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 Post subject: ENB No 246 June 22 2008
PostPosted: Sun Jun 22, 2008 10:35 am 
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The SOCIETY for POPULAR ASTRONOMY
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Electronic News Bulletin No. 246 2008 June 22
===================================================


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
visiting http://tinyurl.com/kogyx



JUNE FIREBALLS
By Alastair McBeath, SPA Meteor Section Director

Fireballs reported to the SPA from the UK and nearby since the last
round-up in ENB 244 ( http://tinyurl.com/5von7z ), have included an
eighth probable sighting of the May 6-7 fireball around 22:16 UT, and
the following fresh events during the first half of June:

Date Time (UT) Magnitude and Notes Observed from
08/06/6-7 23:08 Very bright Guernsey
08/06/6-7 23:16 Very bright Hampshire
08/06/7-8 23:20 -10/-12? Brittany, France
08/06/12-13 ~23:00 Bright Essex

The apparently two events on June 6-7 may have been separate
reports of the same meteor, and clarification is being sought from both
witnesses still at present. There were also media reports from SW
Wales from June 7-8, suggesting either that the Brittany fireball that
night was seen from there too, or that another bright fireball (perhaps
more than one) had happened around 23h UT, probably over the sea
as seen from Pembrokeshire.

Additional observations of these or any other fireballs seen from the
British Isles or points adjacent (a fireball is any meteor that reaches
at least magnitude -3) are always welcomed by the Section. Details to
send are outlined on the "Fireball Observing" page of the SPA website,
at: http://tinyurl.com/l62fh .


METEOR PROSPECTS FOR JULY
By Alastair McBeath, SPA Meteor Section Director

Midsummer's overnight twilight persists for most of the month for Britain,
but by mid to late July, the nights are showing signs of growing darker
again across much of the country, except for the more northerly parts
of Scotland. Twilight and exams often deter watchers from observing till
late July, but as the month's main shower peaks fall in the closing days
of July, a period this year with a waning to new Moon, this is not so
negative as it might seem. Some early Perseid activity should be seen
in any clear, moonless skies after mid July too.

Antihelion Source (ANT) meteors may be seen at any time during July,
typically medium-speed meteors radiating from a large, roughly oval,
region of sky centred about twelve degrees east of the point opposite
the Sun in the sky (hence its name). Positions for this central point can
be found in the Section's online Meteor Showers List, linked from the
Meteor homepage at: http://tinyurl.com/2m335d , while the radiant area
is about 30 degrees in RA (two hours) by 15 degrees in Dec. In July,
the ANT radiant is starting to head northwards from its most southerly
point in June, passing from Sagittarius across Capricornus to just into
Aquarius, an area that is on-view for most of the brief nights,
culminating around 01h UT. In general, one or two meteors per hour is
a good level of ANT activity, and according to the latest International
Meteor Organization analyses, such a low rate is liable to persist for
most of this month. Zenithal Hourly Rates (ZHRs) may be a little better,
maybe 2-3, by late July, by when there will be little moonlight, though the
southerly radiant declination will likely reduce observed numbers from
the UK to negligible proportions for most of the month.

July-August brings one of the stronger annual near-ecliptic meteor
showers, the Delta Aquarids, usually present from about July 12 through
to mid August. Their maximum ZHRs of about 15-20 are usually
achieved for a day or two around July 27, from a radiant then near
22h36m RA, -16 degrees Dec. Though this is not too far southeast of
the ANT's centre, the shower should still be detectable as distinct from
the ANT thanks to its relative best strength and slightly swifter, though
still roughly medium-speed, meteors. Note though that the Delta
Aquarids' southerly radiant declination means observed meteor
numbers are always much lower than the ZHRs from Britain, but meteors
from the shower can be very bright sometimes. As with the ANT, the
Delta Aquarid radiant is above the horizon throughout the short summer
nights, and the waning crescent Moon by the peak will cause no
problems for observers.

