the capture and processing of methane gas. The
models using Pluto and Charon's 248-year orbit around the Sun show
some extreme weather at Charon's poles, where 100 years of continuous
sunlight alternate with a century of continuous darkness. Surface
temperatures during those long winters dip to -257 C, cold enough to
freeze methane solid.
The methane molecules bounce around on Charon's surface until they
either escape back into space or land on the cold pole, where they
freeze solid, forming a thin coating of methane ice that lasts until
sunlight comes back in the spring. But while the methane ice quickly
sublimates away, the heavier hydrocarbons created from it remain on
the surface. Sunlight further irradiates those leftovers into reddish
material -- called tholins -- that has slowly accumulated on Charon's
poles over millions of years. New Horizons' observations of Charon's
other pole, currently in winter darkness -- and seen by New Horizons
only by light reflected from Pluto, or 'Pluto-shine' -- confirmed
that the same activity was occurring at both poles. This study solves
one of the greatest mysteries astronomers found on Charon, Pluto's
giant moon, and opens up the possibility that other small planets in
the Kuiper Belt with moons may create similar, or even more extensive,
'atmospheric transfer' features on their moons.
ASTRONOMERS OBSERVE STAR REBORN IN A FLASH
RAS
Using the Hubble telescope, astronomers have been able to study
stellar evolution in real time. Over a period of 30 years dramatic
increases in the temperature of the star SAO 244567 have been
observed. Now the star is cooling again, having been reborn into an
earlier phase of stellar evolution. That makes it the first reborn
star to have been observed during both the heating and cooling stages
of rebirth. Even though the Universe is constantly changing, most
processes are too slow to be observed within a human life-span. The
new work shows an exception to that rule. SAO 244567 is one of the
rare examples of a star that allows us to witness stellar evolution in
real time. Over only twenty years the star has doubled its
temperature, and it was possible to watch the star ionizing its
previously ejected envelope, which is now known as the Stingray
Nebula. SAO 244567, 2,700 light-years from the Earth, is the central
star of the Stingray Nebula, and has been visibly evolving between
observations made over the last 45 years. Between 1971 and 2002 the
surface temperature of the star skyrocketed by almost 40,000°C. Now
new observations made with the Cosmic Origins spectrograph on the
Hubble telescope have revealed that SAO 244567 has started to cool and
expand. That is unusual, though not unheard-of, and the rapid heating
could easily be explained if one assumed that SAO 244567 had an
initial mass of 3 to 4 times the mass of the Sun. However, the data
show that SAO 244567 must have had an original mass similar to that of
our Sun. Such low-mass stars usually evolve on much longer time-
scales, so the rapid heating has been a mystery for decades.
In 2014 the team proposed a theory that resolved the issue of both SAO
244567's rapid increase in temperature as well as the low mass of the
star. They suggested that the heating was due to what is known as a
helium shell-flash event: a brief ignition of helium outside the
stellar core. That theory has very clear implications for SAO
244567's future: if it has indeed experienced such a flash, then it
would force the central star to begin to expand and cool again --- it
would return back to the previous phase of its evolution. That is
exactly what the new observations confirm. The release of nuclear
energy by the flash forces the already very compact star to expand
back to giant dimensions --- the born-again scenario. It is not the
only example of such a star, but it is the first time that a star has
been observed during both the heating and cooling stages of such a
transformation. Astronomers need refined calculations to explain some
still mysterious details in the behaviour of SAO 244567. Those could
not only help them to understand the star itself better but could also
provide a deeper insight into the evolution of the central stars of
planetary nebulae.
