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Monday, 26 November 2012

SPA ENB No. 343

                The SOCIETY for POPULAR ASTRONOMY
        Electronic News Bulletin No. 343  2012 November 25

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


For a limited time only, new members of the SPA will receive a free
DVD, Eyes on the Skies, produced by the ESA/Hubble team.  The
60-minute programme covers the development of the telescope, from its
invention to the achievements of the Hubble telescope, and is on sale
elsewhere at £14.99.  Joining the SPA has never been better value!


The SPA has a varied range of astronomy related gifts that are
suitable for Christmas.  Included in the selection are ideas for all
members of the family.

Just arrived is a new range of galactic jewellery - pendants and

Also just arrived, and ideal for a cloudy night, is the game

Check out the complete range at the SPA shop -


Astronomers have identified an object that is probably a planet
wandering through space without a parent star.  At a distance of about
100 light-years, it is the closest such object so far discovered.
Its comparative proximity, and the absence of a bright star very close
to it, has allowed the team to study its atmosphere.  The object also
gives astronomers a preview of the exo-planets that they hope future
instruments will image around stars other than the Sun.  Free-
floating planets are planetary-mass objects that roam through space
without any ties to a star.  Possible examples of such objects have
been found before, but without knowledge of their ages, it was not
possible for astronomers to know whether they were really planets or
brown dwarfs ('failed stars') that lack the mass to trigger the
nuclear reactions that make stars shine.  The new object, labelled
CFBDSIR2149, may be associated with the 'AB Doradus moving group',
which is the closest such group to the Solar System.  If so, then it
is a young object, and it is possible to deduce much more about it,
including its temperature, mass, and what its atmosphere is made of.
There remains a possibility that the association with the moving group
is by chance.  Free-floating objects like CFBDSIR2149 are thought to
form either as normal planets that have been ejected from the
vicinities of their parent stars, or as lone objects like the smallest
stars or brown dwarfs.


Over the past three decades, astronomers have discovered hundreds of
dusty discs around stars, but only two -- 49 Ceti is one -- also have
large amounts of gas orbiting them.  Young stars, about a million
years old, have discs of both dust and gas orbiting them, but the gas
almost always dissipates within about 10 million years.  Yet 49 Ceti,
which is thought to be about 40 million years old, is still being
orbited by a large quantity of gas in the form of carbon monoxide
molecules.  Astronomers do not know why there should be so much gas
around an otherwise ordinary star that is that old.  The gas may be in
a massive disc region, perhaps rather similar to the Sun's Kuiper Belt,
although 49 Ceti is a much brighter star than the Sun -- it is more
like Sirius.

The total mass of the various objects that make up the Kuiper Belt,
which lies beyond the orbit of Neptune and includes Pluto, is about
one-tenth the mass of the Earth.  But when the Sun was young, the
Kuiper Belt may have had a mass as much as 40 times the Earth's.  The
possible analogue that orbits around 49 Ceti, however, now has a mass
of about 400 Earth masses.  Innumerable comets may orbit around
49 Ceti and also round one other star whose age is about 30 million
years.  Young comets probably contain more carbon monoxide than
typical comets in our Solar System.  When they collide, the carbon
monoxide escapes as a gas.  It is suggested that the gas seen around
the two stars is the result of an incredible number of collisions
among their comets -- but that begs a question, because the comets
must have been formed from carbon monoxide (among other things) in
the first place.


Astronomers using the Very Large Telescope (VLT) have discovered a
pair of stars orbiting each other at the centre of a remarkable
planetary nebula.  The new result supports a suggestion about what
controls the spectacular and symmetrical appearance of the nebula.
Planetary nebulae are glowing shells of gas around white dwarfs --
formerly-Sun-like stars in the final stages of their evolution.
'Fleming 1' is a beautiful example that has strikingly symmetrical
jets that weave into knotty, curved patterns.  It is in the southern
constellation Centaurus and was discovered just over a century ago by
Williamina Fleming, a former maid who was hired by Harvard College
Observatory after showing an aptitude for astronomy.

