The SOCIETY for POPULAR ASTRONOMY
Electronic News Bulletin No. 350 2013 April 7
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SATURN'S MOONS AND RINGS DATE BACK TO BIRTH OF SOLAR SYSTEM
NASA
Analysis of data from the Cassini spacecraft suggests that Saturn's
moons and rings date from the time of the Solar System's birth.
Though they are tinted on the surface from recent 'pollution', those
bodies formed out of the protoplanetary nebula, the cloud of material
still orbiting the Sun after its ignition as a star. Data from
Cassini's 'visual and infrared mapping spectrometer' (VIMS) have
revealed how water ice and also colours -- which are signs of complex
molecules -- are distributed throughout the Saturnian system. The
spectrometer's data in the visible part of the spectrum show that
colouring on the rings and moons generally is only skin-deep. Using
its infrared range, VIMS also detected abundant water ice -- too much
to have been deposited by comets or other recent means. The authors
deduce that the water ices must have formed around the time of the
origin of the Solar System, because Saturn orbits the Sun beyond the
so-called snow line. There, in the outer Solar System where Saturn
resides, the environment is conducive to preserving water ice, like a
deep-freeze. Within the snow line, nearer to the Sun's warmth, ices
and other volatiles dissipate more easily.
The coloured patina on the ring particles and moons roughly
corresponds to their location in the Saturn system. For Saturn's
inner ring particles and moons, water-ice spray from the geyser-moon
Enceladus has a whitewashing effect. Farther out, the scientists
found that the surfaces of Saturn's moons generally were redder the
further they orbited from Saturn. Phoebe, one of Saturn's outer moons
and an object thought to have originated in the far-off Kuiper Belt,
seems to be shedding reddish dust that eventually rouges the surface
of the nearby moons Hyperion and Iapetus. A rain of meteoroids from
outside the system appears to have turned some parts of the main ring
system -- notably the B ring -- a subtle reddish hue. Some scientists
think that the reddish colour could be oxidized iron or polycyclic
aromatic hydrocarbons. One of the big surprises from this research
was the similar reddish colouring of the potato-shaped moon Prometheus
and nearby ring particles. Other moons in the area are more whitish.
The similar reddish tint may suggest that Prometheus is constructed
from material in Saturn's rings. Scientists had been wondering
whether ring particles could have stuck together to form moons --
since the dominant theory was that the rings basically came from
satellites being broken up. The colouring might suggest that it can
work the other way round, too. Observing the rings and moons with
Cassini gives us a view of the intricate processes at work in the
Saturn system, and perhaps in the evolution of planetary systems more
generally.
CLOSEST STAR SYSTEM FOUND IN ALMOST A CENTURY
Penn State University
A pair of newly discovered stars is the third-closest known star
system to the Sun, the closest one discovered for nearly 100 years.
Both stars in the new binary system are brown dwarfs, 'failed' stars
which never became hot enough to ignite hydrogen fusion, because their
masses are too small. As a result, they are very cool and dim,
resembling a giant planet like Jupiter more than a bright star like
the Sun. Their distance is about 2 parsecs or 6.5 light-years. The
system is named WISE J104915.57-531906 because it was discovered in
a map of the entire sky obtained by the Wide-field Infrared Survey
Explorer (WISE) satellite. (The numbers are its RA and Dec, and the
J implies that they are for epoch 2000.) It is only slightly further
away than the second-closest star, Barnard's star, which was
discovered 6.0 light-years from the Sun in 1916. The closest system
of all consists of the double star Alpha Centauri, found to be a
neighbour of the Sun in 1839 at 4.4 light-years, and its faint and
distant companion Proxima Centauri, discovered in 1917 at 4.2
light-years.
To discover the new star system, astronomers studied the images of the
sky that the WISE satellite had obtained during a 13-month period
ending in 2011. During its mission, WISE observed each point in the
sky 2 or 3 times. In the time-lapse images, they were able to tell
that this system was moving very quickly across the sky, suggesting
that it was probably very close to us (in the astronomical sense, of
course!). By identifying it in older sky surveys, astronomers found
it possible to estimate its parallax (the apparent shift of a star in
the sky as the Earth moves round the Sun). Then the Gemini South
telescope in Chile was used to obtain a spectrum of it, which
demonstrated that it is very cool, a brown dwarf. As an unexpected
bonus, the images from Gemini also revealed that it is actually not
just a single object but is a pair of brown dwarfs orbiting each
other.
