MOONS OF SATURN YOUNGER THAN DINOSAURS
SETI
New research suggests that some of Saturn's icy moons, as well as its
famous rings, might be 'modern' adornments. Their dramatic birth may
have taken place a 'mere' hundred million years ago, more recent than
the reign of many dinosaurs. While Saturn's rings have been known
since the 1600s, there is still debate about their age. The straight-
forward assumption is that they are primordial -- as old as the planet
itself, which is more than four billion [US billion, = 10 to the 9]
years. However, in 2012, French astronomers found that tidal effects
-- the gravitational interaction of the inner moons with fluids deep
in Saturn's interior -- are causing them to spiral to larger orbital
radii comparatively quickly. The implication, given their present
positions, is that those moons, and presumably the rings, are
comparatively recent phenomena. Astronomers used computer modelling
to infer the past dynamic behaviour of Saturn's icy inner moons.
Whereas our own Moon has its orbit around the Earth to itself,
Saturn's many satellites have to share space with each other. All of
their orbits slowly grow owing to tidal effects, but at different
rates. That results in pairs of moons sometimes entering so-called
orbital resonances. Those occur when one moon's orbital period is a
simple fraction (for example, one-half or two-thirds) of that of
another. In those special configurations, even small moons with
weak gravity can strongly affect one another's orbits, making them
more eccentric and/or tilting them out of their original orbital
planes.
By comparing present orbital tilts and those predicted by computer
simulations, the researchers could learn how much the orbits of
Saturn's moons may have grown. It turned out that for some of the
most important satellites -- Tethys, Dione and Rhea -- the orbits are
less dramatically altered than was previously thought. The relatively
small orbital tilts indicate that they have not crossed many orbital
resonances, meaning that they must have formed not far from where they
are now. But how long ago was their birth? The team used results
from the Cassini mission to help answer that question. The Cassini
spacecraft has observed icy geysers on Saturn's moon Enceladus. On
the assumption that the energy powering the geysers comes directly
from tidal interactions, and that Enceladus' level of thermal activity
is more or less constant, then the tides within Saturn are quite
strong. According to the team's analysis, those tides would move the
satellite by the small amount indicated by the simulations in 'only'
about 100 million years. That would date the formation of the major
moons of Saturn, with the exception of more distant Titan and Iapetus,
to the relatively recent (in the history of the Earth) Cretaceous
period, the era of the dinosaurs. So the question arises, what caused
the rather recent birth of the inner moons? The best guess is that
Saturn had a similar collection of moons previously, but their orbits
were disturbed by a special kind of orbital resonance involving
Saturn's motion around the Sun. Eventually, the orbits of neighbouring
moons crossed, and the objects collided. From the rubble, the present
set of moons and rings formed. If that is correct, Saturn's rings may
be more recent than the heyday of the dinosaurs, and we are fortunate
to witness them today.
DUST COUNTER GOT FEW HITS ON PLUTO FLY-BY
University of Colorado at Boulder
A student-built instrument riding on the New Horizons spacecraft found
only a handful of dust grains, the building blocks of planets, when it
shot past Pluto at 31,000 mph last July. Data downloaded and analyzed
by the New Horizons team indicated that the space environment around
Pluto and its moons contained only about six dust particles per cubic
mile. Studying the microscopic dust grains can give researchers clues
about how the Solar System was formed billions of years ago and how it
works today, providing information on planets, moons and comets.
Unfortunately they can not use a microscope on dust that has smashed
onto New Horizons.
Launched in 2006, the New Horizons mission was designed to help
planetary scientists understand the icy worlds at the edge of the
Solar System, including Pluto and the Kuiper Belt. A vast region,
thought to span more than a (US) billion miles beyond Neptune's orbit,
the Kuiper Belt is believed to harbour at least 70,000 objects more
than 60 miles in diameter and contain samples of ancient material
created during the Solar System's violent formation some 4.5 billion
years ago. The dust counter logged thousands of dust-grain hits over
the spacecraft's nine-year journey to Pluto, while most of the other
six instruments slept. Now we are starting to see a slow but steady
increase in the impact rate of larger particles, possibly indicating
that New Horizons has already entered the inner edge of the Kuiper
Belt. The CU-Boulder dust counter is a thin film resting on a honey-
combed aluminium structure the size of a cake tin mounted on the
spacecraft's exterior. A small electronic box functions as the
instrument's 'brain' to assess each individual dust particle that
strikes the detector, allowing the students to infer the mass of each
particle. Images from closest approach were taken from roughly 7,700
miles above Pluto's surface. The spacecraft, about the size of a baby
grand piano, carries six other instruments. The next and final target
of New Horizons is a 30-mile-diameter Kuiper-Belt object named 2014
MU69, which the spacecraft is expected to pass in 2019 January.
