NEW LUNAR CRATER NAMED AFTER AMELIA EARHART
BBC News
Scientists have discovered a large crater on the Earth-facing side of
the Moon -- the first detection of its kind in at least a century.
The previously unknown structure has been named after aviation pioneer
Amelia Earhart -- the first woman to fly solo across the Atlantic.
The buried crater, 200 km in diameter, was found in data from the
Grail spacecraft, which mapped the Moon's gravity field. The find was
made while the scientists were searching the data for evidence of
hollow underground structures known as lava tubes. The Serenitatis
Basin is thought to have been created by a giant impact about 3.9
billion years ago. So Earhart, which lies partially buried under the
debris, must be at least that age, but how much older is not known at
this stage. Earhart crater has been sitting within sight all these
years -- you can see it with a small telescope or a pair of binoculars.
But people did not recognise it without the extra piece of information
-- the gravity field.
MARS LOST AN OCEAN OF WATER
ESO
After a six-year study with the Very Large Telescope, along with
instruments at the Keck Observatory and the Infrared Telescope
Facility, astronomers say that a primitive ocean on Mars held more
water than our Arctic Ocean, and covered a greater portion of the
planet's surface than the Atlantic Ocean does of the Earth. They say
that, about four billion years ago, the young Mars had enough water to
cover its entire surface in a layer about 140 metres deep, but it is
more likely that the water pooled to form an ocean occupying almost
half of the northern hemisphere, and in some regions reaching depths
of more than a mile. The new estimate is based on detailed
observations of two slightly different forms of water in Mars'
atmosphere. One is the familiar form of water, made with two hydrogen
atoms and one oxygen, H2O. The other is HDO, or semi-heavy water, a
naturally occurring variation in which one hydrogen atom is replaced
by its heavy form, deuterium. The deuterated form is less easily lost
into space through evaporation. So, the greater the water loss from
the planet, the greater the ratio of HDO to H2O in the water that
remains. By comparing the ratio of HDO to H2O, scientists can measure
by how much the fraction of HDO has increased and thus estimate how
much water has escaped into space. That in turn allows the amount of
water on Mars at earlier times to be estimated.
In the study, the team mapped the distribution of H2O and HDO
repeatedly over nearly six Earth years -- equal to about three Mars
years -- producing global snapshots of each, as well as their ratio.
The maps reveal seasonal changes and microclimates, even though modern
Mars is essentially a desert. The team was especially interested in
regions near the north and south poles, because the polar ice caps are
the planet's largest known reservoir of water. The water stored there
is thought to document the evolution of Mars' water from the wet
Noachian period, which ended about 3.7 billion years ago, to the
present. The new results show that atmospheric water in the
near-polar region was enriched in HDO by a factor of seven relative to
the Earth's ocean water, implying that water in Mars' permanent ice
caps is enriched eight-fold. Mars must have lost a volume of water
6.5 times larger than the present polar caps to provide such a high
level of enrichment. The volume of Mars' early ocean must have been
at least 20 million cubic kilometres. A likely location for the water
would be the Northern Plains, which have long been considered a good
candidate because of their low-lying ground. An ancient ocean there
would have covered 19% of the planet's surface -- for comparison, the
Atlantic Ocean occupies 17% of the Earth's surface. It is possible
that Mars once had even more water, some of which may have been
deposited below the surface. Because the new maps reveal micro-
climates and changes in the atmospheric water content over time, they
may also prove to be useful in the continuing search for underground
water.
17TH-CENTURY NOVA EXPLAINED
ESO
New observations reveal that the star that European astronomers saw
appear in the sky in 1670 was not a nova, but a much rarer, violent
breed of stellar collision. It was spectacular enough to be easily
seen with the naked eye during its first outburst, but the traces it
left were so faint that very careful analysis of sub-millimetre
observations was needed before its nature could finally be unravelled
now. Some of seventeenth century's greatest astronomers, including
Hevelius -- the father of lunar cartography -- and Cassini, carefully
documented the appearance of a new star in the skies in 1670.
Hevelius described it as 'nova sub capite Cygni' -- a new star below
the head of the Swan -- but astronomers now know it by the name Nova
Vulpeculae 1670. Historical accounts of novae are rare and of great
interest to modern astronomers. Nova Vul 1670 is claimed to be both
the oldest recorded nova and the faintest nova when later recovered.