Another leading, if much less active, late July shower peak is that of the
Alpha Capricornids, which shower lasts from July 3 into mid August. At
best, around July 29-30, ZHRs are liable to be just 4 or 5, though there
was a minor enhancement to ~10, with several fireballs, seen from
Europe in 1995. The radiant at maximum is around 20h28m RA, -10
degrees Dec, a position which means the Alpha Capricornid radiant
actually overlaps that of the ANT's, and we are not sure it will still be
observable as separate from the ANT as a result. With new Moon on
August 1, conditions could scarcely be more favourable for checking
this summer, however. The shower is noted for occasionally producing
wonderfully bright, and always slow, meteors.

A perpetual favourite with British watchers, the swift-moving Perseids
should begin their annual activity by July 17, just before full Moon. For
most of the month, their radiant lies below the "W" of Cassiopeia, not
actually in Perseus at all. Even by the Moon-free skies of late July,
Perseid rates will still be low (their ZHRs usually do not rise above ~10
until the first week of August), but it's always exciting to spot the first
few during July, whenever the Moon allows, at least. The shower's mid
August peak has a waxing gibbous Moon, but could produce stronger
than normal rates in 2008; more about that next month.

Good luck for all your observing, and clear skies!


STRANGE SOLAR SYSTEM BODIES FOUND
New Scientist

Astronomers searching for distant supernovae have stumbled upon two
relatively nearby objects that may shed light on the early Solar
System. Supernova surveys look for changes in faint light sources,
and many of the faint objects that suddenly appear at a new spot in
the sky are not supernovae at all but are actually small bodies in the
Solar System. When astronomers at the University of Washington at
Seattle studied their survey data, they found 14,000 asteroids in the
inner Solar System, including 1300 new objects. Now, the team reports
finding 14 previously unknown objects in the outer Solar System in
data collected by the ESSENCE supernova search from 2002 to 2007. One
particularly interesting object is 2003 UC414, which is about 100 km
across and travels in a nearly circular orbit near the midpoint
between Uranus and Neptune. Few such orbits are stable; only two
other objects have been found in that zone, and they appear not to be
in stable orbits. However, 2003 UC414 is close to two 'islands of
stability' where objects could orbit for a billion years, but with
only two months of observations the orbit can not be placed definitely
in a stable zone. Another interesting object is 2004 VN112, which is
thought to be about 300 km across. Its orbit is tilted 25° to the
plane of the Solar System and is of high eccentricity, its distance
from the Sun ranging from about 47 to 600 AU.


'PLUTOID' CHOSEN AS NAME FOR PLUTO-TYPE OBJECTS
International Astronomical Union

Almost two years after the IAU introduced the category of dwarf
planets, it has decided on a name for trans-neptunian dwarf planets
similar to Pluto. The name plutoid was approved by the IAU Executive
Committee at its recent meeting in Oslo. Plutoids are celestial
bodies that are in orbit around the Sun at a distance greater than
that of Neptune, that have sufficient mass for their self-gravity to
overcome rigid-body forces so that they assume a near-spherical shape,
and that have not cleared the neighbourhood around their orbits. The
two so-far-known and named plutoids are Pluto and Eris. The dwarf
planet Ceres is not a plutoid, as it is located in the asteroid belt
between Mars and Jupiter. Since Ceres appears to be the only object
of its kind, no proposal is made for a separate category of Ceres-like
dwarf planets.


