The SOCIETY for POPULAR ASTRONOMY
Electronic News Bulletin No. 358 2013 August 4
Here is the latest round-up of news from the Society for Popular
Astronomy. The SPA is Britain's liveliest astronomical society, with
members all over the world. We accept subscription payments online
at our secure site and can take credit and debit cards. You can join
or renew via a secure server or just see how much we have to offer by
SPITZER OBSERVES GAS EMISSION FROM COMET ISON
Astronomers using the Spitzer space telescope have observed what are
probaably strong carbon-dioxide emissions from Comet ISON, which is
due to pass through the inner Solar System later this year. Images
from Spitzer's infrared camera indicate that an estimated 1000 tons of
gas (probably carbon dioxide) and 500 tons of dust are being emitted
from the nucleus every day and forming a tail about 300,000 km long.
Comet ISON, which is thought to be less that 3 miles in diameter, was
about 312 million miles from the Sun when the observations were made.
Like other comets, ISON is probably a 'dirty snowball' made up of dust
and frozen gases such as water, ammonia, methane and carbon dioxide.
The comet is believed to be inbound on its first passage from the
distant 'Oort Cloud', a postulated collection of comets and comet-like
structures that is supposed to exist between one-tenth and 1 light-
year from the Sun. It will pass within 724,000 miles (less than one
solar diameter) of the Sun on Nov. 28. As it warms up as it gets
closer to the Sun, different gases that have existed on it in a frozen
state are heated up to the point of evaporation, revealing themselves
to instruments in space and on the ground. Carbon dioxide is thought
to be the gas that powers the emission from most comets between the
orbits of Saturn and the asteroids.
ENIGMA OF THE MISSING WAVES ON TITAN
One of the most surprising discoveries of the past 10 years is how
much the landscape of Saturn's moon Titan resembles that of the Earth.
Like ours, Titan's surface is dotted with lakes and seas; it has river
channels, islands, mud, rain clouds and maybe even rainbows. It is
wet, but the liquid on Titan is not water. With a surface temperature
dipping to -180C, Titan is far too cold for liquid water. Instead,
the liquid on Titan is thought to be a mixture of methane, ethane,
and other hard-to-freeze hydrocarbons. The idea that Titan is
a wet world with its own alien 'waters' is widely accepted by
planetary scientists. Nothing else can account for the observations:
the Cassini spacecraft has flown by Titan more than 90 times since
2004, charting it with radar and mapping its lakes and seas. The
Huygens probe parachuted to the surface of Titan in 2005, descending
through humid clouds and actually landing in moist soil. However, if
Titan is really so wet, where are the waves? Here on Earth, bodies of
water are rarely still. Breezes blowing across the surface cause
waves to ripple and break; raindrops striking sea surfaces also
provide some roughness. Yet on Titan, the lakes are eerily smooth,
with no discernible wave action down to the millimetre scale,
according to radar data. Scientists know that here is wind on
Titan -- there are magnificent sand dunes to prove it. Add to that
the low gravity -- only 1/7th that of the Earth -- which offers so
little resistance to wave motion, and there is a real puzzle.
Researchers have toyed with several explanations. Perhaps the lakes
are frozen. That is unlikely, however, because we see evidence of
rainfall and of surface temperatures well above the melting point of
methane. Or maybe the lakes are covered with a tar-like substance
that damps wave motion. The answer might be found in the results of a
study published in the 2013 July issue of the journal Icarus. Taking
into account the gravity of Titan, the low viscosity of liquid
hydrocarbons, the density of Titan's atmosphere, and other factors,
the authors calculated how fast the wind on Titan would have to blow
to stir up waves. They found that a walking-pace breeze of only 1 to
2 mph should be enough. But perhaps the winds just haven't been
blowing hard enough. Since Cassini reached Saturn in 2004, Titan's
northern hemisphere (where most of the lakes are located) has been
locked in the grip of winter. Cold heavy air barely stirs, and seldom
reaches the threshold for wave-making. But now the seasons are
changing. In 2009 the Sun crossed Titan's equator heading north.
