The SOCIETY for POPULAR ASTRONOMY
Electronic News Bulletin No. 205 2006 October 1
Here is the latest round-up of news from the Society for Popular
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By Alastair McBeath, SPA Meteor Section Director
Things calmed down rather after the flurry of UK fireball sightings
in late August and early September, as reported in the last couple
of ENBs, but one new event happened in morning twilight at
04:50 UT on September 16-17, as seen by two witnesses in
different parts of Wiltshire. The event was somewhere around
magnitude -4 to -9. Any additional reports of this or other
fireballs would be welcomed by the Section. Information on what
to send and where to can be found via the Section's "Fireball
Observing" page at:
OCTOBER'S METEOR PROSPECTS
By Alastair McBeath, SPA Meteor Section Director
Full Moon on October 7 is very unfavourable for checking on
possible Draconid activity, most likely between 14h UT on October
8 to 07h UT on October 9, if anything happens at all. Commonly,
only years when the Draconid parent comet, 21P/Giacobini-Zinner,
is near perihelion and fairly close to the Earth in early October,
does anything much happen, as last in 1998, when an outburst
with estimated Zenithal Hourly Rates (ZHRs) of ~700 happened
briefly. The comet was at perihelion most recently in July 2005, and
although a relatively minor outburst happened last October (even
so, with ZHRs about 30-40; see ENBs 184 and 185), it is thought
too distant now to influence the shower's activity this autumn. An
unexpected minor outburst happened in 1999 however, showing
there are still features to watch out for with this shower, so alert
observers may be rewarded if something unanticipated chances-by,
and survives the bright sky.
By contrast, new Moon on October 22 creates a perfect viewing
opportunity right across the Orionid maximum, with the main peak
due on October 21 this autumn. The radiant, in Orion's "Club"
asterism then, is usefully observable from about 23h UT onwards.
ZHRs are likely to be about 20-25 at best this year, with rates
sometimes sustained at a little below maximum levels for several
days over the main peak. A strong sub-maximum, with ZHRs
similar to the main one, is possible on October 17-18, but this has
been definitely reported in only two years so far, 1993 and 1998.
The waning crescent Moon is in Leo on October 17 and 18, only
a small distraction for covering any possible repeat this year.
Orionids are normally present from October 2 to November 7,
and the meteors are fast, often bright, and frequently trained.
Unfortunately, it is unlikely any unusual activity from the twin Taurid
showers will happen again in late October this year, as occurred last,
when increased activity and very healthy numbers of fireballs were
spotted, from the closing days of October into mid-November See
ENBs 185, 186, 188, 191 and the two special meteor ENBs of
2005 November 5 and December 27 for details. This is because
the next autumnal Taurid meteoroid "swarm" return, thought to
have produced the 2005 activity, as well as that in 1995 and 1998,
among other years, is not due till 2008. Even so, worth keeping an
eye out just in case, particularly with a helpful first quarter Moon
on October 29. More details on October's expected meteor
activity can be found at:
Good luck, and clear skies!
MILKY WAY'S SPIRAL MAY HAVE BEEN LATE ADDITION
Some astronomers using ESO's's Very Large Telescope think that they
see evidence that the middle of our galaxy formed separately and at a
different time from the spiral arms. They find that the stars in the
Milky Way's central bulge have a different chemical composition from
those in the arms of the galaxy. The galactic bulge is largely made
up of the oldest stars in the galaxy, dating back 10 billion
years. The arms, meanwhile, are populated by stars of all ages.
The chemical makeup of stars gives astronomers clues to their pasts.
Stars rich in elements such as oxygen and iron are probably second- or
even third-generation -- that is, their material has been in stars
previously. Massive stars can end their lives in a number of
different types of supernovae. It takes a type-II supernova to
produce most oxygen, while iron is produced in type-Ia explosions.
Thus, the amounts of each element reveal something about the ancestry
of the star.
The astronomers studied fifty giant stars in four regions of the
Galaxy close to the central bulge. They found the amount of oxygen in
disc and bulge stars to be significantly different, suggesting that
the two portions of the galaxy are 'genetically different'. They
infer from that that the bulge must have formed more rapidly than the
disc, probably in less than a billion years and when the Universe was
still very young. The team found that for a given amount of iron,
stars in the disc contain less oxygen than their bulge counterparts,
suggesting that the bulge stars formed independently and did not
originate in the disc and then migrate inward to build up the bulge.