When it first appeared, Nova Vul 1670 was easily visible to the naked
eye and varied in brightness over the course of two years. It then
disappeared and reappeared twice before vanishing for good. Although
it was well documented for its time, the astronomers of the day did
not understand the apparent nova's peculiar performance. During the
twentieth century, astronomers came to understand that most novae
could be explained by the runaway explosive behaviour of close binary
stars. But Nova Vul 1670 did not fit that model well at all. Even
with ever-increasing telescopic power, the event was believed for a
long time to have left no trace, and it was not until the 1980s that a
team of astronomers detected a faint nebula surrounding the suspected
location of what was left of the star. While those observations
offered a tantalising link to the sighting of 1670, they did not shed
any new light on the true nature of the event. Astronomers have now
observed the area at sub-millimetre and radio wavelengths and have
found that the surroundings of the remnant are bathed in a cool gas
rich in molecules, with a very unusual chemical composition. What the
team discovered was that the mass of the cool material was too great
to be the product of a nova explosion, and in addition the isotope
ratios around Nova Vul 1670 were different from those expected from a
nova. But if it wasn't a nova, then what was it? The answer is a
spectacular collision between two stars, more brilliant than a nova,
but less so than a supernova, which produces something called a red
transient. Those are very rare events in which stars explode owing to
a merger with another star, spewing material from the stellar
interiors into space, eventually leaving behind only a faint remnant
embedded in a cool environment, rich in molecules and dust. That
newly recognised class of eruptive stars fits the profile of Nova Vul
1670 almost exactly.
NEWBORN STARS AT THE EDGE OF MILKY WAY
RAS
Brazilian astronomers have discovered a cluster of stars forming on
the very edge of our Milky Way Galaxy, which has a barred spiral
shape, with arms of stars, gas and dust winding out from a central
bar. Viewed from the side, the Galaxy would appear relatively flat,
with most of the material in a disc and the central regions. Stars
form inside massive and dense clumps of gas in so-called giant
molecular clouds (GMCs) that are mainly located in the inner part of
the Galactic disc. With many clumps in a single GMC, most (if not
all) stars are born together in clusters.
The team looked at data from the orbiting Wide-Field Infrared Survey
Explorer (WISE) observatory. They not only found GMCs thousands of
light-years above and below the Galactic disc, but that one of them
unexpectedly contained two clusters of stars. This is the first time
that astronomers have found stars being born in such a remote
location. The new clusters, named Camargo 438 and 439, are within the
molecular cloud HRK 81.4-77.8. That cloud is thought to be about 2
million years old and is around 16000 light-years beneath the Galactic
disc, an extraordinary distance away from the usual regions of star
formation. Astronomers have thought up two possible explanations.
In the first case, the Chimney Model, violent events such as supernova
explosions eject dust and gas out of the Galactic disc. The material
then falls back, in the process merging to form GMCs. The other idea
is that the interaction between our Galaxy and its satellites, the
Magellanic Clouds, may have disturbed gas that falls into the Galaxy,
again leading to the creation of GMCs and stars.
NEW DWARF GALAXIES FOUND ORBITING MILKY WAY
University of Cambridge
A team of astronomers has identified nine new dwarf satellites
orbiting the Milky Way, the largest number ever discovered at once.
The findings, by the Dark Energy Survey, may throw light on 'dark
matter', a hypothetical invisible substance credited with holding
galaxies together. (The Dark Energy Survey is a five-year effort to
photograph a large portion of the southern sky with the Dark Energy
Camera, which it claims to be the most powerful digital camera in the
world.) The new results also mark the first discovery of dwarf
galaxies -- small celestial objects that orbit larger galaxies -- for
years, after dozens were found in 2005 and 2006 in the skies above the
northern hemisphere. The new satellites were found in the southern
hemisphere near the Large and Small Magellanic Clouds, the largest and
best-known dwarf galaxies in the Milky Way's orbit. The newly
discovered objects are a billionth* the brightness of the Milky Way,
and a millionth of the mass. The closest is about 95,000 light-years
away, and is located in the constellation Reticulum, while the most
distant is more than a million light-years away in Eridanus.
According to the Cambridge team, three of the discovered objects are
definite dwarf galaxies, while others could be either dwarf galaxies
or globular clusters -- objects with somewhat similar visible
properties to dwarf galaxies, but not held together with dark matter.
Dwarf galaxies are the smallest galaxy structures observed, the
faintest of which contain just 5000 stars -- the Milky Way, in
contrast, contains hundreds of billions of stars. Standard
cosmological models of the Universe predict the existence of hundreds
of dwarf galaxies in orbit around the Milky Way, but their dimness and
small size makes them very difficult to find. Since they contain up
to 99% dark matter and just 1% observable matter, dwarf galaxies are
ideal for testing whether existing dark-matter models are correct.
Perhaps they were once satellites that orbited the Magellanic Clouds
and have been thrown out by the interaction of the Small and Large
Clouds. Perhaps they were once part of a gigantic group of galaxies
that -- along with the Magellanic Clouds -- are falling into our Milky
Way Galaxy.
MILKY WAY 50% LARGER THAN PREVIOUSLY THOUGHT
Rensselaer Polytechnic Institute (RPI)
The Milky Way Galaxy is at least 50% larger than is commonly believed,
according to new findings that reveal that the galactic disc is
contoured into several concentric ripples. The research revisits
astronomical data from the Sloan Digital Sky Survey which, in 2002,
established the presence of a bulging ring of stars beyond the known
plane of the Milky Way. In essence, the team found that the disc of
the Milky Way is not just a disc of stars in a flat plane -- it is
corrugated. Outward from the Sun, at least four ripples can be seen
in the disc of the Milky Way. While only part of the Galaxy can be
seen in the data, it is assumed that the same pattern is going to be
found throughout the disc. Importantly, the findings show that the
features previously identified as rings are actually part of the
Galactic disc, extending the known diameter of the Milky Way from
100,000 to 150,000 light-years. Astronomers had observed that the
number of Milky Way stars diminishes rapidly about 50,000 light-years
from the centre of the Galaxy, and then a ring of stars appears at
about 60,000 light-years from the centre. Now it seems that that
apparent ring is actually a ripple in the disc, and it may well be
that, further out, there are more ripples which we have not yet seen.