Summer is coming, bringing light, heat and wind to Titan's lake
country. According to climate models, winds will pick up as the
solstice, due in 2017, approaches, and should be strong enough for
waves. If waves appear, Cassini should be able to detect them. Radar
reflections from wavy lake surfaces can tell researchers a great deal.
Wave dimensions, for instance, may reveal the viscosity of the
underlying fluid, and thus constrain its chemical composition. That
would be quicker and cheaper (though not in the long run more
satisfactory) than sending a spacecraft to retrieve a sample and
return it to us. Also, wave speeds would offer a clue to the speed of
the overlying winds, providing some check on Titan climate models.
MOST CENTAURS MAY BE COMETS
Centaurs are small Solar-System bodies that exist in the same region
of the Solar System as the giant planets. They have unstable orbits
that cross (or have crossed) the orbits of one or more of the giant
planets, and have dynamical lifetimes only of a few million years.
Centaurs typically behave with characteristics of both asteroids and
comets. It has been estimated that there are around 44,000 Centaurs
in the Solar System with diameters larger than 1 km. The first
Centaur to be discovered was 944 Hidalgo in 1920, but they were not
recognized as a distinct population until the discovery of 2060 Chiron
in 1977. The largest one known is 10199 Chariklo which, at 260 km in
diameter, is as big as a mid-sized main-belt asteroid; it was
discovered in 1997. No Centaur has been photographed from close by,
although there is evidence that Saturn's moon Phoebe, imaged by the
Cassini probe in 2004, may be a captured Centaur. In addition, the
Hubble telescope has gleaned some information about the surface
features of 8405 Asbolus. Before the observations reported below,
three Centaurs had been found to display cometary comas: Chiron,
60558 Echeclus, and 166P/NEAT. Chiron and Echeclus are therefore
classified as both asteroids and comets. Other Centaurs such as
52872 Okyrhoe have been suspected of showing cometary activity.
Until now, astronomers were not certain whether Centaurs are asteroids
flung out from the inner Solar System or comets travelling in towards
the Sun from afar. Because of their dual nature, they take their name
from the creature in Greek mythology whose head and torso are human
and whose legs are those of a horse. Data from NEOWISE point to a
cometary origin for most of the objects, suggesting that they come
from further out in the Solar System. The expression 'cometary origin'
is intended to refer to an object that is likely to have been made
from the same material as a comet, may have been an active comet in
the past, and may be active again in the future. The findings come
from an infrared survey of Centaurs and their more distant cousins,
called scattered-disc objects. NEOWISE, the asteroid-hunting portion
of the WISE mission, gathered infrared images of 52 centaurs and
scattered-disc objects. Fifteen of the 52 are new discoveries. Since
the Centaurs and scattered-disc objects orbit in an unstable belt,
ultimately the gravities of the giant planets will fling them either
closer to the Sun or farther away from their current locations.
Although astronomers previously observed some Centaurs with dusty
haloes (a common feature of outgassing comets), and the Spitzer
telescope also found some evidence for comets in the group, they had
not been able to estimate the numbers of comets and asteroids.
Infrared data from NEOWISE provided information on the objects'
albedos to help astronomers sort the population. NEOWISE can tell
whether a Centaur has a matte and dark surface or a shiny one that
reflects more light. Astronomers combined the albedo information with
what was already known about the colours of the objects. Visible-
light observations have shown Centaurs generally to be either
blue-grey or reddish in hue. A blue-grey object could be either an
asteroid or a comet. NEOWISE showed that most of the blue-grey
objects are dark, a characteristic of comets. A reddish object is
more likely to be an asteroid. Comets have a dark, soot-like coating
on their icy surfaces, making them darker than most asteroids. The
results indicate that at any rate two-thirds of the Centaur population
are comets, which come from the frigid outer reaches of the Solar
System. It is not clear whether the rest are asteroids.
NEW CATALOGUE OF X-RAY SOURCES
University of Leicester
Since the Earth's atmosphere blocks out all X-rays, only a telescope
in space can detect and study celestial X-ray sources. Scientists led
by some at the University of Leicester have published a new catalogue
of cosmic X-ray sources, called '3XMM', from observations made by the
X-ray satellite XMM-Newton. The sources in the 3XMM catalogue are
mostly a by-product of the satellite's principal observing programme.