Astronomers have wondered how the symmetrical jets could be created,
but no consensus has been reached.  Now, an effort has been made to
combine new observations of Fleming 1 with computer modelling to
explain how the bizarre shapes came about.  The team used the VLT to
study the light coming from the central star and found that Fleming 1
is likely to have two white dwarfs at its centre, circling each other
every 1.2 days.  Although binary stars have been found at the hearts
of planetary nebulae before, systems with two white dwarfs orbiting
one another are rare.  Astronomers have suggested a binary star
before, but it was thought that in that case the pair would be well
separated, with an orbital period of tens of years or longer.  The new
observations found the pair to be very much closer.  When a star
with a mass up to eight times that of the Sun approaches the end of
its evolution, it begins to lose mass by blowing off its outer shells.
That exposes the hot inner core of the star, which radiates strongly,
causing the outward-moving cocoons of gas to glow brightly as a
planetary nebula.  Many such nebulae are strikingly complex, with
knots, filaments, and intense jets of material forming intricate
patterns.  The new study indicates that the patterns for Fleming 1 are
the result of the close interaction between the pair of stars -- the
surprising swan song of a stellar couple.  This is the most
comprehensive case yet of a binary central star for which simulations
have correctly reproduced the shape of the surrounding nebula.


The current model for the evolution of the Universe supposes that
stars began to form about 13.4 billion years ago, or around three
hundred million years after the Big Bang.  Many of the first stars are
thought to have been monsters by today's standards, and were probably
hundreds of times more massive than the Sun.  Such objects would have
aged very quickly, exhausted their fuel, and exploded as supernovae
within a million years or so.  Lower-mass stars, in contrast, have
much longer lives and last for billions of years.  Much of the dust
and gas from stellar explosions was (and still is) recycled to form
fresh generations of stars.  Our Sun, for example, is thought to be a
third-generation star, and has a very typical mass by today's
standards.  But regardless of their mass and properties, stars are key
ingredients of galaxies like our own Milky Way.  Unveiling the history
of star formation across cosmic time is fundamental to understanding
how galaxies form and evolve.

In a new study, scientists used the UK Infrared Telescope (UKIRT), the
VLT and the Subaru telescope to carry out a survey of star-forming
galaxies at different distances, with around ten times the data of any
previous effort.  With the range of distances, the time taken for the
light to reach us means that we see identically selected galaxies at
different periods in the history of the Universe, so we may be able to
understand how conditions changed over time.  By looking at the light
from clouds of gas and dust in galaxies where stars are forming, the
team is able to assess the rate at which stars are being born.  It
seems that the production of stars in the Universe as a whole has been
continuously declining over the last 11 billion years, being 30 times
lower today than at its peak 11 billion years ago.  If the decline
continues, then no more than 5% more stars will form over the
remaining history of the cosmos, even if it lasts for ever.  The
research suggests that we live in a Universe dominated by old stars.
Half of them were born in the boom that took place between 11 and 9
billion years ago, and it took five times as long to produce the rest.

Space Telescope Science Institute (STScI)

A new record has been claimed by 'CLASH' (Cluster Lensing And
Supernova Survey with Hubble) for finding the most distant galaxy,
which appears as a diminutive blob that is only a tiny fraction of the
size of our Milky Way galaxy.  The CLASH survey is a multi-wavelength
census of 25 massive clusters of galaxies.  The programme uses the
'gravitational lensing' effects of such clusters to identify amplified
images of objects behind them.  The very distant galaxy, called
MACS0647-JD, is seen as it appeared when its light left it 420 million
years after the 'big bang' when the Universe was only 3% of its
present age of 13.7 billion years.  Its light has travelled 13.3
billion years to reach us.  Along the way, 8 billion years into its
journey, it found multiple paths around the massive galaxy cluster
MACS J0647+7015.  In the absence of the cluster, astronomers would not
have seen it, but because of gravitational lensing, the CLASH research
team was able to observe three magnified images of MACS0647-JD with
the Hubble telescope.  The cluster's gravity boosted the light from
the far galaxy, making the images appear about eight, seven, and two
times brighter than they otherwise would be.  MACS0647-JD is less than
600 light-years across; for comparison, the Large Magellanic Cloud (a
dwarf-galaxy companion to the Milky Way) is 14,000 light-years across.
MACS0647-JD has a redshift of 11, the highest yet observed.