SUPERNOVA SPLIT INTO FOUR IMAGES
Space Telescope Science Institute (STScI)
Astronomers using the Hubble telescope have observed for the first
time a distant supernova split into four images. The multiple images
of the exploding star are caused by the powerful gravity of a fore-
ground elliptical galaxy embedded in a massive cluster of galaxies.
This observation will help astronomers refine their estimates of the
amount and distribution of dark matter in the lensing galaxy and
cluster. The gravity of both the elliptical galaxy and the galaxy
cluster distorts and magnifies the view of the supernova behind
them, an effect called gravitational lensing. First predicted by
Einstein, the effect is similar to a glass lens bending light to
magnify and distort the image of an object behind it. The multiple
images are arranged around the elliptical galaxy in a cross-shaped
pattern called an Einstein Cross, a name originally given to a
particular multiply imaged quasar, the bright core of an active
galaxy. The elliptical galaxy and its cluster, MACS J1149.6+2223, are
5 billion light-years away, and the supernova behind it is 9.3 billion
light-years away. Although astronomers have discovered dozens of
multiply-imaged galaxies and quasars, they have never seen a supernova
resolved into several images. Astronomers predict that, when the four
images fade away, they will have a rare opportunity to catch a rerun
of the supernova. That is because the current four-image pattern is
only one part of the lensing display. The supernova may have appeared
as a single image some 20 years ago elsewhere in the cluster field,
and it is expected to reappear once more within the next five years.
That prediction is based on computer models of the cluster, which
describe the various paths the supernova light is taking through the
maze of clumpy dark matter in the galactic grouping. Each image takes
a different route through the cluster and arrives at a different time,
owing, in part, to differences in the lengths of the pathways that the
light follows to reach the Earth. The four supernova images captured
by Hubble, for example, appeared within a few days or weeks of each other.
Measuring the time delays between images offers clues to the type of
warping of the space that the supernova's light had to cross, and will
help the astronomers to construct the models that map out the
cluster's mass. While making a routine search of the team's data,
researchers noticed the four images of the exploding star in 2014
November. They had been searching for such highly magnified
supernovae since 2013, and this object is their most spectacular
discovery. The supernova appears about 20 times brighter than its
natural brightness, owing to the combined effects of two overlapping
lenses. The dominant lensing effect is from the massive galaxy
cluster, which focuses the supernova light along at least three
separate paths. A secondary lensing effect occurs because one of
those light paths happens to be precisely aligned with a specific
elliptical galaxy within the cluster. The dark matter of that
individual galaxy then bends and refocuses the light into four more
paths, generating the rare Einstein Cross pattern currently observed.
Astronomers spent a week analyzing the object's light, confirming that
it was the signature of a supernova. They then asked the Keck
Observatory in Hawaii to measure the distance to the supernova's host
galaxy. The astronomers nicknamed the supernova Refsdal in honour of
Norwegian astronomer Sjur Refsdal, who, in 1964, first proposed using
time-delayed images from a lensed supernova to study the expansion of
the Universe.
MASSIVE BLACK HOLE DISCOVERED AT COSMIC DAWN
University of Arizona
Scientists have discovered the brightest quasar in the early Universe,
powered by the most massive black hole yet known from that time. The
discovery of the quasar, named SDSS J0100+2802, marks an important
step in understanding how quasars, the most powerful objects in the
Universe, have evolved from the earliest epoch, 'only' 900 million
years after the Big Bang, which is thought to have happened 13.7
billion years ago. The quasar is at a distance of 12.8 billion
light-years. The quasar dates from a time close to the end of an
important cosmic epoch that astronomers referred to as the 'epoch of
reionization': the cosmic dawn when light from the earliest
generations of galaxies and quasars is thought to have ended the
'cosmic dark ages' and transformed the Universe into how we see it
today. Discovered in 1963, quasars are the most powerful objects
beyond our Milky Way galaxy, beaming vast amounts of energy across
space as the super-massive black holes in their centres suck in matter
from their surroundings. Thanks to the new generation of digital sky
surveys, astronomers have discovered more than 200,000 quasars, with
ages ranging from 0.7 billion years after the Big Bang to today. The
new quasar is seven times brighter than the most distant quasar known
(which is 13 billion light-years away). It harbours a black hole with
mass of 12 billion solar masses, making it to be the most luminous
quasar with the most massive black hole among all the known high-
redshift (very distant) quasars. By comparison, our own Milky Way
galaxy has a black hole with a mass of 'only' 4 million solar masses
at its centre.
Topic: Late March Astronomy Bulletin (Read 1311 times)