MASSES OF COOLEST BROWN DWARFS
Topix

Brown dwarfs, 'failed stars', are a class of objects between the
lowest-mass stars and gas-giant planets such as Jupiter and Saturn.
They are the faintest objects that can be directly observed outside
the Solar System, emitting as little as 1/300,000th of the energy of
the Sun and some having surface temperatures as low as 700 K.
Hundreds of them are known within 100 light-years, and about 15% of
them are binary systems. Astronomers have now reported a project in
which the Keck and Hubble telescopes were used in an effort to
determine the masses of just two pairs of brown dwarfs that occur
together in binaries. The total mass of any binary system can be
determined by measuring the orbit's size and how long it takes for the
two objects to complete one orbital cycle. Those can be difficult
measurements, because brown-dwarf binaries have tiny separations on
the sky and orbit one another very slowly. One of the binaries, known
as 2MASS 1534-2952AB, is composed of two 'methane' brown dwarfs, the
coolest type of brown dwarf, which is characterized by the presence of
methane gas in its atmosphere. The total mass of the system proved to
be only 6% of the Sun's mass, so each brown dwarf in it has a mass of
about 3% of the Sun's (about 30 times the mass of Jupiter). The other
binary system, HD 130948BC, is a pair of slightly warmer 'dusty' brown
dwarfs with a total mass of only 11% of the sun's mass. The two
binaries, located in the constellations of Libra and Bootes
respectively, are about 45-60 light-years from Earth. The two
components of each binary have a typical separation of about 2 AU and
their orbital periods are about 10-15 years.


SMALL PLANET DISCOVERED ORBITING SMALL STAR
University of Notre Dame

An international team of astronomers has discovered an extra-solar
planet of about three Earth masses orbiting a star with a mass near
the lower limit for nuclear-burning stars. The planet, referred to as
MOA-2007-BLG-192Lb, has the lowest mass yet found for an extra-solar
planet orbiting a normal star. The star is at a distance of 3,000
light years and has the lowest mass known for any star having a
companion with a planetary mass ratio. The mass of the star is about
6% of the mass of the Sun, which corresponds to that of a brown dwarf,
slightly below the mass needed to sustain nuclear reactions in the
core, but the measurement uncertainty also permits a mass slightly
above 8% of a solar mass, which would allow MOA-2007-BLG-192L to be a
normal hydrogen-burning star. The planet's orbital radius is similar
to that of Venus, but since the star is very faint the top of the
planet's atmosphere is likely to be colder than Pluto.


A TRIO OF SUPER-EARTHS
ESO

A group of astronomers using the HARPS instrument at La Silla
Observatory has discovered a system of three super-Earths around the
rather normal star HD 40307, which is less massive than the Sun and is
located 42 light-years away towards the southern constellation Pictor.
Precise measurements of the radial velocity of the star over the last
five years indicate the presence of three planets. They have 4.2,
6.7, and 9.4 times the mass of the Earth and orbit the star with
periods of 4.3, 9.6, and 20.4 days, respectively. The perturbations
induced by the planets are really tiny - the mass of the smallest
planet is 1/100,000 of that of the star. Indeed, each planet induces
a motion of the star of only a few metres per second. The team also
announced the discovery of two other planetary systems. In one, a
super-Earth (7.5 Earth masses) is in a 9-day orbit around the star HD
181433, which also has a Jupiter-like planet with a period close to 3
years. The second system contains a 22-Earth-mass planet having a
period of 4 days and a Saturn-like planet with a 3-year period as
well.


NEWLY BORN TWIN STARS ARE FAR FROM IDENTICAL
Vanderbilt University

The analysis of the youngest pair of supposedly identical twin stars
yet discovered has revealed surprising differences in brightness,
surface temperature and possibly even the size of the two. The study
suggests that one of the stars formed significantly earlier than its
twin. Because astrophysicists have assumed that binary stars form
simultaneously, the discovery provides an important new test for
star-formation theories, forcing theorists back to the drawing board
to see if their models can produce binaries with stars that form at
different times. The twins were discovered in the Orion Nebula, a
well-known stellar nursery that is 1,500 light-years away, and are
about one million years old.