BBC Online

The 16th-Century Danish astronomer Tycho Brahe is unlikely to have
been poisoned, according to a researcher studying his remains.  His
body was exhumed in 2010 in a bid to establish the cause of his death.
Tycho was born Tyge Ottesen Brahe in 1546 in Scania, which at the time
was a Danish province, and studied astronomy at the University of
Copenhagen, as well as at German academic institutions.  He catalogued
more than 1,000 new stars, and his stellar and planetary observations
helped lay the foundations of modern astronomy.  Brahe's fame is also
partly due to his personal life.  He lost the bridge of his nose in a
duel while at the University of Rostock in 1566, and wore a metal
prosthetic for the rest of his life.  Tests now indicate that the
prosthetic was in fact made of brass, not gold and silver as accounts
had suggested.

On his death in 1601, he was buried at Tyn Church near Prague's Old
Town Square.  His body has been exhumed previously, in 1901.  It has
been thought that he died of a bladder infection, but traces of
mercury were found in hair taken from his beard in 1901.  However, the
recent tests have found that the level of mercury was not high enough
to have killed him.  There has been speculation that he was killed on
the orders of the Danish king, or even by fellow astronomer Johannes
Kepler.  A team of Danish and Czech scientists has been working to
solve the problem by analysing bone, hair and clothing samples.  The
description given by Kepler of Brahe's death at the age of 54 matches
well the progression of a severe bladder infection.  One widely told
story is that his bladder burst at a royal banquet when he had not
dared to leave the table to relieve himself.  Accounts say that he
died 11 days later.

By Tony Markham, SPA Meteor Section Director

The 2012 Geminids are very well placed for observation, with their
December peak being almost coincident with New Moon.  Although the
Perseids of August attract more observers, the Geminids of December
produce higher rates.  Observing the Geminids does, however, pose the
challenges of enduring the cold December nights and a (usually) lower
chance of clear skies than for the Perseids.

The Geminid meteor shower is active from around Dec 7 to Dec 16.  This
year's peak is due during the night of Dec 13-14 (Thurs-Fri), most
likely within a few hours of 2330UT.  Rates will also be high during
Dec 12-13, but will have dropped off by Dec 14-15.  The peak Geminid
ZHR is usually in excess of 100, although actual observed hourly rates
will, of course, depend on the radiant altitude and the darkness of
your observing site.  The Geminids are observable throughout the
night.  Good numbers of Geminids can be seen from around 8pm onwards,
with the best observed rates likely to be seen between midnight and

In some ways the Geminids are like the Perseids, being rich in bright
meteors.  In other ways they are different, with very few Geminids
leaving persistent trains -- a consequence of the Geminids being more
robust particles derived from asteroid 3200 Phaethon whereas the
Perseids are more fragile icy material derived from comet Swift-

The Geminid radiant is located near the star Castor.  A chart showing
the radiant location can be found at

By Richard Bailey, SPA Solar Section Director

Rotation Nos. 2128, 2129

Forecasts have been made for many months by several Solar experts that
the Maximum of current Cycle 24 will be of lesser intensity than
others in recent times.  Records of observations made by Section
members have shown the Sun to have been less active than hoped for at
this stage in the  Cycle ( the Maximum is due about mid-2013). The MDF
and R figures for this month hardly vary from those for September. It
will be interesting to see how the data goes from now through to
May/June next year,  to confirm, (or otherwise ) the forecasts.  Each
day at least one AR was on show, the SH again heading the activity
ratings. No large AR's were seen, the biggest being in mid-month, NHAR
1589 and NH 1591. 1589 was an open cluster of small spots, about 13 at
its best.  The Sun was quietest on the 7th., 8th.,  and 9th., From the
10th, activity in both hemispheres increased, concentrated near the
equator.  Around the 18th. NH AR1591 was very close to the equator, 7
degrees N.  The  month ended with a row  4 AR's across the SH, none in
the NH though.  Limb faculae were seen regularly.  Poor observing
weather in many parts of the UK again restricted observing time, and
the Director is again very grateful for all the work done by Section
members in such conditions.