VIOLENT PAST FOR MILKY WAY
RAS
The black hole at the centre of the Milky Way is dormant, and existing
stars are peacefully circling. Although conditions are favourable,
there doesn't even seem to be much new star-formation going on. Now a
team of astronomers suggests how a single event -- a violent collision
and merger between the Galactic black hole and an intermediate-sized
black hole in a putative small satellite galaxy that used to circle
the Milky Way -- could have produced the features that point to a more
violent past for the Galactic core. The most dramatic of the clues
are the Fermi bubbles, lobes of high-energy radiation caused by
particles moving at nearly the speed of light, extending some 10
kiloparsecs (30,000 light-years) above and below the Milky Way centre.
If they were glowing in visible light they would fill about half the
night sky, but they radiate X-ray and gamma-ray light, so you would
need X-ray vision to see them. Another puzzling characteristic of the
Galactic Centre is the fact that it contains the three most massive
clusters of young stars known in the entire Galaxy. The Central,
Arches and Quintuplet clusters each contain hundreds of young, hot
stars that are much larger and very much brighter than the Sun. Such
stars typically burn out in a few million years because of their
extreme brightness, so there has to have been a relatively recent
burst of star formation right near the Galactic Centre.
The supermassive black hole at the centre of the Milky Way is about
four million solar masses and is roughly 40 light-seconds in diameter,
only nine times the size of the Sun. Such an object produces intense
gravitational tides. So astronomers were surprised to discover a
number of clumps of bright new stars within one parsec (three
light-years) of the black hole. It would not be surprising if the
disruptive tidal forces prevented stars from forming near the hole,
but the indications are that they did form in their present place.
For that to happen, the clouds of dust and gas from which they formed
must have been exceptionally dense, perhaps 10,000 times denser than
the other molecular clouds in the Galactic Centre. While there is an
excess of young hot stars in that vicinity, there is also a surprising
dearth of older stars. Theoretical models would like the density of
old stars to increase closer to the black hole, but in fact there are
very few old stars found within several light-years of it. The
proposed scenario begins about 13 billion years ago, when the path of
one of the smaller satellite galaxies orbiting the Milky Way was
diverted so that it began drifting inwards towards the core.
According to recent speculation, such a thing may have happened dozens
of times in the lifetime of the Milky Way. As the satellite galaxy (a
collection of stars and gas with an intermediate-sized black hole of
its own with a mass of about 10,000 Suns) spiralled in, most of its
mass was gradually stripped away, finally leaving the black hole and a
handful of gravitationally bound stars.
About 10 million years ago, the stripped-down core of the satellite
galaxy finally reached the Galactic Centre. When two black holes
merge, they first go through an elaborate dance, so the smaller one
would have circled the Galactic black hole for several million years
before it was ultimately consumed. As it circled closer and closer,
it would have churned up the dust and gas in the vicinity and pushed
enough material into the Galactic hole in the process to produce the
Fermi bubbles. The violent gravitational tides produced by the
process could easily have compressed the molecular clouds in the core
to the super densities required to produce the young stars that are
now located dangerously close in. In addition, the vigorous churning
would have swept out the existing stars from the area surrounding the
massive central black hole. In fact, the astronomers' model predicts
that the black holes' merger dance should have flung a large number of
the missing old stars out into the Galaxy at high velocities, thus
explaining the absence of old stars immediately around the hole. The
gravitational pull of the satellite galaxy's hole could have thrown
nearly 1,000 stars out of the Galactic Centre. Those stars should
still be racing through space, about 10,000 light-years away from
their original orbits. It should be possible to detect them with
large surveys like the Sloan Digital Sky Survey because they would be
travelling at much higher velocities than stars that have not
undergone such an interaction. So the discovery of a large number of
stars racing outwards through the Galaxy would strongly support such a
picture of the Milky Way and satellite-galaxy merger. Unfortunately
for the speculation, however, no such discovery has been made so far.
ASTRONOMERS FIND 'LOST' SUPERNOVA
Harvard-Smithsonian Center for Astrophysics
Supernova explosions of massive stars are common in spiral galaxies
like the Milky Way, where new stars are forming all the time. They
are almost never seen in elliptical galaxies where star formation has
nearly ceased. So astronomers were surprised to find a young-looking
supernova in an old galaxy. Supernova PS1-12sk, discovered with the
Pan-STARRS telescope on Haleakala, is rare in more ways than one.
From the presence of helium and other features, PS1-12sk is classified
as a very rare 'Type Ibn' supernova -- only the sixth such example
found. Although the origin of that type of supernova is uncertain,
the most likely cause seems to be the explosion of a massive star that
previously ejected massive amounts of helium, much like Eta Carinae's
Homunculus Nebula. That origin is supported by the fact that the
five previous Type Ibn supernovae were all found in galaxies like the
Milky Way that are actively forming stars. (Since massive stars don't
live long, they can't stray far from where they are born before
exploding.) PS1-12sk is different. It was found on the outskirts of
a bright elliptical galaxy about 780 million light-years away. The
site of the explosion shows no signs of recent star-formation, and a
supernova from a massive star has never before been seen in a galaxy
of that type. The finding suggests that the dim and distant host
galaxy might be concealing a star factory, allowing it to form massive
stars where none was expected. But less improbably, PS1-12sk might
have an entirely different origin such as a collision of two white
dwarfs, one of which was helium-rich.