HUNDREDS OF UNDISCOVERED BLACK HOLES
University of Surrey
New research has shone light on a globular cluster of stars that could
host several hundred black holes, a phenomenon that until recently was
thought impossible. Globular clusters are spherical collections of
stars which orbit around a galactic centre such as that of our Milky-
Way galaxy. Using computer simulations, astronomers were able to see
the un-seeable by mapping the globular cluster NGC 6101, from which
the existence of black holes within the system was deduced. The black
holes are a few times larger than the Sun, and form in the gravita-
tional collapse of massive stars at the end of their lives. It was
previously thought that such black holes would almost all be expelled
from their parent cluster owing to the effects of supernova explo-
sions, during the death of a star. Black holes are impossible to see
with a telescope, because no photons can escape. In order to find
them astronomers look for their gravitational effect on their
surroundings. Using observations and simulations they are able to
spot the distinctive clues to their whereabouts and therefore
effectively 'see' the un-seeable. It is only as recently as 2013 that
astrophysicists found individual black holes in globular clusters
through rare phenomena in which a companion star donates material to
the black hole. That work has shown that in NGC 6101 there could be
several hundred black holes, overturning old theories as to how black
holes form.
The work is intended to help answer fundamental questions related to
dynamics of stars and black holes, and the recently observed
gravitational waves. Those are emitted when two black holes merge,
and if the interpretation is right, the cores of some globular
clusters may be where black-hole mergers take place. The researchers
chose to map that particular ancient globular cluster because of its
recently-found distinctive make-up, which suggested that it could be
different from other clusters. Compared with other globular clusters,
NGC 6101 appears dynamically young, in contrast to the ages of the
individual stars. Also the cluster appears inflated, with the core
being under-populated by observable stars. Using computer simulation,
the team recreated every individual star and black hole in the cluster
and their behaviour. Over the whole lifetime of thirteen billion
years the simulation demonstrated how NGC 6101 has evolved. It was
possible to see the effects of large numbers of black holes on the
visible stars, and to reproduce what was observed in NGC 6101. From
that, the researchers showed that the otherwise inexplicable dynamical
apparent youth is an effect of the large black-hole population. The
results show that globular clusters like NGC 6101, which were always
considered boring, are in fact the most interesting ones, possibly
each harbouring hundreds of black holes.
RARE FOSSIL RELIC OF MILKY WAY
ESO
Astronomers have found a fossilized remnant of the early Milky Way
harbouring stars of hugely different ages. That stellar system
resembles a globular cluster, but is like no other cluster known.
It contains stars remarkably similar to the most ancient stars in the
Milky Way, and bridges the gap in understanding between our Galaxy's
past and its present. Terzan 5, 19,000 light-years distant in the
constellation Sagittarius (the direction of the Galactic Centre), has
been classified as a globular cluster for the 40-odd years since its
detection. Now, an Italian-led team has discovered that Terzan 5 is
like no other globular cluster known. The team scoured data from the
'Multi-conjugate Adaptive Optics Demonstrator' installed on the VLT,
as well as from a suite of other ground-based and space telescopes.
They found compelling evidence that there are two distinct kinds of
stars in Terzan 5 which not only differ in the elements they contain,
but have an age gap of roughly 7 billion years. The ages of the
two populations indicate that the star-formation process in Terzan 5
was not continuous, but was dominated by two distinct bursts. That
requires the Terzan 5 ancestor to have large amounts of gas for a
second generation of stars and to have a mass at least 100 million
times the mass of the Sun. Its unusual properties make Terzan 5 the
ideal candidate for a fossil from the early days of the Milky Way.
Current theories on galaxy formation assume that vast clumps of gas
and stars interacted to form the primordial bulge of the Milky Way,
merging and dissolving in the process. Some remnants of such gaseous
clumps could remain relatively undisrupted and keep existing, embedded
within the Galaxy. Such galactic fossils allow astronomers to
reconstruct an important piece of the history of our Milky Way.
While the properties of Terzan 5 are uncommon for a globular cluster,
they are very similar to those of the stellar population which can be
found in the Galactic bulge, the tightly packed central region of the
Milky Way. Those similarities could make Terzan 5 a fossilized relic
of galaxy formation, representing one of the earliest building blocks
of the Milky Way. That idea is strengthened by the original mass of
Terzan 5 necessary to create two stellar populations -- a mass similar
to those of the huge clumps which are assumed to have formed the bulge
during galaxy assembly around 12 billion years ago. Somehow Terzan 5
has managed to survive being disrupted for billions of years, and has
been preserved as a remnant of the distant past of the Milky Way.