MDF 3.89  R  48.66  H-ALPHA

The MDF was up slightly on the September value of 4.86. mirroring the
steady state of solar activity seen in white light.  No flares were
seen , nor special plaging activity. Filaments were usually ordinary,
small prominences scattered around the perimeter showed on each
observation. Larger ones stood out on the 9th, 10th and 20th., in the W.

MDF 5.31

Check out impressive pictures and drawings by Section members on the
Solar link  from the SPA Homepage, under Monthly Reports. Click on the
thumbnails to see the originals.

Bulletin compiled by Clive Down

(c) 2012 the Society for Popular Astronomy

Good Clear Skies
Colin James Watling
Various Voluntary work-Litter Picking for Parish Council (Daytime) and also a friend of Kessingland Beach (Watchman)
Real Astronomer and head of the Comet section for LYRA (Lowestoft and Great Yarmouth Regional Astronomers) also head of K.A.G (Kessingland Astronomy Group) and Navigator (Astrogator) of the Stars (Fieldwork)

Monday, 19 November 2012

Shuttle Atlantis plastic wrapped / Issues on SpaceX cargo flight

NEWSALERT: Saturday, November 17, 2012 @ 1400 GMT
The latest news from Spaceflight Now


We're proud to help launch Future Space(TM) in a new direction.

Pratt & Whitney Rocketdyne's half-century of proven, dependable rocket engines has powered missions that have changed life on Earth. Our employees are ready to lead the charge into Future Space(TM). To innovate solutions, embrace new opportunities and make giant leaps.


The space shuttle Atlantis has been encased in a protective plastic, a wrap that will keep the spacecraft dust-free while construction crews finish building the exhibit hall to showcase her to the public.


Engineers are combing through data from SpaceX's October cargo mission to the International Space Station, examining a rocket engine failure, electronics glitches from suspected radiation, and a power loss that could have imperiled precious medical samples returned from the outpost, NASA officials said Wednesday.


Europe's ion-powered GOCE satellite, devised to skim through the outer reaches of Earth's atmosphere, is lowering its altitude in a precarious attempt to map Earth's gravity field with unmatched precision.


Weather sensors on the Curiosity rover have picked up signs of dust devils brushing by the six-wheeled robot, researchers said Thursday, but the rover's cameras have not yet caught sight of the passing whirlwinds.


Lockheed Martin and Raytheon have submitted bids to the U.S. Air Force for construction of a network of radars to scan the sky and detect small fragments of space debris with unprecedented precision, the companies announced this week.


Astronomy Now and Spaceflight Now have created an interactive iPad guide to the Curiosity rover mission. Learn more about the mission, explore the rover's components and preview Europe's plans for the next Mars rover destined to visit the Red Planet. Now updated with a timeline of the triumphs and disasters of Mars exploration.


"The Final Mission" - NASA emblem developed for the Shuttle Carrier Aircraft crew and their support teams to deliver the orbiters to their final destinations at museums. +++

Forward email

Spaceflight Now | Launch Complex 39 Press Site | Kennedy Space Center | FL | 32899


Good Clear Skies
Colin James Watling
Various Voluntary work-Litter Picking for Parish Council (Daytime) and also a friend of Kessingland Beach (Watchman)
Real Astronomer and head of the Comet section for LYRA (Lowestoft and Great Yarmouth Regional Astronomers) also head of K.A.G (Kessingland Astronomy Group) and Navigator (Astrogator) of the Stars (Fieldwork)

Fwd: Skywatch

Well, tonight, Saturday, was a washout. No Leonid meteor shower to see through this thick cloud. The weather forecast for Sunday night is a clear sky, a mild SW Wind at 5 knots so Richard, Leonard and myself will aim to be there about  6.30 to 7.00 pm




Usual spot, on the little green on Pakefield Cliff behind the Trowel and Hammer in Pakefield Street, free parking available.