In each of the 600-700 observations made each year, around 70 extra
sources are detected in addition to the object of particular interest,
which usually takes up only a small fraction of the field of view.
Covering observations from 2000 to 2012, the catalogue contains more
than half a million source detections relating to some 372000
different objects. It includes some rare and extreme phenomena, such
as tidal disruption events -- when a black hole swallows a star,
producing a prodigious outburst of X-rays. There are many more common
objects such as active galactic nuclei, clusters of galaxies,
interacting compact binaries, and active stellar coronae.
MISSING MASSIVE GALAXIES
One ambitious goal of contemporary astronomy is to understand the ways
in which galaxies grow and evolve, a key question being star
formation: what determines the number of new stars that will form in a
galaxy? NGC 253 is a spiral galaxy seen in the southern constellation
Sculptor. At a distance of about 11 million light-years, it is one of
our closer intergalactic neighbours, and one of the closest starburst
galaxies visible from the southern hemisphere. Using the Atacama
Large Millimetre Array (ALMA), astronomers can see massive
concentrations of cold gas being thrown out of the galactic disc in
expanding shells by the pressure created by young stars. The amount
of gas measured provides good evidence that some growing galaxies
expel more gas than they accrete. We may be seeing a present-day
example of a very common occurrence in the early Universe. These
results may help to explain why astronomers have found surprisingly
few high-mass galaxies throughout the cosmos. Computer models show
that older, redder galaxies should have considerably more mass and a
larger number of stars than we currently observe. It seems that the
galactic winds or outflow of gas are so strong that they deprive the
galaxy of the fuel for the formation of the next generation of stars.
These features trace an arc that is almost perfectly aligned with the
edges of the previously observed hot, ionised gas outflow, allowing
astronomers to see the step-by-step progression of starburst to
outflow. The researchers determined that quantities of molecular gas
nearly ten times the mass of our Sun each year (possibly much more)
were being ejected from the galaxy at velocities of 40--300 km/s. The
total amount of gas ejected would be more than actually went into
forming the galaxy's stars in the same time. At that rate, the galaxy
could run out of gas in as few as 60 million years.
By Tony Markham, SPA Meteor Section Director
We are now closing in on the maximum of the Perseid meteor shower.
Although Perseid activity started in mid July, observations up to the
end of July were hindered by moonlight. However, with the Moon now out
of the way, observing conditions have become more favourable.
Perseid maximum is predicted for the early evening of August 12th and
so it it is likely that the highest observed rates will be seen late
in the nights of Aug 11-12 (Sun-Mon) and Aug 12-13 (Mon-Tue). Good
rates should also be seen during the nights of Aug 10-11 and Aug 13-14.
Nights earlier and later than this tend to be the territory of more
dedicated meteor observers, but casual observers may spot a few
Perseids. Additional meteor activity may also be seen during this
period from a number of minor showers in Capricornus and Aquarius and
from the sporadic background whose rates are quite high during August.
Perseid meteors can be seen at any time of the night. The shower is
quite rich in bright meteors and around a third of Perseids leaving
persistent trains. Observed rates will increase as the night
progresses due to Perseus getting higher in the sky. At maximum, the
Perseid radiant is located at RA 03h04m, Dec +58 (about 5 degrees
north of gamma Persei). A chart showing its position on other nights
can be found at http://tinyurl.com/pejc6hb
To maximise the number of Perseids that you see, find as dark an
observing site as possible with an unobstructed view of the sky. Centre
your field of view on an area of sky around 30 degrees away from the
radiant and at an altitude of around 50 degrees above the horizon.
Bulletin compiled by Clive Down
(c) 2013 the Society for Popular Astronomy
The Society for Popular Astronomy has been helping beginners in
amateur astronomy -- and more experienced observers -- for 60 years.
If you are not a member then you may be missing something. Membership
rates are extremely reasonable, starting at just £18 a year in the UK.
You will receive our bright bi-monthly magazine Popular Astronomy,
help and advice in pursuing your hobby, the chance to hear top
astronomers at our regular meetings, and other benefits. You can join
online right now with a credit card or debit card at our lively
Current and previous news bulletins from the SPA
1 post • Page 1 of 1