Topic: Mid April Astronomy News Bulletin

[Clive]

SPOTLESS SUN
Spaceweather.com

NASA has asserted that the Sun has plunged into the deepest solar
minimum in nearly a century.  Sunspots have all but vanished.  In
2008, the Sun had no spots 73% of the time, a 95-year low.  In 2009,
sunspots are even scarcer, with the 'spotless rate' currently 87%.
The situation is unusual but not unprecedented: similarly deep
solar minima were common in the late-19th and early-20th centuries,
and each time the Sun recovered with a fairly robust solar maximum.


JUPITER'S RED SPOT
Cosmos Online

New high-resolution maps of Jupiter have provided evidence that the
Great Red Spot -- the biggest storm in the Solar System -- is
shrinking.  Scientists at Berkeley have collected data over a number
of years from space probes such as Galileo and Cassini and have used
them to create detailed maps of wind speeds in the Great Red Spot.
They have shown that, from 1996 to 2006, the spot's diameter shrank at
an average rate of a kilometre a day.  The Red Spot is twice as large
as the Earth; it has lasted for at least the 300 years since
observations began, and may be much older.  The Spot is made up of
gases such as hydrogen, helium, ammonia, methane and water vapour,
just like the rest of the Jovian atmosphere.  What gives it its
distinctive red colour is still not agreed, but some scientists
believe it may result from material drawn up from deeper in Jupiter's
atmosphere, below the ammonia clouds.

Previously, changes in the size of the Spot were estimated by looking
at cloud patterns created by the storm.  To get a more accurate
measure, software has been developed to follow the movement of cloud
patterns over long periods of time.  The shrinking of the Spot has
relevance to the energy balance in the surrounding atmosphere.  The
amount of energy leaving the Red Spot does not appear to be balanced
by the energy the storm is gaining, but the reason for any discrepancy
is not obvious and neither is its significance.


UNUSUAL SUPERNOVA PROGENITOR STAR
STScI

Astronomers using the Hubble telescope have identified a star that
was a million times brighter than the Sun before it exploded as a
supernova in 2005.  According to current theories of stellar
evolution, the star ought not to have self-destructed so early in its
life.  The star, which is estimated to have had about 100 times the
Sun's mass, was not mature enough, according to theory, to have
evolved a massive iron core of nuclear-fusion ash, which is supposedly
the prerequisite for the core collapse that triggers a supernova blast.
The explosion, called supernova SN 2005gl, was seen in the barred-
spiral galaxy NGC 266 on 2005 October 5.  Pre-explosion pictures from
the Hubble archive, taken in 1997, reveal the progenitor as a very
luminous point source with an absolute visual magnitude of -10.3.
That is so bright that the star seems likely to have belonged to the
class of stars called Luminous Blue Variables (LBVs).  As an LBV
evolves it sheds much of its mass through a violent stellar wind.
Only at that point does it develop a massive iron core, and then it
explodes as a core-collapse supernova.  Extremely massive and luminous
stars of more than 100 solar masses, such as Eta Carinae in our own
Milky Way, are expected to lose their entire hydrogen envelopes prior
to their ultimate explosions as supernovae.  The new progenitor
identification shows that, at least in that case, the star exploded
before losing most of its hydrogen envelope, suggesting that the
evolution of the core and the evolution of the envelope are less
coupled than previously seemed to be implied by stellar-evolution
theory.  One possibility is that the progenitor to SN 2005gl was
really a binary system that merged, making it look more luminous and
less evolved than it really was.  The observations indicated that
only a small part of the star's mass was flung off in the explosion;
most of the material was probably drawn into the collapsing core that
may now be a black hole estimated at at least 10 to 15 solar masses.


ERRATIC BLACK HOLE
Chandra X-ray Center

There is believed to be a class of black holes, with masses in the
approximate range 7--25 solar masses, that has arisen in binary
systems.  Some such objects emit powerful jets of particles and
radiation, rather analogous to those seen in quasars, and are called
'micro-quasars'.  A new study looks at a famous micro-quasar in our
own Galaxy, and regions close to its event horizon.  The system, GRS
1915+105, contains a black hole about 14 times the mass of the Sun
that is feeding off material from a nearby companion star.  As the
material swirls toward the black hole, an accretion disc forms.  The
system shows remarkably unpredictable and complicated variability on
time-scales ranging from seconds to months, including 14 different
patterns of variation.

Since its launch in 1999, the Chandra X-Ray observatory has observed
GRS 1915+105 eleven times. The studies indicate that its jet may be
periodically choked off when a hot wind, seen in X-rays, is driven off
the accretion disc around the black hole.  The wind is believed to
shut down the jet by depriving it of the matter that would otherwise
have fuelled it.  Conversely, once the wind dies down, the jet can
re-emerge.  The latest Chandra results also show that the wind and the
jet carry about the same amount of matter away from the black hole,
possibly suggesting that the black hole is somehow regulating its
accretion rate, which may be related to the toggling between mass
expulsion via either a jet or a wind from the accretion disc.


MOST DETAILED MAP OF NEARBY UNIVERSE
Anglo-Australian Observatory

A survey of galaxies, called the 'Six-degree-field galaxy survey', has
been carried out with the 1.2-m UK Schmidt telescope (now part of the
Anglo-Australian Observatory).  The Schmidt's wide field of view --
5.7 degrees, or 11 times the width of the Full Moon -- enabled the
survey to cover as much as 80% of the southern sky in a reasonable
time.  From conception to delivery, the survey has taken almost a
decade.  It has recorded the positions of more than 110,000 galaxies
over more than 80% of the southern sky, out to about two billion
light-years (a redshift of 0.15).  As well as participating in the
overall expansion of the Universe, galaxies have their own individual
'peculiar' motions.  Assessment of the peculiar velocities may be
possible for about 10% of the galaxies surveyed.  It is done by
comparing a galaxy's distance predicted by its redshift with its
distance estimated from its internal properties.  The technique
depends upon measuring the width of spectral lines in the galaxy
concerned.  That has been done with a purpose-built spectrograph, the
'Six-degree-field' instrument, which allows 150 spectra to be taken
simultaneously.  The light from each of the 150 individual galaxies is
brought to line up on the entrance slit of the spectrograph by means
of a set of flexible optical fibres whose front ends are positioned on
the galaxy images by a robotic fibre-positioner.


DISCS AROUND COOL STARS HAVE DIFFERENT CHEMICAL MIXES
JPL

A new study from the Spitzer space telescope finds that the abundances
of certain molecules in planet-forming discs around young stars differ
according to the luminosities of the stars concerned.  Researchers
examined planet-forming discs around 44 Sun-like stars and 17 cooler
ones (M dwarfs and brown dwarfs).  The stars are all about 1-3 million
years old, an age when planets are thought to be growing.  The
astronomers tried to measure the abundance of hydrogen cyanide with
respect to that of a baseline molecule, acetylene.  They found that
the cool stars, both M-dwarf stars and brown dwarfs, showed no
hydrogen cyanide at all, while 30% of the Sun-like stars did.  They
speculate that ultraviolet light, which is much stronger around the
Sun-like stars, may drive the production of the hydrogen cyanide.