I will bring a telescope. There should be a crescent moon, shortly to set, and Jupiter rising in the East.


John P



Good Clear Skies
Colin James Watling
Various Voluntary work-Litter Picking for Parish Council (Daytime) and also a friend of Kessingland Beach (Watchman)
Lyra Website:
Real Astronomer and head of the Comet section for LYRA (Lowestoft and Great Yarmouth Regional Astronomers) also head of K.A.G (Kessingland Astronomy Group) and Navigator (Astrogator) of the Stars (Fieldwork)

Monday, 12 November 2012

Taurid Meteor Shower

Space Weather News for Nov. 11, 2012

TAURID METEOR SHOWER: Earth is passing through a stream of gravelly debris from Comet Encke, source of the annual Taurid meteor shower. Because the debris stream is not very congested, Taurid meteor rates are low, no more than 5 per hour. The special thing about Taurids is that they tend to be fireballs. Check for the latest videos and images of the display, which is expected to peak around Nov. 12th.

Don't just watch meteors, wear them:  Authentic meteorite jewelry is available in the Space Weather Store:

Good Clear Skies
Colin James Watling
Various Voluntary work-Litter Picking for Parish Council (Daytime) and also a friend of Kessingland Beach (Watchman)
Lyra Website:
Real Astronomer and head of the Comet section for LYRA (Lowestoft and Great Yarmouth Regional Astronomers) also head of K.A.G (Kessingland Astronomy Group) and Navigator (Astrogator) of the Stars (Fieldwork)

SPA ENB no. 342

                  The SOCIETY for POPULAR ASTRONOMY
          Electronic News Bulletin No. 342  2012 November 11

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


For a limited time only, new members of the SPA will receive a free
DVD, Eyes on the Skies, produced by the ESA/Hubble team.  The
60-minute programme covers the development of the telescope, from its
invention to the achievements of the Hubble telescope, and is on sale
elsewhere at £14.99.  Joining the SPA has never been better value!


Amateur and professional astronomers alike have been following Comet
168P/Hergenrother, which has been traversing the constellations of
Andromeda and Lacerta.  Over the past several weeks it has produced a
series of outbursts of dusty material.  Now the comet's nucleus has
taken another step in its fragmentation: it has separated into at
least four distinct pieces, resulting in a large increase in dust
material in its coma.  With more material to reflect the Sun's rays,
the coma has brightened considerably.  The fragments are considerably
fainter than the nucleus.  The fragmentation event was initially
detected on October 26 by observers using the Faulkes Telescope North
on Haleakala, Hawaii.


The Curiosity rover has made its first analysis of soil on Mars.
A preliminary operation served to 'cleanse the palate' of the rover's
sample-collection systems, which may have brought contaminants from
the Earth.  With that out of the way, Curiosity used a technique
called X-ray diffraction, in which X-rays are shot into samples that
include crystalline materials.  The precise ways in which the X-rays
scatter off the crystals gives clear information as to their chemical
makeup, and hints as to their structure.  The 'CheMin' experiment
first sieves a soil sample, separating out the components smaller than
150 microns.  It then gives the sifted soil a shake while firing
X-rays at it.  The sample was found to contain the minerals feldspar,
olivine and pyroxene, such as might be found in weathered basaltic
materials of volcanic origin, like those in Hawaii.  So far, the
materials that Curiosity has analyzed are consistent with initial
ideas of the deposits in Gale Crater (the large equatorial crater
where it landed in August), recording a transition through time from a
wet to a dry environment.  In the weeks since its arrival on Mars, the
rover has already examined some larger rocks.  The ancient rocks, such
as the conglomerates, suggest flowing water, while the minerals in the
younger soil are consistent with limited interaction with water.  The
next step is to deliver soil samples into another experiment within
the rover ('Sam', or 'Sample Analysis at Mars') instrument, which will
look for the presence of carbon-containing molecules.