Eclipsing binaries are pairs of stars that revolve in orbits that we
see edge-on; their eclipses demonstrate their orbital periods, which
in some cases allow determination of their masses. The newly
discovered twins are found to have nearly identical masses 41% that of
the Sun. According to current theories, mass and composition are the
two factors that determine a star's physical characteristics and
dictate its entire life cycle. Because the two stars condensed from
the same cloud of gas and dust they should have the same composition.
With identical mass and composition, they should be identical in every
way. So the astronomers were surprised to find that the twins
exhibited significant differences in brightness, surface temperature
and possibly size. A major difference in the depths of the two
eclipses that occur in every orbital cycle showed that one of the
stars is twice as bright as the other and has a surface temperature
about 300 degrees higher than that of its twin. There is also an
indication that one of the stars is about 10% larger than the other.
The easiest way to explain the differences is if one star was formed
about 500,000 years before its twin.


LIGO LOOKS AT THE CRAB NEBULA
California Institute of Technology

The unsuccessful search for gravitational waves has provided new
information about the core of one of the most famous objects in the
sky, the pulsar in the Crab Nebula. An analysis by the international
LIGO (Laser Interferometer Gravitational-wave Observatory)
collaboration has shown that no detectable contribution to the energy
loss of the pulsar is made by the emission of gravitational waves.
The nebula was formed relatively recently, in 1054, in a supernova
explosion. Chinese astronomers at the time reported that it was
visible during the daytime for three weeks. Some reports suggest that
it may have been brighter at its peak than the Full Moon.

At the centre of the Crab Nebula is a pulsar, a neutron star that is
emitting radio waves and other radiation and is spinning at a rate of
about 30 Hz. In comparison with other pulsars, its rate of rotation
is decreasing rapidly -- an indication that it is radiating energy at
a prodigious rate. The mechanism for the energy loss is not known,
but the gravitational-wave enthusiasts hoped that a substantial part
of it would be due to the emission of gravitational waves that they
could detect with LIGO. Gravitational waves, described as being
ripples in the fabric of space-time itself, are a prediction of
Einstein's general theory of relativity. Neutron stars are incredibly
dense, and gravity must attempt to compress them into near-perfect
spherical shapes. Yet, owing to their high mass, even a slight
asymmetry could result in significant perturbations in the fabric of
space-time during their high-speed rotations. Since the neutron star
in the Crab Nebula is so young, maybe (it was thought) it has not had
time to become an exactly spherical body and might be emitting gravity
waves that carry away a lot of energy and would be mainly responsible
for the rapid braking that the star is undergoing. Results from LIGO,
however, have shown no sign of gravitational waves. The results put
an upper limit of 4% to the proportion of the pulsar's energy loss
that could be due to gravitational radiation.


YOUNG SUPERNOVA REMNANT FOUND IN MILKY WAY
European Space Agency

The expanding cloud of debris that a supernova explosion leaves behind
is known as a supernova remnant (SNR), and is in many cases a bright
source of X-rays and radio waves. When astronomers took the first
high-resolution radio images of a celestial object known as G350.1-0.3
in the 1980s, they saw an irregular knot of gases that did not seem to
meet the expectations for an SNR, and it was classified as a probable
background galaxy and quietly forgotten. Now astronomers at the
University of Sydney have observed it with the orbiting X-ray
observatory XMM-Newton and found that G350.1-0.3 is indeed the debris
of an exploded star despite its misshapen configuration. In fact, it
turns out to be one of the youngest and brightest supernova remnants
in the Milky Way.

Radio surveys show that G350.1-0.3 exploded next to a dense cloud of
gas about 15,000 light-years away; the cloud prevented the blast from
expanding evenly in all directions, resulting in a misshapen supernova
remnant. G350.1-0.3 is very small and young in astronomical terms,
only eight light-years across and about 1000 years old. Few such
young SNRs are known, so even having one more is important; they are
highly luminous, with the newly-formed chemical elements glowing
brightly, making them easier to study. The light from the supernova
would have reached the Earth about the time of William the Conqueror,
but it is thought that it would have passed unnoticed. The X-ray data
tell us that there is a lot of dust in the line of sight, so it would
have been invisible to the unaided eye.


