SATURN'S RINGS CONTAIN LESS MATTER THAN WAS THOUGHT
NASA
It seems intuitive that an opaque material should contain more stuff
than a more translucent substance. For example, muddier water has
more suspended particles of dirt in it than clearer water. Likewise,
you might think that, in the rings of Saturn, more-opaque areas must
contain a greater concentration of material than places where the
rings seem more transparent. But that intuition does not always
apply, according to a recent study of the rings, using data from the
Cassini mission. In their analysis, scientists found surprisingly
little correlation between how dense a ring might appear to be -- in
terms of its opacity and reflectiveness -- and the amount of material
it contains. The new results concern Saturn's B ring, the brightest
and most opaque of Saturn's rings, and are consistent with previous
studies that found similar results for Saturn's other main rings.
The scientists found that, while the opacity of the B ring varied by
a large amount across its width, the mass per unit area did not vary
much from place to place. They determined the mass density of the
nearly opaque centre of the B ring for the first time, in several
places, by analyzing spiral density waves. Those are fine-scale
ring features created by the gravity of Saturn's moons acting on ring
particles, and the planet's own gravity. The structure of each wave
depends directly on the amount of mass in the part of the rings where
the wave is located.
Research on the mass of Saturn's rings has important implications for
their age. A less-massive ring would evolve faster than a ring
containing more material, becoming darkened by dust from meteorites
and other cosmic sources more quickly. Thus, the less massive the
B ring is, the younger it might be -- perhaps a few hundred million
years instead of a few thousand million. By 'weighing' the core of
the B ring for the first time, the study makes a meaningful step in
the quest to determine the age and origin of Saturn's rings. While
all of the giant planets in the Solar System (Jupiter, Saturn, Uranus
and Neptune) have ring systems, Saturn's are clearly different in
scale from the others. Explaining why Saturn's rings are so bright
and vast is an important challenge in understanding their formation
and history. For scientists, the density of material packed into each
section of the rings is a critical factor in ascribing their formation
to a physical process. An earlier study by members of Cassini's
infrared-spectrometer team had already suggested the possibility that
there might be less material in the B ring than researchers had
thought. The new analysis is the first to determine the density of
mass in the ring and to demonstrate that that is the case.
FIRST DETECTION OF GASES AT SUPER-EARTH
RAS
The first successful detection of gases in the atmosphere of a super-
Earth reveals the presence of hydrogen and helium, but no water
vapour. The exotic exo-planet, 55 Cancri e, is over eight times the
mass of the Earth and has previously been dubbed the 'diamond planet'
because models based on its mass and radius have led some astronomers
to speculate that its interior is carbon-rich. Now, using new
processing techniques on data from the Hubble telescope, a UCL-led
team of European researchers has been able to examine the atmosphere
of 55 Cancri e in further detail. Super-Earths are thought to be the
most common planetary type in our galaxy and are so called because
they have masses larger than the Earth's but still much less than
those of the gas giants in the Solar System. The Wide-Field Camera 3
(WFC3) on Hubble has already been used to observe the atmosphere of
two super-Earths, but no spectral features were seen in those previous
studies. The planet is close to its star and has a 'year' that lasts
only 18 hours; temperatures on its surface are thought to reach around
2000C. It is located in a system around 55 Cancri, about 40 light-
years from us. Because 55 Cancri is such a bright star, the team was
able to use new analysis techniques to extract information about its
planetary companion.
Observations were made by scanning WFC3 quickly across the star to
create a number of spectra. By combining those observations, the
researchers were able to retrieve the spectral fingerprints of
55 Cancri e superimposed on the starlight. The result gives a first
insight into the atmosphere of a super-Earth and provides clues as to
what the planet is currently like, and how it might have formed and
evolved; that has important implications for 55 Cancri e and other
super-Earths. Intriguingly, the data also hinted at a signature for
hydrogen cyanide, a marker for carbon-rich atmospheres. Such an
amount of hydrogen cyanide would indicate an atmosphere with a high
ratio of carbon to oxygen.