LIGHTWEIGHT GIANT PLANET
Harvard-Smithsonian Center For Astrophysics
Using a network of small automated telescopes known as HAT,
Smithsonian astronomers have discovered a planet, designated HAT-P-1,
whose density is remarkably small. With a radius about 1.38 times
Jupiter's, it is the largest known planet, but its mass is only half
that of Jupiter. It orbits one member of a pair of stars called ADS
16402, 450 light-years away in the constellation Lacerta, every 4.5
days in an orbit one-twentieth the size of the Earth's. Once each
orbit, it transits in front of its parent star, causing the star to
dim by about 1.5% for more than two hours.
HAT-P-1 is not alone in having a low density. The first planet ever
found to transit its star, HD 209458b, is 20% larger than
theoreticians would like; HAT-P-1 is 24% larger. The only way to puff
up giant planets beyond the sizes calculated from planetary-structure
equations seems to be to supply additional heat to their interiors.
Simple heating of the surface by the host star would not work -- if it
could, all close-in transiting giant planets should be expanded, not
just two of them.
CASSINI FINDS ETHANE ON TITAN
Scientists suspect that ethane falls as rain or snow on Saturn's moon
Titan, after the Cassini spacecraft identified a huge cloud of the
hydrocarbon in its atmosphere. Researchers had hoped to find lakes of
ethane on Titan, but now suspect the substance is all frozen solid at
the poles. The discovery of the ethane cloud has added weight to that
hypothesis, since it was identified at the very edges of Titan's
arctic circle at altitudes between 29 and 60 km. Astronomers believe
that ethane is raining or, if temperatures are low enough, snowing on
the north pole right now. If the polar conditions are cold enough,
the ethane could easily accumulate as ice.
EARLIEST RECORDED SUPERNOVA IDENTIFIED
Some astronomers think that they have identified a supernova remnant,
RCW 86, with a 'guest star' recorded in 185 AD by Chinese astronomers.
The Chinese noted that it sparkled like a star and did not appear to
move in the sky, arguing against its being a comet. Also, the
observers noticed that it took about eight months to fade, a fact
consistent with modern observations of supernovae. The researchers
worked out the age of the remnant by measuring its expansion velocity
and calculating backwards. They got a much shorter age than was at
one time thought, because the expansion velocity they derived was much
faster than some earlier studies of the remnant had indicated, and the
team said that that was probably owing to the nature of the space into
which it is expanding. Before the star exploded, it would have sent
out a massive shock wave, effectively creating a bubble of stellar
wind in the area around the star. Some of the exploded material is
still within that irregularly shaped area, but some has hit denser
material beyond it and slowed down. The faster moving material in the
bubble gives a better indication of the supernova's age, and it is
that material that the team has now measured.
SCIENTISTS IMAGE BROWN DWARF SYSTEM
Scientists using the Spitzer space telescope have discovered and
directly imaged a small brown dwarf star, 50 times the mass of
Jupiter, orbiting with a planet around a Sun-like star. Such an
arrangement has not been seen before but might be common, according to
the scientists. The brown dwarf is one of the coolest such
objects, called T dwarfs. It is called HD 3651 B and is located in
the constellation Pisces. It is in a system containing a star
slightly less massive than our Sun that is orbited by a planet
slightly smaller than Saturn. The planet's orbit around the Sun-like
star is highly elliptical. Other extra-solar planets have been
discovered with highly elliptical orbits; the Spitzer discovery might
support an idea that small companions such as T dwarfs can hide in
such systems and cause the orbits of planets to be highly eccentric.
The planet's orbit is similar to Mercury's, but the T dwarf has
an orbit over ten times larger than Pluto's.
SUPERNOVA IS 'TOO BRIGHT'
A supernova more than twice as bright as others of its type has been
observed, suggesting that it arose from a white dwarf that managed to
grow more massive than the theoretical limit. Perhaps such supernovae
are not such good 'standard candles' for determining distances as has
Supernovae fall into several classes differentiated by their spectra.
Type-Ia supernovae are thought to form when a white dwarf receives
enough matter from a nearby evolving companion that it explodes in a
runaway thermonuclear reaction. That was thought to occur when the
white dwarf had reached 1.4 times the mass of the Sun -- a limit known
as the Chandrasekhar mass. The mass cut-off was thought to make all
such supernovae explode with about the same intrinsic brightness,
allowing astronomers to calculate their distances on the basis of
their magnitudes. In fact, it was observations of type Ia supernovae
that led to the allegation in 1998 that some mysterious entity was
causing the Universe's expansion to speed up. Now, astronomers at the
University of Toronto have found a type-Ia supernova that is
2.2 times as bright as others of its class. It lies about 3 billion
light years away, a distance that was verified from the spectrum of
its host galaxy. Its brightness, along with other clues from its
spectrum, suggests that the white dwarf exploded with 2.1 solar masses
of material -- significantly above the Chandrasekhar mass.