STUDENTS MAP MILKY WAY WITH DWARF STARS
RAS
Two astronomy students from Leiden University have mapped the entire
Milky Way Galaxy in dwarf stars for the first time. They show that
there is a total of 58 (US) billion dwarf stars, of which seven per
cent reside in the outer regions of the Galaxy. The Milky Way, the
galaxy we live in, consists of a prominent, rather flat, disc with
relatively closely spaced bright stars, and a halo, a sphere of stars
with a much lower density, around it. Astronomers assume that the
halo is the remnant of the first galaxies that fused together to form
our Galaxy. To find out exactly what the Milky Way looks like,
astronomers have previously made maps using counts of the stars in the
night sky. The students used the same technique in their research.
Rather than studying bright stars, they used Hubble-telescope data
from 274 dwarf stars, which were serendipitously observed by the
orbiting observatory while it was looking for the most distant
galaxies in the early Universe. The particular type of star they
looked at were red dwarfs of spectral class M. Dwarf stars are
under-sized and often have too low a mass to burn hydrogen. As warm,
rather than hot, objects, they are best viewed with near-infrared
cameras. Astronomers believe that there are very many such stars.
That makes them quite suitable for mapping the Galaxy, even though
they are so hard to find.
To find the distribution of the M dwarfs, the students used three
density models that astronomers use to describe the flat disc and
halo, both separately and combined. To calculate which model best
describes the structure of the Milky Way, the students then applied
the 'Markov Chain Monte Carlo' method. That works by letting a
computer programme test all possible values of each parameter of the
model. It then fixes the value which corresponds best with the data.
The model that includes both disc and halo was the best match. From
the positions of the 274 known M dwarfs in their sample, the students
inferred the existence of 58 billion dwarf stars! They were also able
to estimate the number of dwarfs in the halo, calculating a fraction
of 7 per cent, higher than astronomers have previously found for the
whole Milky Way. The results are important for future research with
ESA's proposed 'Euclid' space telescope, intended to be launched in
2020. Like Hubble, Euclid will image the whole sky in near-infrared.
HUBBLE UNVEILS MONSTER STARS
RAS
Astronomers using the ultraviolet capabilities of the Hubble telescope
have have identified dozens of stars exceeding 50 solar masses and
nine monster stars with masses over 100 times the mass of the Sun in
the star cluster R136. That makes it the largest sample of very
massive stars identified to date. The international team used two
Hubble instruments -- the Wide-Field Camera 3 (WFC3) and the Space
Telescope Imaging Spectrograph (STIS) -- to observe the young star
cluster R136 in ultraviolet light for the first time. R136 is only a
few light-years across and is located in the Tarantula Nebula within
the Large Magellanic Cloud, about 170,000 light-years away. The young
cluster hosts many extremely massive, hot and luminous stars whose
energy is mostly radiated in the ultraviolet, which is why the
scientists looked at the ultraviolet emission of the cluster.
However, the current record-holder R136a1 does keep its place as the
most massive star known in the Universe, at over 250 solar masses.
The detected stars are not only extremely massive, but also extremely
bright. Together the nine most-massive stars outshine the Sun by a
factor of 30 million. The scientists were also able to investigate
outflows from those behemoths, which are most readily studied in the
ultraviolet. They eject up to an Earth mass of material per month at
a speed approaching one per cent of the speed of light, resulting in
extreme mass loss throughout their brief lives.
In 2010 astronomers showed the existence within R136 of four stars
each with over 150 times the mass of the Sun. At that time the
extreme properties of those stars came as a surprise, as they exceeded
the upper mass limit that was generally accepted for stars at that
time. Now, the new census has shown that there are five more stars
with more than 100 solar masses in R136. The results gathered from
R136 and from other clusters also raise many new questions about the
formation of massive stars, as the origin of such behemoths remains
unclear. There have been suggestions that they result from the merger
of less-extreme stars in close binary systems. From what we know
about the frequency of massive mergers, such a scenario can not
account for all the really massive stars that we see in R136, so it
would appear that such stars can originate directly from the star-
formation process. The team is continuing to work on the problems
connected with the massive stars and their origins. Analysis of new
optical STIS observations will also allow it to search for close
binary systems in R136, which could produce massive black-hole
binaries which would ultimately merge, producing gravitational waves.