DISTANCE TO THE LARGE MAGELLANIC CLOUD
ESO
An international team of astronomers has measured the distance to our
neighbouring galaxy, the Large Magellanic Cloud, more accurately than
before. Astronomers survey the scale of the Universe by first
measuring the distances to 'close-by' objects and then using them as
standard candles to pin down distances farther and farther out into
the cosmos. But this chain is, at best, only as accurate as its
weakest link. Up to now finding an accurate distance to the Large
Magellanic Cloud (LMC), one of the nearest galaxies to the Milky Way,
has proved elusive. As stars in that galaxy are used to fix the
distance scale for more remote galaxies, it is crucially important.
But careful observations of a rare class of double star have now
allowed a team of astronomers to deduce a more precise value for the
LMC distance: 163,000 light-years.
The improvement in the measurement of the distance to the LMC also
gives better distances for many Cepheid variable stars. Those bright
pulsating stars are used as standard candles to measure distances out
to more remote galaxies and to determine the expansion rate of the
Universe -- the Hubble Constant. That in turn is the basis for
surveying the Universe out to the most distant galaxies that can be
seen with current telescopes. So the more accurate distance to the
LMC immediately reduces the inaccuracy in current measurements of
cosmological distances. The astronomers worked out the distance to
the LMC by observing eclipsing binary stars. By tracking their
changes in brightness and also measuring the stars' orbital speeds, it
is possible to determine the stars' sizes and masses, and their
orbits. Those quantities, in combination with measurements of the
total brightness and colours of the stars, provide remarkably accurate
distances. The method has been used before, but only with hot stars,
in whose cases certain assumptions that increase the possibility of
error have to be made. But now there have been identified eight
eclipsing binaries where both stars are cooler red giants. They yield
distances accurate to about 2%.
RARE TRIPLE QUASAR
RAS
A rare triple quasar system has been discovered. Systems with
multiple quasars are believed to be the product of galaxies colliding.
They are difficult to resolve because their angular separations are
very small. Observers found the triple quasar, called QQQ J1519+0627,
by combining observations from the ESO New Technology Telescope and
from Calar Alto Observatory in Spain. The light from the three
quasars has travelled 9 billion light-years to reach us -- their light
was emitted when the Universe was only a third of its current age.
No evidence was seen near them of any ultra-luminous infrared galaxy,
such as is often associated with quasars.
UNIVERSE OLDER THAN PREVIOUSLY THOUGHT
NASA
Planck is a satellite that was launched in 2009 and has been mapping
the cosmic microwave background, the afterglow of the theorized Big
Bang that started the Universe. That relic radiation provides a
snapshot of the Universe 370,000 years after the Big Bang. A map
based on the first 15 months of observations (seemingly now rather out
of date -- ED), reveals tiny temperature fluctuations in the cosmic
microwave background, ancient light that has travelled for billions of
years from the very early Universe to reach us. It suggests that the
Universe is expanding slightly more slowly than was thought, and is
13.8 billion years old, 100 million years older than previous
estimates. As that ancient light travels to us, matter acts like an
obstacle course, getting in its way and changing the patterns
slightly. The Planck map reveals not only the very young Universe,
but also matter, including dark matter, everywhere in the Universe.
The cosmic microwave background is remarkably uniform over the entire
sky, but tiny variations are suggested to indicate the imprints of
sound waves triggered by quantum fluctuations in the Universe just
moments after it was born. Those imprints, appearing as splotches in
the Planck map, are the seeds from which matter grew, forming stars
and galaxies.
PLANETARY SECTION REPORT
By Alan Clitherow, SPA Planet Section Director
Saturn has been an early-morning object over the last few months, but
is now becoming increasingly visible in the late-evening sky and in
late April will be observable all night long. It will then be in the
east-south-eastern sky as the Sun sets and will be due south at
midnight UT (1am BST). On the 28th the planet will be at opposition,
that is, 180° from the Sun (in celestial longitude). Saturn takes
about 30 years to complete a single orbit around the Sun, and (like
all the planets) as it travels approximately round the ecliptic its
maximum height above the horizon changes markedly over the orbital
period. This year Saturn is not very high above the horizon as seen
from the UK, around 25° at its highest point from mid-UK latitudes.