ANOTHER SATURN RING
Images taken by the Cassini spacecraft on September 17 show a
previously unobserved ring around Saturn. The images were obtained
during the longest solar occultation of Cassini's four-year mission.
In such an event, the Sun passes directly behind Saturn, so Cassini
lies in the shadow of Saturn while the rings are brilliantly backlit.
Usually an occultation lasts only about an hour, but this time it was
12 hours. The occultation allowed Cassini to map the presence of
microscopic particles that are not normally visible across the ring
system. As a result, Cassini saw the entire inner Saturnian system in
a new light.
The new ring is a tenuous feature, visible outside the brighter main
rings of Saturn and inside the G and E rings, and coincides with the
orbits of Saturn's moons Janus and Epimetheus. Scientists expected
that meteoroid impacts on Janus and Epimetheus might knock particles
off the moons' surfaces and inject them into Saturn orbit, but they
were surprised that a well-defined ring structure exists at that
location. Saturn's extensive, diffuse E ring, the outermost ring, had
previously been imaged one small section at a time, but the 12-hour
occultation enabled the entire structure to be seen in one view. The
moon Enceladus was seen sweeping through the E ring, extending wispy,
finger-like projections into the ring; they very probably consist of
tiny ice particles being ejected from Enceladus' south-polar geysers
and entering the E-ring.
HUBBLE DISCOVERS NEW OLD GALAXIES
NASA astronomers analyzing two of the deepest images obtained by the
Hubble telescope have noted more than 500 galaxies that they claim to
have existed less than a billion years after the Big Bang, when the
Universe was less than 7% of its present age. The fact that most of
them are dwarf galaxies is considered to be evidence for galaxies
building up from small pieces.
THE TRUTH OF MARS HILL
New images of the 'face on Mars' have been obtained by Europe's Mars
Express spacecraft. The 'face' appeared first in a photo of Mars'
Cydonia region taken in 1976 by Viking 1. Scientists believed from
the beginning that the feature was simply a hill that happened to look
like a face because of the way the Sun cast shadows across it at the
time the photo was taken. However, the image sparked speculation that
the face was built by aliens and that NASA was attempting a cover-up.
The agency used the Mars Global Surveyor spacecraft to take new images
of the region in 1998 and 2001. Those much more detailed images
showed a hill with no particular resemblance to a face, but since the
Mars Express spacecraft arrived at Mars in 2003, certain unconvinced
members of the general public have been asking mission scientists to
take more images of the feature. Mission controllers have actually
been trying to get images of the region since 2004 but had been
thwarted until recently by dust and haze in the Martian atmosphere.
Finally, on July 22, they obtained clear images of the region. By
making observations from different angles, they have been able to
build a 3D map of the face and the surrounding area. The hill that
sparked such speculation is clearly seen in the new images to be a
natural feature sculpted by erosion.
FIRST SUPERWASP PLANETS FOUND
The UK-led SuperWASP (Wide Angle Search for Planets) programme has
found its first two planets. Using wide-angle camera lenses backed
by CCD cameras, the team has been repeatedly surveying several
million stars over large areas of the sky, looking for the tiny dips
in the starlight caused when a planet transits in front of its star.
Confirmation of the new finds came earlier this month when the team
joined forces with the Swiss and French users of SOPHIE, a powerful
new instrument at the Observatoire de Haute-Provence. SOPHIE was able
to detect slight wobbles in both stars' motions as the planets orbited
around them. Together the two types of observation confirmed the
existence and nature of the planets. Both are of a type known as 'hot
Jupiters -- giant gas planets, like Jupiter, but much closer to their
parent stars. One is only 6 million km from its star and orbits every
2.5 days, while the other is only 4.5 million km from its star and
orbits in 2 days. The very close orbits mean that the planets must
be much hotter, at over 1800 C, even than Mercury in our Solar System,
which is nearly 60 million km from the Sun and has a surface
temperature of over 400Â°C. Both planets show signs that they are
losing their atmospheres to space.
SKA GIANT RADIO TELESCOPE
Australia and Southern Africa have been short-listed as the countries
to host the Square Kilometre Array (SKA), a giant next-generation
radio telescope being developed by scientists in 17 countries. The
decision was made by the International SKA Steering Committee,
following advice from an external committee of 7 scientists from 5
countries that examined the four site bids. The SKA will be a set of
thousands of antennae, not a single giant instrument, spread over
3,000 km, but with half of the antennae located in a central region 5
km across. The SKA is intended to be 50 times more sensitive than the
most powerful radio telescopes we now have.
Bulletin compiled by Clive Down
(c) 2006 the Society for Popular Astronomy
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