PLANET FORMATION AROUND BINARY STAR
National Radio Astronomy Observatory
Using ALMA, astronomers have taken a new, detailed look at the very
early stages of planet formation around a binary star. Embedded in
the outer reaches of a double star's proto-planetary disc, the
researchers discovered a striking crescent-shaped region of dust that
is conspicuously devoid of gas. Astronomers struggle to understand
how planets can form in binary-star systems. Early models suggested
that the gravitational effects of the two stellar bodies would send
young planets into eccentric orbits, possibly ejecting them completely
from the system or else causing them to collide with their stars.
Observational evidence, however, shows that planets do indeed form and
maintain surprisingly stable orbits around double stars. In an effort
to understand how such systems form and evolve, astronomers using the
Atacama Large Millimeter/submillimeter Array (ALMA) took a new,
detailed look at the planet-forming disc around HD 142527, a binary
system about 450 light-years away in a cluster of young stars known as
the Scorpio-Centaurus Association. The HD 142527 system consists of a
main star a little more than twice the mass of the Sun and a smaller
companion star only about a third the mass of the Sun. They are
separated by approximately one billion miles, a little more
than the distance from the Sun to Saturn. Previous ALMA studies of
the system revealed surprising details about the structure of the
system's inner and outer discs. Planets form from the discs of dust
and gas that surround young stars. Small dust grains and pockets of
gas eventually come together under gravity, forming larger and larger
agglomerations and eventually asteroids and planets. The details of
that process, however, are not well understood. By studying a wide
range of proto-planetary discs with ALMA, astronomers hope to under-
stand better the conditions that set the stage for planet formation
across the Universe.
ALMA's new, high-resolution images of HD 142527 show a broad
elliptical ring around the double star. The disc begins remarkably
far from the central star -- about 50 times the Sun-Earth distance.
Most of it consists of gases, including carbon monoxide, but there is
a noticeable dearth of gases within a huge arc of dust that extends
nearly a third of the way around the star system. That crescent-
shaped dust cloud may be the result of gravitational forces unique to
binary stars and may also be the key to the formation of planets. Its
lack of free-floating gases may be the result of their freezing out
and forming a thin layer of ice on the dust grains. The temperature
is so low that the gas freezes and sticks to the grains. That process
is thought to increase the capacity for dust grains to stick together,
making it a strong catalyst for the formation of planetesimals, and
ultimately of planets.
GALACTIC-CENTRE GAMMA-RAYS
Princeton University
Bursts of gamma-rays from the centre of our Galaxy are not likely to
be signals of dark matter but rather other astrophysical phenomena
such as fast-rotating stars called millisecond pulsars, according to
two new studies, one from a team based at Princeton University and
the Massachusetts Institute of Technology and another based in the
Netherlands. Previous studies suggested that gamma-rays coming from
the dense region of space in the inner Milky Way galaxy could be
caused when invisible dark-matter particles collide. But using new
statistical analysis methods, the two research teams independently
found that the gamma-ray signals are not characteristic of those
expected from dark matter. The analysis suggests that what we are
seeing is evidence for a new astrophysical source of gamma-rays at the
centre of the Galaxy. That is a very complicated region of the sky,
and there are other astrophysical signals that could be confused with
dark-matter signals. The centre of the Milky Way galaxy is thought to
contain dark matter because it is home to a dense concentration of
mass, including dense star clusters and a black hole. A conclusive
finding of dark-matter collisions in the Galactic Centre would be a
step forward in our understanding of the Universe by helping
astronomers to understand the relationship between dark matter and
ordinary matter.
To tell whether the signals were from dark matter versus other
sources, the Princeton/MIT research team turned to image-processing
techniques. They looked at what the gamma-rays should look like if
they indeed come from the collision of hypothesized dark-matter
particles known as weakly-interacting massive particles, or WIMPs.
For the analysis, researchers studied images of gamma-rays captured by
the Fermi gamma-ray space telescope, which has been observing the rays
since 2008. Dark-matter particles are thought by some astronomers to
make up about 85% of the mass in the Universe, but they have never
been directly detected. The collision of two WIMPs, according to a
widely accepted model of dark matter, causes them to annihilate one
another to produce gamma-rays, which are the highest-energy form of
light. According to that model, the high-energy particles of light,
or photons, should be smoothly distributed among the pixels in the
images captured by the Fermi telescope. In contrast, other sources,
such as pulsars, release periodic bursts of light that show up as
isolated, bright pixels. The researchers applied their statistical
method of analysis to images collected by the Fermi telescope and
found that the distribution of photons was clumpy rather than smooth,
indicating that the gamma-rays were unlikely to be caused by dark-
matter particle collisions. The obvious next step, trying to
determine what the sources actually are, has yet to succeed.