NASA/Goddard Space Flight Center.

A second look at data from the Hubble telescope is reviving the claim
(reported in ENB 256, 2008 November 30) that the 'nearby' star
Fomalhaut hosts a massive planet.  The study suggests that the planet,
named Fomalhaut b, is completely shrouded by dust.  Fomalhaut is the
brightest star in the constellation Piscis Austrinus and lies 25
light-years away.  In 2008, Hubble astronomers announced the
exo-planet as the first one ever directly imaged in visible light
around another star.  The object was imaged just inside a ring of
debris surrounding but offset from the host star.  The planet's
location and mass -- no more than three times Jupiter's -- seemed just
right for its gravity to explain the ring's appearance.  Recent
studies have claimed that that planetary interpretation is incorrect.
On the basis of the object's apparent motion and the lack of an
infrared detection by the Spitzer space telescope, the studies have
suggested that the object is a short-lived dust cloud unrelated to any
planet.  A new analysis, however, brings the planet conclusion back to
life.  Although the results seriously challenge the original discovery
paper, they do so in a way that actually makes the object's
interpretation much cleaner and leaves intact the core conclusion,
that Fomalhaut b is indeed a massive planet.  The discovery study
reported that Fomalhaut b's brightness varied by about a factor of two
and cited that as evidence that the planet was accreting gas.  Follow-
up studies then interpreted the variability as evidence that the
object actually was a transient dust cloud instead.  In the new study,
astronomers re-analyzed Hubble observations of the star from 2004 and
2006.  They easily recovered the planet in observations taken in red
light, and made a new detection in violet light.  In contrast to the
earlier research, the team found that the planet remained at constant

The team attempted to detect Fomalhaut b in the infrared with the
Subaru telescope in Hawaii, but was unable to do so.  The non-
detections with Subaru and Spitzer imply that Fomalhaut b must be less
than twice the size of Jupiter.  Another contentious issue has been
the object's orbit. If Fomalhaut b is responsible for the ring's
offset and sharp interior edge, then it must follow an orbit aligned
with the ring and must now be moving at its slowest speed.  The speed
implied by the original study appeared to be too fast.  Additionally,
some researchers argued that Fomalhaut b follows a tilted orbit that
passes through the ring plane.  Using the Hubble data, the team
established that Fomalhaut b is moving with a speed and direction
consistent with the original idea that the planet's gravity is
modifying the ring.  The team also addressed studies that interpret
Fomalhaut b as a compact dust cloud not gravitationally bound to a
planet.  Near Fomalhaut's ring, orbital dynamics would spread out or
completely dissipate such a cloud in as little as 60,000 years.  The
dust grains experience additional forces, which operate on much faster
time-scales, as they interact with the star's light.  Given what we
know about the behaviour of dust and the environment where the planet
is located, scientists think that they are seeing a planetary object
that is completely embedded in dust rather than a free-floating dust
cloud.  Because astronomers detect Fomalhaut b by the light of
surrounding dust and not by light or heat emitted by its atmosphere,
it no longer ranks as a 'directly imaged exo-planet'.  But because it
has the right mass and is in the right place to affect the ring, it
could perhaps be allowed the weasel-worded claim to be a 'planet
identified from direct imaging'!


Yale University

Researchers have discovered a band, or stream, of stars that they
believe to be the remnant of an ancient star cluster slowly being
ingested by the Milky Way galaxy.  It was found by searching a region
recently surveyed by the Sloan Digital Sky Survey III (SDSS-III), an
international collaboration that is mapping the sky through wide-field
photometry.  Galaxies are believed to form hierarchically through the
merger of smaller galaxies and still smaller star clusters, which are
disrupted by the gravitational force of the principal galaxy.  That
process may be the primary way galaxies such as the Milky Way grow in
mass.  The Milky Way is constantly accreting small galaxies and star
clusters, because the more powerful gravity of our Milky Way pulls
them apart and their stars then become part of the Milky Way itself.
Researchers have previously found evidence of the Milky Way
assimilating dwarf galaxies, but it is thought that the newly found
stellar stream, called the Triangulum stream, is the remnant of a star
cluster rather than of a larger galaxy, because the stream is very



