Topic: Early October Astronomy Bulletin

[Clive]

BIG SUN-GRAZING COMET DISCOVERED
Spaceweather.com

The newly-discovered Comet Ison, currently still beyond the orbit of
Jupiter, is heading for a very close encounter with the Sun next year.
In 2013 November, it will pass less than 0.012 AU (1.8 million km)
from the solar surface.  The fierce heating it will experience could
turn the comet into a bright naked-eye object -- but comets can and
often do fizzle out!  Meanwhile, comet researcher John Bortle has
pointed out a similarity between the orbit of Comet Ison and that of
the Great Comet of 1680. "Purely as speculation," he says, "perhaps
the two bodies could have been one a few revolutions ago."


SHOCK WAVES SHAPED OUR SOLAR SYSTEM?
RAS

Certain scientists think that shock waves that might have been emitted
from the Sun when it was very young might have caused the planets in
the Solar System to form at different times.  It seems that the waves
might have created a series of debris rings that eventually accreted
into planets, the outermost ones first.


DAWN SUGGESTS SPECIAL DELIVERY OF HYDRATED MATERIAL TO VESTA
RAS

The mechanism by which water was incorporated into the terrestrial
planets is a matter of debate.  Now, observations of Vesta by the Dawn
mission suggest that hydrous materials were delivered to the asteroid
mainly through a build-up of small particles during an epoch when the
Solar System was rich in dust.  Vesta's surface shows distinct areas
enriched with hydrated materials.  Those regions are not dependent on
solar illumination or temperature, as we find in the case of the Moon.
The uneven distribution is unexpected and indicates ancient processes
that differ from those believed to be responsible for delivering water
to other airless bodies, like the Moon.  Analysis of data from Dawn's
mapping spectrometer showed large regional concentrations of hydroxyl
(a hydrogen and an oxygen atom bound together) clearly associated with
features including ancient, highly-cratered landforms and the Oppia
crater.  Hydroxyl on the surface of the Moon is thought to be created
continuously by the interaction of protons from the solar wind with
the lunar regolith.  Highest concentrations are found in areas near
the lunar poles and in permanently shadowed craters where it is very
cold.  By contrast, the distribution of hydroxyl on Vesta is not
dependent on significant shadowing or unusually cold temperatures.
It is also stable over time, so its origin does not appear to be due
to short-term processes.

The hydroxyl-rich regions on Vesta broadly correspond to its oldest
surfaces.  Around relatively large and young impact craters, hydroxyl
detections are weak or absent, suggesting that the delivery of
hydroxyl is not an ongoing process.  The spectrometer evidence
suggests that much of Vesta's hydroxyl was delivered by small
particles of primitive material, less than a few centimetres in
diameter, over a time-limited period, but it is not clear what period
that was.


SUPERNOVA 1006
ScienceDaily

Astronomers all over the world observed the supernova of the year
1006.  Some of them, including Chinese astronomers, highlighted the
fact that it was visible for three years.  The most explicit record,
made by the Egyptian Ali ibn Ridwan (988-1061), notes that the
phenomenon was about three times brighter than Venus, and that it
emitted light of a quantity equivalent to almost a quarter of the
Moon's brightness.  Now a team of astronomers has studied the existing
stars in the area, regarding distance and possible contamination by
elements of the supernova, and the results show that there is no star
that could be considered the progenitor of the explosion and that the
event was probably the result of a collision and merger of two white
dwarf stars of similar mass.


MILKY WAY IS SURROUNDED BY HALO OF HOT GAS
Chandra X-ray Observatory / Harvard-Smithsonian Center for
Astrophysics

Astronomers have found evidence that the Milky Way Galaxy is embedded
in an enormous halo of hot gas that extends for hundreds of thousands
of light-years.  The estimated mass of the halo is comparable to the
mass of all the stars in the Galaxy.  If the size and mass of such a
gas halo is confirmed, it could also be an explanation for what is
known as the 'missing baryon' problem for the Galaxy.  Baryons are
particles, such as protons and neutrons, that make up more than 99.9%
of the mass of atoms found in the cosmos.  Measurements of extremely
distant gas haloes and galaxies indicate that the baryonic matter
present when the Universe was only a few billion years old represented
about one-sixth the mass and density of the existing unobservable, or
dark, matter.  In the current epoch, about 10 billion years later, a
census of the baryons present in stars and gas in our galaxy and
nearby galaxies suggests that at least half the baryons are
unaccounted for.  In a recent study, a team of astronomers used data
from Chandra, the XMM-Newton space observatory and the Suzaku
satellite to set limits on the temperature, extent and mass of the hot
gas halo.  Chandra observed eight bright X-ray sources located far
beyond the Galaxy at distances of hundreds of millions of light-
years.  The data indicated that X-rays from those distant sources are
absorbed selectively by oxygen ions in the vicinity of the Galaxy.
The scientists determined the temperature of the absorbing halo is
between 1 million and 2.5 million °C.

Other studies have shown that the Milky Way and other galaxies are
embedded in 'warm' gas with temperatures between 100,000 and 1 million
degrees.  Studies have indicated the presence of a hotter gas with a
temperature greater than 1 million degrees.  The new research provides
evidence that the hot gas halo enveloping the Milky Way is much more
massive than the warm gas halo.  In an effort to estimate the size and
mass of the halo, the authors supplemented Chandra data on the amount
of absorption produced by the oxygen ions with XMM-Newton and Suzaku
data on the X-rays emitted by the gas halo.  They concluded that the
mass of the gas is equivalent to the mass in more than 10 billion
Suns, perhaps as large as 60 billion Suns.  The work shows that, for
reasonable values of parameters and with reasonable assumptions, the
Chandra observations imply a huge reservoir of hot gas around the
Milky Way.  It may extend for a few hundred thousand light-years
around the Milky Way or it may extend further into the surrounding
local group of galaxies.  Either way, its mass appears to be very
large.  The estimated mass depends on factors such as the amount of
oxygen relative to hydrogen, which is the dominant element in the gas.
Nevertheless, the estimation represents an important step in solving
the question of the missing baryons, which has puzzled astronomers.
Although there are uncertainties, the work provides the best evidence
yet that the Galaxy's missing baryons have been hiding in a halo of
million-degree gas that envelopes the Galaxy.  The estimated density
of the halo is so low that similar haloes around other galaxies would
have escaped detection.  It must be noted that the weighty conclusions
proposed in this item all stem from a mere eight observations, and may
accordingly be open to some revision.


ULTRA-DISTANT GALAXY DISCOVERED
Johns Hopkins University

A team of astronomers has discovered what could be the most distant
galaxy ever detected.  The young galaxy observed by the Hubble
telescope and Spitzer shone when the Universe was just 500 million
years old.  The far-off galaxy existed within an important era when
the Universe began to emerge from the so-called 'Dark Ages', as it
went from a dark, starless expanse to a recognizable cosmos full of
galaxies.  Technically speaking, the galaxy has a redshift of 9.6.
It is observable only because it has been enhanced by a 'gravitational
lens'.


FIRST TWO JAMES WEBB TELESCOPE MIRRORS DELIVERED
NASA

The first two of the 18 primary-mirror elements for the James Webb
space telescope have arrived at the Goddard Space Flight Center.  The
rest will make their way from Ball Aerospace to Goddard over the next
12 months.  Each of the 18 hexagonal tiles that make up the primary
mirror measures more than 1.3 metres across, and weighs about 40 kg.