HIDDEN GALAXIES DISCOVERED BEHIND MILKY WAY
International Centre for Radio Astronomy Research
Despite being 'just' 250 million light-years away -- rather 'close'
in inter-galactic terms -- the new galaxies had been hidden from view
until now by our own Galaxy, the Milky Way. Using the Parkes radio
telescope equipped with an innovative receiver, an international team
of scientists has been able to see past the stars and dust of the
Milky Way into a previously unexplored region of space. The discovery
may help to explain the 'Great Attractor' region, which appears to be
drawing the Milky Way and hundreds of thousands of other galaxies
towards it with a gravitational force equivalent to 10 to the 15 Suns.
The team found 883 galaxies, a third of which had never been seen
before. Scientists have been trying to understand the nature of the
mysterious Great Attractor since major deviations from universal
expansion were first discovered in the 1970s and 1980s. Astronomers
do not actually understand what is causing the apparent gravitational
acceleration of the Milky Way or where it's coming from. We know that
in that region there are a few very large collections of galaxies that
we call clusters or super-clusters, and our whole Milky Way is moving
towards them at more than 500 km/s. The research identified several
new structures that could help to explain the movement of the Milky
Way, including three galaxy concentrations (named NW1, NW2 and NW3)
and two new clusters (named CW1 and CW2). Astronomers have been
trying to map the galaxy distribution hidden behind the Milky Way for
decades. They have used a range of techniques, but only radio
observations have really succeeded in allowing us to see through the
thickest foreground layers of dust and stars. An average galaxy
contains 100,000 million stars, so finding hundreds of new galaxies
hidden behind the Milky Way points to a lot of mass that we did not
know about until now.
NGC 4889 HARBOURS SUPER-MASSIVE BLACK HOLE
ESA/Hubble Information Centre
The placid appearance of NGC 4889 can fool the unsuspecting
observer. But at the heart of that elliptical galaxy is one of the
most massive black holes ever discovered. Located about 300 million
light-years away in the Coma Cluster, the giant galaxy is the
brightest and largest galaxy in the cluster and harbours a black hole
21,000 million times the mass of the Sun. It has an event horizon --
the surface from within which even light cannot escape its
gravitational grasp -- with a diameter of approximately 130,000
million kilometres, about 15 times the diameter of Neptune's orbit
round the Sun. By comparison, the super-massive black hole at the
centre of our own Galaxy, the Milky Way, is believed to have a mass
about four million times that of the Sun and an event horizon just
one-fifth the diameter of the orbit of Mercury. But the time when
NGC 4889's black hole was swallowing stars and devouring dust is past.
Astronomers believe that the gigantic black hole has stopped feeding,
and is currently resting. The environment within the galaxy is now so
peaceful that stars are forming from its remaining gas and orbiting
undisturbed around the black hole. When it was active, NGC 4889's
super-massive black hole was fuelled by the process of hot accretion.
When galactic material -- such as gas, dust and other debris -- fell
inwards towards the black hole, it accumulated and formed an accretion
disc. Orbiting the hole, the spinning disc of material was
accelerated by the hole's immense gravitational pull and heated to
millions of degrees. The heated material also expelled gigantic and
very energetic jets. During the active period, astronomers would have
classified NGC 4889 as a quasar and the disc around the hole would
have emitted up to a thousand times the energy output of the Milky
Way. Although it is impossible to observe a black hole directly, its
mass can be indirectly determined. Using instruments on the Keck II
and Gemini North telescopes, astronomers measured the velocity of the
stars circulating around NGC 4889's centre. Those velocities -- which
depend on the mass of the object they orbit -- revealed the immense
mass of the black hole.