Nonetheless, when seeing conditions are good the planet will present a
wonderful sight. The rings are well presented to view; at opposition
they will be tilted some 18 degrees away from edge-on. Most
telescopes will show both the outer A ring and the brighter, inner B
ring. In good seeing, quite a small telescope such as a 3-inch
refractor will show the thin dark line, known as the Cassini Division,
separating the two rings. Larger instruments *may* allow you to
glimpse the tiny Encke Division a little way in from the outer edge of
the A ring, splitting that ring into uneven parts. Closer to the
planet, inside the B ring lies the ghostly C ring, sometimes called
the Crepe Ring; it was the astronomer William Lassell who gave it that
name, as it reminded him of a crepe veil in front of Saturn's
globe. The C ring may not be visible against the blackness of space,
viewed between the B ring and the limb of the planet, but it appears
as a thin grey veil dimming the pale yellow of the planet where it
passes in front of it.
If you have been observing Saturn you will have noticed the shadow of
the planet falling on the rings behind the globe. As we move towards
opposition the shadow will disappear, as it will be directly behind
the planet and out of our line of sight, but it will re-appear on the
opposite side shortly after opposition. An interesting effect, known
as the 'Opposition Surge' or 'Seeliger Effect', occurs for a few days
either side of opposition (and not just for Saturn). It consists of
an appreciable rise in the brightness of the Saturn system,
particularly the rings. When the Sun shines on any rough surface, or
(equivalently) on randomly distributed reflecting surfaces of any
sort, much of the parts of the surface(s) that you see are shadowed by
other parts, *except* when the Sun is directly behind your head, when
you are looking in exactly the same direction as that from where the
illumination is coming. *Then*, *everything* that you see is
illuminated. You do not need to look at Saturn to see the effect: it
can be seen very conspicuously as a sort of halo around the shadow of
your head if that shadow is cast on a rough surface. It can be quite
extreme when seen, for example, on a corn field when the Sun is low,
and is very often conspicuous when one is in an aircraft, when it is
seen around the shadow of that aircraft when it is flying over rough
country, even if the plane is so high that the shadow itself is
invisible - it is like a bright patch on the ground, moving along and
keeping up with the plane. When seen on the Earth, the effect is
often called the 'heiligenschein'. That, and many other interesting
natural sights to be seen by an alert (or alerted) observer, are
described in a book by the Dutch astronomer M. G. J. Minnaert; an
English translation appeared in a Dover reprint under the title
'Light and Colour in the Open Air'.
Any observations that might demonstrate and quantify the Seeliger
effect on Saturn would be very welcome.
Having dwelt (not literally!) on Saturn I must also say that Jupiter,
while fading in the western sky, is still reasonably well placed for
observation in April. The planet's atmosphere continues to change and
evolve, with a number of interesting features on view. While Saturn
is sitting on the border between the constellations of Virgo and
Libra, Jupiter is obvious, close by the horns of Taurus the Bull
shortly after sunset, and represents a good starting point for
planetary observation early in the evening.
SPA SOLAR SECTION MARCH 2013
By Richard Bailey, SPA Solar Section Director
Rotation Nos. 2134, 2135
Two to be or one to be, that is the question ! As mentioned last
month, Solar Cycle 24 was due to be at its Maximum about now, but the
Sun has not yet shown the expected activity. Some experts are
wondering whether this will be a double-header, with two of reduced
activity. Yet more poor weather across much of the UK, and reduced
solar observation. The worst periods were from the 6th to the 10th,
and the 21st to the 25th when observation chances were about zero.
This month's report will not therefore feature a weekly analysis. The
MDF and R figures are slightly up on last month's.
WHITE LIGHT From reports received, the NH was the busiest. No large
AR's were reported. From the 12th to the 19th the Sun was the most
active, and fortunately weather conditions allowed many observations
to be made. On the 16th 9 AR's were seen, 7 in the NH. Those in the
NH formed a line across the disc and three had good umbras and
penumbras. NH AR1695 was E of AR 1692 (in the centre of the disc) and
AR 1696 W of centre. Light bridging was seen to the umbra of 1695. On
the 31st SH AR 1689 returned at the E limb, to be numbered SH AR1711.
On the 7th, as SH AR 1689, it was a line of mixed small spots just in
from the E limb. On the 31st it had a large umbra and penumbra.
Faculae were seen in each observation.
MDF 4.56 R 58.22 H-ALPHA.
The MDF is up a bit on February's. No flares were seen, but varied
prominencies and filaments were on show in each observation. On the
11th a tall prominence stood on the E limb, and on the 30th and 31st
high ones off the S, N and SE limbs. A long filament curved eastwards
from NH AR 1704 on the 28th. and 29th.
MDF 6.40
The full March report will be on show, with pictures, on the Solar
link from the SPA Homepage.
Bulletin compiled by Clive Down
(c) 2013 the Society for Popular Astronomy
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Various Voluntary work-Litter Picking for Parish Council (Daytime) and also a friend of Kessingland Beach (Watchman)
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