SUPERNOVAE CAUGHT IN ACT OF EXPLODING
University of Notre Dame
Two supernovae have been caught in the act of exploding by an
international team of astrophysicists. Stars 10 to 20 times the mass
of our Sun often puff up to supergiants before ending their 'lives'
as supernovae. Such stars are so large that the Earth's orbit would
easily fit inside them. Using the Kepler space telescope, the team
spent three years observing 50 (US) trillion stars for the chance to
watch as supersonic shockwaves reached their surfaces after explosions
deep in the core. For the first time, a 'shock breakout' in an
exploding supergiant star was observed at visible wavelengths. The
flash from a breakout should last about an hour, so astronomers have
to be very lucky or continuously stare at millions of stars just to
catch one flash. In 2011, two such massive red supergiants exploded
while in Kepler's view. The first, KSN 2011a, is nearly 300 times the
size of the Sun and 'only' 700 million light-years away. The second,
KSN 2011d, is roughly 500 times the size of the Sun and some 1200
million light-years away. Supernovae like them -- known as Type II --
begin when the internal furnace of a star runs out of nuclear fuel,
causing its core to collapse as gravity takes over. Understanding the
physics of such explosions allows scientists to understand better how
the seeds of chemical complexity and life itself have been scattered
in space and time in the Milky Way galaxy. The Kepler telescope is
famous for its discoveries of extra-solar planets, some of which may
have the right conditions to harbour life. But Kepler can also look
at galaxies beyond the Milky Way. A team of astrophysicists from
Notre Dame, Maryland, Berkeley and Australia have formed the 'Kepler
ExtraGalactic Survey', or KEGS, specifically to apply the power of
Kepler to the study of galaxies and supernovae.
MOST 'OUTRAGEOUSLY' LUMINOUS GALAXIES
University of Massachusetts at Amherst
Astronomers have observed the most luminous galaxies ever seen in the
Universe, objects so bright that established descriptors such as
'ultra-' and 'hyper-'luminous, used to describe the previously
brightest-known galaxies don't even come close. The researchers have
taken to calling them 'outrageously luminous' because there is no
more-scientific term to apply. The research group uses the 50-m
'Large Millimeter Telescope' (LMT), the largest, most sensitive
single-aperture instrument in the world for studying star formation.
It is located on the summit of Sierra Negra, a 15,000-foot extinct
volcano in the central state of Puebla, a subsidiary summit to
Citlaltepetl, Mexico's highest mountain. The team also used the
latest generation of satellite telescope and a cosmology experiment on
the Planck satellite that detects the glow of the Big Bang and
microwave background for this work. It estimates that the newly
observed galaxies are about 10 (US) billion years old and were formed
only about 4 billion years after the Big Bang. In categorizing
luminous sources, astronomers call an infrared galaxy 'ultra-luminous'
when it has a rating of about 1 (US) trillion (10 to the power 12)
solar luminosities, and that rises to about 10 trillion solar
luminosities at the 'hyper-luminous' level. The newly discovered
galaxies were not predicted by theory to exist; they are too big and
too bright, so no one really looked for them before. Discovering them
may help astronomers to understand more about the early Universe.
Knowing that they really do exist and how much they have grown in the
first 4 billion years since the Big Bang helps astronomers to estimate
how much material was there for them to work with. Their existence
teaches us about the process of collecting matter and of galaxy
formation. They suggest that that process was more complex than many
people thought. The newly observed galaxies are not as large as they
appear, the researchers point out. Follow-up studies suggest that
their extreme brightness arises from a phenomenon called gravitational
lensing that magnifies light passing near massive objects, as
predicted by Einstein's general relativity. As a result, from the
Earth they look about ten times brighter than they really are.
Gravitational lensing of a distant galaxy by another galaxy is quite
rare, so finding as many as eight potential lensed objects as part of
this investigation is another potentially important discovery. The
team also reckoned that the galaxies' brightness is most likely due
solely to their amazingly high rate of star formation. The Milky Way
produces a few solar masses of stars per year, whereas the objects of
interest here appear to be forming stars at a rate of about one an
hour. We still do not know how many tens to hundreds of solar masses
of gas can be converted into stars so efficiently in these objects.
For this work, the team used data from powerful international
facilities -- Planck, Herschel and the LMT. The all-sky coverage of
Planck is the only way to find these rare but exceptional objects, but
the much higher resolutions of Herschel and the LMT are needed to
pinpoint their exact locations.
Topic: Early April Astronomy Bulletin (Read 2141 times)