GLOBULAR CLUSTERS GAIN GAS FROM OUTSIDE THE CLUSTERS THEMSELVES
Northwestern University
Among the most striking objects in the sky are the dense swarms of
stars known as globular clusters. For a long time astronomers thought
that globular clusters formed their millions of stars in bulk at
around the same time, with each cluster's stars having very similar
ages. Recent discoveries of young stars in old globular clusters
have upset that tidy picture. Instead of having all their stellar
progeny at once, globular clusters can somehow bear second or even
third sets of thousands of sibling stars. Using observations by the
Hubble telescope, a research team has for the first time found within
globular clusters young populations of stars that have apparently
developed by courtesy of star-forming gas flowing in from outside
the clusters themselves. That process stands in contrast to the
conventional idea of the clusters' initial stars shedding gas as they
age to enable future rounds of star birth. Globular clusters are
spherical, densely packed groups of stars orbiting the outskirts of
galaxies. Our home Galaxy, the Milky Way, has several hundred of
them. Most of those local, massive clusters are quite old, however,
so the research team turned its attention to young and intermediate-
aged clusters found in two nearby dwarf galaxies, the Magellanic
Clouds. They used Hubble observations of the globular clusters
NGC 1783 and 1696 in the Large Magellanic Cloud, along with NGC 411
in the Small Cloud. Scientists routinely infer the ages of stars by
looking at their colours and brightnesses (their colour--magnitude
diagrams). Within NGC 1783, for example, astronomers identified an
initial population of stars aged 1400 million years, along with two
newer populations that formed 890 million and 450 million years ago.
What is the most straightforward explanation for those unexpectedly
differing stellar ages? Some globular clusters might retain enough
gas and dust to produce multiple generations of stars, but that seems
unlikely. Once the most massive stars form, they are like ticking
time bombs, with only about 10 million years left to go before they
explode in powerful supernovae that clear out any remaining gas and
dust. Afterwards, the lower-mass stars, which last longer and end in
less violent ways, may allow the cluster to build up gas and dust once
again. The research team proposes that globular clusters can sweep up
stray gas and dust that they encounter while moving about their
respective host galaxies. The theory of newborn stars arising in
clusters as they 'adopt' interstellar gases actually dates back to a
1952 paper. More than half a century later, that once-speculative
idea suddenly has key evidence to support it. In the study, the
researchers analyzed Hubble observations of the star clusters, and
then carried out calculations that show that that theoretical
explanation is actually possible in the globular clusters that the
team studied. Future studies will aim to extend the findings to other
globular clusters in the Magellanic Clouds as well as in the Milky Way.
SCIENTISTS GIVE UP ON PHILAE
CNN
Scientists from the German Aerospace Center (DLR) have given up hope
of establishing further contact with the comet lander, Philae,
currently perched on Comet 67P/Churyumov-Gerasimenko. The probability
of Philae re-establishing contact with the Lander Control Center is
almost zero, and they will no longer be sending any commands. While
it is sad, the announcement is perhaps only to be expected, given that
a last-ditch attempt to wake Philae ended in failure last month.
Ground control believes that the probe is probably ice-free, but most
likely covered in dust and unable to function owing to the extremely
cold environment. Comet 67P is now over 350 million kilometres away
from the Sun, where night temperatures can fall below -180 C -- far
colder than the -50 C limit at which it was designed to operate.
Philae was deployed from its mother ship Rosetta in 2014 November, and
despite a successful landing its position in the shade made powering
the onboard solar-driven batteries problematic. The probe last made
contact last July when it sent information to European engineers, who
were again unable to stabilize contact. It is believed that a failure
in the lander's transmitter is the cause of its irregular communica-
tions. Meanwhile, the Rosetta craft will continue to orbit the comet,
undertaking scientific experiments until September. Led by ESA in a
consortium that includes NASA, Rosetta followed Comet 67P around the
Solar System for a decade, in a quest to find answers about our own
origins in the Universe. Since bouncing on to the surface, Philae has
provided valuable scientific data including the dramatic discovery of
16 carbon- and nitrogen-rich organic compounds, supporting the theory
that the building blocks of life could have been brought to Earth by
comets.
Topic: Late February Astronomy Bulletin (Read 1824 times)