MILKY WAY HAS ONLY TWO MAIN SPIRAL ARMS
Science Daily

Since the 1950s, astronomers have produced maps of the Milky Way. The
early models were based on radio observations of gas in the Galaxy,
and suggested a spiral structure with four major star-forming arms,
called the Norma, Scutum-Centaurus, Sagittarius and Perseus arms. In
addition to arms, there are bands of gas and dust in the central part
of the Galaxy. Our Sun lies near a small, partial arm called the
Orion Arm, or Orion Spur, located between the Sagittarius and Perseus
arms. Large infrared sky surveys in the 1990s led to some major
revisions of the models, including the discovery of a large bar of
stars in the middle of the Milky Way. Infrared light can penetrate
dust, so telescopes that observe in the infrared are better able to
map the dusty and crowded Galactic Centre. In 2005, astronomers used
the Spitzer infrared satellite telescope to obtain detailed
information about our Galaxy's bar, and found that it extends farther
out from the centre of the Galaxy than previously thought.

The team now has infrared imagery from Spitzer of a long swath of the
Milky Way, stretching 130 degrees across the sky and one degree above
and below the Galaxy's mid-plane. Star counts show an increase in the
direction of the Scutum-Centaurus Arm, as would be expected for a
spiral arm, but in the directions of the Sagittarius and Norma arms
there is no jump in the number of stars. The fourth arm, Perseus, is
seen from here in the anti-centre direction which is not included in
the Spitzer map. The findings suggest that the Milky Way has two
major spiral arms, a common structure for galaxies with bars. Those
major arms, the Scutum-Centaurus and Perseus arms, have the greatest
densities both of young, bright stars, and of older, red-giant stars.
The two minor arms, Sagittarius and Norma, are filled with gas and
pockets of young stars. The two major arms seem to connect up nicely
with the near and far ends of the Galaxy's central bar.


ULTRAVIOLET GIVES VIEW INSIDE DYING STAR
University of Oxford

An international team of astronomers combined data from ground-based
telescopes observing visible light from a supernova with data from a
space telescope looking for an earlier peak in ultraviolet light from
an associated dying star. They saw signs of the shock wave that forms
within a star before it explodes as a supernova. Usually we do not
know what a star looks like just before it explodes as a supernova,
and the explosion destroys almost all evidence of the original star.
Scientists have observed stars nearing the end of their lives, and
supernova explosions and their afterglow, but have had little firm
evidence of what happens in between. The new observations give a
first glimpse of what happens inside a star during the final hours of
its existence.

Out of all the supernovae looked at by the team, one was found that
was preceded by a dramatic flash of ultraviolet light given off by a
red supergiant star in a galaxy around a billion light years away.
The flash occurred about a fortnight before the object was detected as
a normal supernova. Astronomers believe that the ultraviolet light,
emanating from deep within the star, was generated after its core
collapsed and compressed the gas surrounding it to around one million
degrees Kelvin. Around four hours after the light was observed a
shock-wave from the collapsed core, travelling at more than 10,000
km/s, would have hit the surface of the star and blown it apart.
However, it was almost a fortnight before the resulting explosion was
observed by supernova hunters.


GLOBAL NETWORK CREATES 6,000-MILE TELESCOPE
Cornell University Communications

The Arecibo Observatory in Puerto Rico has joined other telescopes in
North America, South America, Europe and Africa in observing the same
targets simultaneously, simulating a telescope more than 6,800 miles
in diameter. The telescopes are all members of the 'Express
Production Real-time e-VLBI Service' (EXPReS) project which uses
multiple radio telescopes simultaneously to observe the same region of
sky, creating in effect an instrument as big as the separation of the
dishes. VLBI can generate images of cosmic radio sources with up to
100 times better resolution than images from the best optical
telescopes.



The SPA Electronic News Bulletins are sponsored by the Open University.
Bulletin compiled by Clive Down


(c) 2008 the Society for Popular Astronomy

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