Some characteristics of Terzan 5 resemble those detected in the giant
clumps we see in star-forming galaxies at high redshift, suggesting
that similar assembling processes occurred in the local and in the
distant Universe at the epoch of galaxy formation. Hence, this
discovery paves the way for a better and more complete understanding
of galaxy assembly. Terzan 5 could represent an intriguing link
between the local and the distant Universe, a surviving witness of the
Galactic-bulge assembly process.
BLACK HOLE CHANGES BEHAVIOUR
ESO
The mystery of a rare change in the behaviour of a super-massive
black hole at the centre of a distant galaxy has been solved by an
international team of astronomers using the VLT along with the Hubble
telescope and the Chandra X-ray observatory. It seems that the black
hole has fallen on hard times and is no longer being fed enough fuel
to make its surroundings shine. Many galaxies are found to have
extremely bright cores powered by supermassive black holes. The cores
make 'active galaxies' some of the brightest objects in the Universe.
They are thought to shine so brightly because hot material is glowing
fiercely as it falls into the black hole, a process known as
accretion. The brilliant light can vary hugely between different
active galaxies, so astronomers classify them into several types on
the basis of the properties of the light they emit. Some of such
galaxies have been observed to change dramatically over the course of
only 10 years --- a blink of an eye in astronomical terms. However,
the active galaxy in the new study, Markarian 1018, stands out by
having changed type a second time, reverting back to its initial
classification within the last five years. A handful of galaxies has
been observed to make such a full-cycle change, but never before has
one been studied in such detail. The discovery of Markarian 1018's
fickle nature was a chance by-product of the Close AGN Reference
Survey (CARS), a collaborative project to gather information on 40
nearby galaxies with active cores. Routine observations of Markarian
1018 with the Multi-Unit Spectroscopic Explorer (MUSE) on the VLT
revealed the surprising changes in the light output of the galaxy.
The chance observation of the galaxy so soon after it began to fade
was an unexpected opportunity to learn what makes such galaxies tick.
Astronomers were lucky to have detected the event just 3-4 years after
the decline started, so they could begin monitoring campaigns to study
details of the accretion physics of active galaxies that cannot be
studied otherwise. The research team made the most of that
opportunity, making it their first priority to pinpoint the process causing
Markarian 1018's brightness to change so wildly. That could have been
caused by any one of a number of astrophysical events, but they could
rule out the black hole pulling in and consuming a single star and
cast doubt on the possibility of obscuration by intervening gas.
But the true mechanism responsible for Markarian 1018's surprising
variation remained unsolved after the first round of observations.
However, the team was able to gather extra data after being awarded
observing time to use the Hubble telescope and the Chandra X-ray
observatory. With the new data it was able to solve the mystery ---
the black hole was slowly fading because it was being starved of
accretion material. It is possible that that starvation is because
the inflow of fuel is being disrupted, and an intriguing possibility
is that that could be due to interactions with a second supermassive
black hole. Such a black-hole binary system is a distinct possibility
in Markarian 1018, as the galaxy is the product of a major merger of
two galaxies, each of which probably contained a super-massive black
hole in its centre.
GAIA TELESCOPE PLOTS ONE BILLION STARS
BBC Science
Astronomers using the Gaia space telescope have released a first
tranche of data recording the positions and brightnesses of over a
billion stars. For some two million of those objects, their
distance and sideways motion across the heavens have also been
accurately plotted. Gaia's mapping effort is already unprecedented in
scale, but it still has several years to run. Remarkably, scientists
say the store of information even now is too big for them to sift,
and they are appealing for the public's help in making discoveries.
To give one example of the scope of Gaia, of the 1.1 billion light
sources in the data release, something like 400 million have never
been recorded in any previous catalogue. A web portal has been opened
where anyone can play with Gaia data and look for novel phenomena.