It has been suggested that most of such people as think about galaxies
at all picture them as huge islands of stars, gas and dust that
populate the Universe in visual splendour.  Theory, however, has
allowed that there could be other types of galaxies that are devoid of
stars and are made predominantly of dense gas.  Such 'dark' galaxies
would be unseen against the black backdrop of space.  Some models have
proposed that dark galaxies were common in the early Universe when
galaxies had more difficulty forming stars -- partly because their
density of gas was not sufficient to form stars -- and only later did
they begin to ignite stars, becoming like the galaxies we see today.
According to a current theory of galaxy formation, big galaxies form
from the merger of smaller ones.  Dark galaxies bring to big galaxies
a lot of gas, which then accelerates star formation in the bigger
galaxies.  Now, an international team of astronomers has actually
detected several dark galaxies by observing the fluorescent glow of
their hydrogen gas, illuminated by the ultraviolet light of 'nearby'


Astronomers using data from the Fermi gamma-ray space telescope have
made a measurement that enables them to estimate the total amount of
light from all of the stars that have ever shone, accomplishing a
primary mission goal.  The optical and ultraviolet light from stars
continues to travel throughout the Universe even after the stars
themselves have ceased to shine, so it forms a sort of fossil
radiation field that can be explored by observations of gamma rays
from distant sources.  Gamma rays are the most energetic form of
light.  Since Fermi's launch in 2008, it has observed the entire sky
in high-energy gamma rays every three hours, creating a detailed map
of the Universe at those energies.  The total sum of starlight in the
cosmos is known to astronomers as the extragalactic background light
(EBL).  To gamma rays, the EBL functions as a kind of cosmic fog.
Astronomers investigated the EBL by studying gamma rays from 150
blazars, or galaxies powered by black holes, that were strongly
detected at energies greater than 3 billion electron volts (3 GeV), or
more than a billion times the energy of visible light.  With more than
a thousand detected so far, blazars are the most common sources
detected by Fermi, but gamma rays at those energies are few and far
between, which is why it took four years of data to make the analysis.

As matter falls toward a galaxy's central super-massive black hole,
some of it is somehow accelerated outward at almost the speed of light
in jets pointed in opposite directions.  When one of the jets happens
to be aimed in the direction of the Earth, the galaxy appears
especially bright and is classified as a blazar.  During their
journey, which may be across billions of light-years, the gamma rays
pass through a sort of fog of visible and ultraviolet light emitted by
stars throughout the history of the Universe.  Occasionally, a gamma
ray collides with starlight and transforms into a pair of particles --
an electron and its antimatter counterpart, a positron.  Once that
occurs, the gamma-ray light is lost.  In effect, the process dampens
the gamma-ray signal in much the same way as fog dims a distant light

From studies of nearby blazars, scientists have determined how many
gamma rays should be emitted at different energies.  More distant
blazars show fewer gamma rays at higher energies -- especially above
25 GeV -- thanks to absorption by the cosmic fog.  Thus the farthest
blazars are missing most of their highest-energy gamma rays. The
researchers then determined the average gamma-ray attenuation across
three distance ranges between 9.6 billion years ago and today.  From
that measurement, the scientists were able to estimate the fog's
thickness.  To account for the observations, the average stellar
density in the cosmos has to be about 1.4 stars per 100 billion cubic
light-years, which implies that the average distance between stars in
the Universe is about 4000 light-years.

Bulletin compiled by Clive Down

(c) 2012 the Society for Popular Astronomy

Good Clear Skies
Colin James Watling
Various Voluntary work-Litter Picking for Parish Council (Daytime) and also a friend of Kessingland Beach (Watchman)
Lyra Website:
Real Astronomer and head of the Comet section for LYRA (Lowestoft and Great Yarmouth Regional Astronomers) also head of K.A.G (Kessingland Astronomy Group) and Navigator (Astrogator) of the Stars (Fieldwork)