WATER DETECTED ON DWARF PLANET CERES
NASA
The spacecraft called Dawn, which has spent more than a year orbiting
the large asteroid Vesta, is now on its way to Ceres, where it is
scheduled to arrive in the spring of 2015. For more than a century,
Ceres was known as the largest asteroid in the Solar System, but in
2006 the International Astronomical Union, the governing organization
responsible for naming planetary objects, re-classified it as a 'dwarf
planet' because of its large size -- it is roughly 950 km in diameter.
(But it is still an asteroid, for all that!) When it was discovered
in 1801, astronomers thought that it was a planet orbiting between
Mars and Jupiter. Later, other cosmic bodies with similar orbits were
found, marking the discovery of the Solar System's main belt of
asteroids. Scientists think that Ceres contains rock in its interior,
with a thick mantle of ice that, if melted, would amount to more fresh
water than exists on the Earth. The materials making up Ceres
probably date from the first few million years of the Solar System's
existence and accumulated before the planets formed.
Scientists using the Herschel infrared space observatory have recently
detected water vapour on Ceres. It appears that plumes of water
vapour shoot up when portions of its icy surface warm slightly.
Until now, ice had been thought to exist on Ceres but had not been
detected conclusively. Herschel did not see water vapour every time
it looked: although it observed water vapour four different times, on
one occasion there was no such signature. Scientists think that, when
Ceres is in the part of its orbit that is closest to the Sun, some of
its icy surface becomes warm enough to cause water vapour to escape in
plumes at a rate of about 6 kilograms per second, while in the colder
part of its orbit no water escapes. The strength of the signal also
varied over hours, weeks and months, because of the water-vapour
plumes rotating in and out of Herschel's view as Ceres spun on its
axis. That enabled the scientists to localize the sources of water to
two dark spots on the surface of Ceres, previously seen by the Hubble
Space Telescope and ground-based telescopes. The dark spots might be
more likely to outgas because dark material absorbs heat better than
light material. When Dawn arrives, we can hope to learn a lot more.
MILKY WAY MAY HAVE FORMED OUTWARDS FROM THE INSIDE
University of Cambridge
Observations of the chemical composition of stars in the Milky Way's
disc, particularly, in the study reported here, the fast-produced
element magnesium, allow astronomers to estimate how rapidly different
parts of the Milky Way were formed. The research suggests that stars
in the inner regions of the Galactic disc were the first to form,
supporting ideas that our Galaxy grew from the inside out. Using data
from the 'Very Large Telescope' in Chile, an international team of
astronomers observed stars with a wide range of ages and locations to
determine their 'metallicity' -- the amount of chemical elements in a
star other than hydrogen and helium, the two dominant elements.
Immediately after the Big Bang, the Universe consisted almost entirely
of hydrogen and helium, with levels of metallic elements growing over
time. Consequently, older stars in general have lower metallicities.
The different chemical elements are created at different rates -- some
in massive stars which live fast and die young, and others in Sun-like
stars with more sedate multi-billion-year lifetimes. Massive stars,
which have short lives and die as 'core-collapse supernovae', produce
huge amounts of magnesium when they explode. The team has shown that
older, 'metal-poor' stars inside the Solar Circle -- the orbit of the
Sun around the centre of the Milky Way, which takes roughly 250
million years to complete -- are far more likely to have high levels
of magnesium than those outside. The higher levels of the element
inside the Solar Circle suggest that that area contained more stars
that 'lived fast and died young' in the past. The stars that lie
outside the Solar Circle are predominantly younger, both 'metal-rich'
and 'metal-poor', and have surprisingly low magnesium levels compared
to their general metallicity.
That observation may signify important differences in stellar
evolution across the Milky Way's disc, with short star-formation
time-scales occurring inside the Solar Circle whereas outside the
Sun's orbit star-formation took much longer. That supports certain
theoretical models for the formation of disc galaxies, which predict
that galactic discs grow from the inside out. The new research also
bears on an apparent 'double structure' in the Milky Way's disc -- the
so-called 'thin' and 'thick' discs. The thin disc hosts spiral arms,
young stars, giant molecular clouds -- all objects which are young, at
least in the context of the Galaxy. But astronomers have suspected
that there is another disc, which is thicker, shorter and older, and
has many old stars that have low metallicity. In the new research,
the team found that stars in the young, 'thin' disc aged between 0 and
8 billion years all have a similar degree of metallicity, regardless
of age in that range, with many of them considered 'metal-rich'. Then
there is a steep decline in metallicity for stars aged over 9 billion
years, typical of the 'thick' disc, with no detectable 'metal-rich'
stars found at all over that age. But stars of different ages and
metallicity can be found in both discs; there is no clear separation
between the thin and thick discs. The proportion of stars with
different properties is not the same in both discs -- that's why we
think that the two discs probably exist -- but they could have very
different origins. The study provides evidence that the inner parts
of the Milky Way's thick disc formed much more rapidly than did the
thin-disc stars, which dominate in our neighbourhood.