The European Space Agency launched the Gaia mission in 2013; its goal
was to update and extend the work of the Hipparcos satellite from the
1990s, which mapped the positions, brightnesses, distances and proper
motions of 100,000 stars. Gaia, with its first release of data, has
just increased that haul 20-fold. The new mission actually carries
two telescopes, which it scans across the Milky Way from a location
about 1.5 million km from the Earth. The telescopes' mirrors form an
image on a 1 billion-pixel camera detector connected to a trio of
instruments. The specification was to determine the brightest
objects' coordinates within an error of just seven micro-arcseconds.
In addition to their positions and proper motions, the stars are
having their physical properties analyzed by Gaia. Not all of that
information can be gleaned at once. It will take repeat viewing, but
by the end of five years of operations the 100,000 stars fully
profiled by Hipparcos should become at least a billion in the Gaia
catalogue.
If one thing is clear from the new data it is that Gaia is seeing many
more faint stars than anyone anticipated. Once the project is
complete it may have plotted 2-3 billion light sources. One
eagerly anticipated measurement is the radial velocity of stars, which
describe the movements they make towards or away from Gaia. If that
measurement is combined with the stars' proper motions, it will lay
bare the dynamics of the Milky Way. It should be possible, for
example, to make a kind of time-lapse movie --- to run forwards to see
how the Galaxy might evolve into the future, or to run backwards to
see how our cosmic neighbourhood came to be the shape it is today. At
the outset of the mission, scientists had hoped to get radial-velocity
data on about 150 million stars. But that was thrown into doubt when
it was realized, soon after Gaia's launch, that unexpected stray light
was getting into the telescope. That made the observation of the
faintest stars and their colours far more challenging. Engineers now
think that they understand the problem: in part it is caused by the
way sunlight diffracts past the 10-metre shade that Gaia uses to keep
its telescopes in shadow. The good news according to the scientists
is that they think they can work around the difficulties. The longer
the mission runs, they believe, the closer will Gaia get to its target
of 150 million radial-velocity measurements.
CASSINI BEGINS FINAL YEAR AT SATURN
NASA
After more than 12 years studying Saturn, its rings and moons, the
Cassini spacecraft has entered the final year of its voyage. The
conclusion of the historic scientific odyssey is planned for 2017
September, but not before the spacecraft completes a daring two-part
end-game. Beginning on November 30, Cassini's controllers will send
the spacecraft just past the outer edge of the main rings, starting a
series of 20 orbits, called the F-ring orbits. During those weekly
orbits, Cassini will approach to within 7,800 kilometres of the centre
of the narrow F ring, with its peculiar kinked and braided structure.
During the F-ring orbits astronomers expect to see the rings, along
with the small moons and other structures embedded in them, as never
before. Cassini's final phase -- called the Grand Finale -- begins
next April. A close fly-by of Saturn's large moon Titan will re-shape
the spacecraft's orbit so that it passes through the gap between
Saturn and the rings - an unexplored space only about 2,400 kilometres
wide. The spacecraft is expected to make 22 plunges through that gap.
During the Grand Finale, Cassini will make the closest-ever observa-
tions of Saturn, mapping the planet's magnetic and gravity fields and
returning ultra-close views of the atmosphere. Scientists also hope
to gain new insights into Saturn's interior structure, the precise
length of a Saturn day, and the total mass of the rings --- which may
finally help settle the question of their age. The spacecraft will
also make direct analyses of dust-sized particles in the main rings
and sample the outer reaches of Saturn's atmosphere --- both first-
time measurements for the mission. The Grand Finale will come to a
dramatic end on 2017 September 15, as Cassini dives into Saturn's
atmosphere, returning data about the planet's chemical composition
until its signal is lost. Friction with the atmosphere will cause
the spacecraft to burn up like a meteor soon afterwards.
Topic: Late September Astronomy Bulletin (Read 2065 times)