DISTANT QUASAR ILLUMINATES FILAMENT OF THE 'COSMIC WEB'
University of California - Santa Cruz
Astronomers have discovered a distant quasar illuminating a vast
nebula of diffuse gas, revealing for the first time part of the
network of filaments thought to connect galaxies in a 'cosmic web'.
Using the 10-m Keck I telescope in Hawaii, the researchers detected a
gaseous nebula extending about 2 million light-years across
intergalactic space. It is an exceptional object, at least twice as
large as any nebula detected before, and it extends well beyond the
galactic environment of the quasar. The standard cosmological model
of structure formation in the Universe would like galaxies to be
embedded in a cosmic web of matter, most of which (about 84%) is
invisible dark matter. The web appears in the results from computer
simulations of the evolution of structure in the Universe, which show
the distribution of dark matter on large scales, including the
dark-matter haloes in which galaxies form and the cosmic web of
filaments that connect them. Gravity causes ordinary matter to follow
the distribution of dark matter, so filaments of diffuse, ionized gas
are expected to trace a pattern similar to that seen in dark-matter
simulations. Until now, however, such filaments have never been seen.
Intergalactic gas has been detected by its absorption of light from
bright background sources, but those results do not show how the gas
is distributed. In this study, the researchers detected the
fluorescent glow of hydrogen gas resulting from its illumination by
intense radiation from the quasar. The quasar is illuminating diffuse
gas on scales beyond any seen before, giving us the first picture of
extended gas between galaxies. If the cosmic web is as ubiquitous
as some theoreticians would have us believe, however, it seems
strange that, with all the interest that there has been in quasars,
none of them has ever seemed to have illuminated such material before.
The hydrogen gas illuminated by the quasar emits far-ultraviolet light
known as Lyman-alpha radiation. The distance to the quasar is so
great (about 10 billion light-years) that the wavelength of the
emitted light is red-shifted by the expansion of the Universe into the
observable spectrum by the time it reaches here. The researchers
imaged the quasar through a filter that passed just the light that had
started out at the Lyman-alpha wavelength. The light from the quasar
is like a torch beam, and in this case they were lucky that the beam
is pointing towards the nebula and making the gas glow. It has been
suggested that the nebula may be even bigger than it appears, but we
only see the part of the filament that is illuminated by the beamed
emission from the quasar.
INVENTOR OF THE POPULAR DOBSONIAN TELESCOPE DIES AT 98
Universe Today
John Dobson, famous as the creator of the simple, low-cost Dobsonian
telescope, passed away on 2014 Jan. 15 at the age of 98. Dobson was
born in Beijing, but moved with his parents to San Francisco in 1927. After
spending 23 years in a monastery, some of which time was spent sneaking
out to build telescopes and observe the night sky, he left to co-found the
San Francisco Sidewalk Astronomers in 1968, a group dedicated to showing
people on the street the wonders of the night sky using what were (for the
time) large telescopes.
Dobson's interest in astronomy started in the early 1950s when he
built a small telescope from parts obtained from a junk shop. He
wanted to see for himself what the Universe looked like. By 1956, he
got hold of a 12-inch slab of porthole glass and ground it into a
mirror following instructions from Allyn J. Thompson?s classic book
'Making Your Own Telescope'. Dobson shook up the amateur telescope-
making universe with an innovative design based on simplicity. Most
telescopes of his day were small refracting telescopes or small to
modest-sized reflectors with metal tubes and heavy equatorial mounts.
Neither was exactly user-friendly nor offered much light. John used
simple materials like porthole glass, cardboard tubes and wooden
altitude-azimuth mounts to build large telescopes that were incredibly
easy to use. While such mounts were nothing new, Dobson combined
cheap materials, large mirrors and a simpler approach to mountings
that made his telescope style unique.
Topic: February Astronomy Bulletin (Read 1129 times)