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Author Topic: Early May Astronomy Bulletin  (Read 504 times)

Offline Clive

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Early May Astronomy Bulletin
« on: May 05, 2018, 21:26 »

So far in 2018, the Sun has been blank more than half the time.  Whole weeks
have gone by without a single sunspot.  Although forecasters have been
expecting sunspots to disappear with the approach of Solar Minimum, it is
happening faster than predicted.  Solar minimum is of course a normal part
of the sunspot cycle.  Sunspots have been disappearing (or nearly so) every
~11 years since 1843, when German astronomer Samuel Heinrich Schwabe
discovered the periodic nature of solar activity.  Sometimes they go away
for decades, as happened during the Maunder Minimum of the 17th century.
We've seen it all before.  Or have we?  Researchers are keeping a wary eye
on the Sun now because of what happened the last time sunspots disappeared.
The solar minimum of 2008-2009 was unusually deep.  The Sun set Space-Age
records for low sunspot number, weak solar wind, and depressed solar
irradiance.  When the Sun finally woke up a few years later, it seemed to
have 'solar minimum hangover'.  The bounce-back Solar Max of 2012-2015 was
the weakest solar maximum of the Space Age, prompting some to wonder if
solar activity is entering a phase of sustained quiet.  The faster-than-
expected decline of the sunspot cycle now may support that idea.

Newcomers to the field are often surprised to learn that a lot happens
during solar minimum: the Sun dims, albeit slightly.  NASA recently launched
a new sensor (TSIS-1) to the International Space Station to monitor that
effect.  With less extreme-UV radiation coming from the Sun, the Earth's
upper atmosphere cools and shrinks.  That allows space junk to accumulate in
low Earth orbit.  The most important change, however, could be the increase
in cosmic rays.  Flagging solar wind pressure during solar minimum allows
cosmic rays from deep space to penetrate the inner Solar System.  Right now,
space-weather balloons and NASA spacecraft are measuring an increase in
radiation owing to that effect.  Cosmic rays may alter the chemistry of
the Earth's upper atmosphere, trigger lightning, and seed clouds.  Air
travellers are affected, too.  It is well known that cosmic rays penetrate
aeroplanes.  Passengers on long commercial flights receive doses similar to
dental X-rays during a single trip, while pilots have been classified as
occupational radiation workers by the International Commission on
Radiological Protection (ICRP).  Ongoing measurements by Spaceweather.com
and Earth to Sky Calculus show that dose rates at cruising altitudes of
35,000 feet are currently ~40 times greater than on the ground below, values
which could increase as the solar cycle wanes.

Southwest Research Institute
Scientists posit a violent birth of the tiny Martian moons Phobos and
Deimos, but on a much smaller scale than the giant impact thought to have
resulted in the Earth-Moon system.  Their work shows that an impact between
proto-Mars and a dwarf-planet-sized object probably produced the two moons.
The origin of the Red Planet's small moons has been debated for decades.
The question is whether the bodies were asteroids captured intact by Mars'
gravity or whether the tiny satellites formed from an equatorial disc of
debris, as is most consistent with their nearly circular and co-planar
orbits.  The production of a disc by an impact with Mars seemed promising,
but prior models of that process were limited by low numerical resolution
and over-simplified modelling techniques.  The new Mars model invokes a much
smaller impactor than considered previously.  Our Moon may have formed when
a Mars-sized object crashed into the nascent Earth 4.5 billion years ago,
and the resulting debris coalesced into the Moon.  The Earth's diameter is
about 8,000 miles, while Mars' diameter is just over 4,200 miles.  The Moon
is just over 2,100 miles in diameter, about one-fourth the size of the
Earth.  While they formed in the same time frame, Deimos and Phobos are very
small, with diameters of only 7.5 miles and 14 miles respectively, and orbit
very close to Mars.  The proposed Phobos-Deimos-forming impactor would be
between the size of the asteroid Vesta, which has a diameter of 326 miles,
and the dwarf planet Ceres, which is 587 miles across.  These findings are
important for the Japan Aerospace Exploration Agency (JAXA) Mars Moons
eXploration (MMX) mission, which is planned to be launched in 2024 and will
include a NASA-provided instrument.  The MMX spacecraft will visit the two
Martian moons, land on the surface of Phobos and collect a surface sample to
be returned to Earth in 2029.


Even after decades of observations and a visit by NASA's Voyager 2
spacecraft, Uranus held on to one critical secret -- the composition of its
clouds.  Now, one of the key components of the planet's clouds has finally
been verified.  A global research team has analyzed an infrared spectrum of
Uranus obtained by the 8-metre Gemini North telescope on Hawaii's Mauna Kea.
They found hydrogen sulphide in Uranus' cloud tops.  The detection of
hydrogen sulphide high in Uranus' cloud deck (and presumably Neptune's) is a
striking difference from the gas-giant planets located closer to the Sun --
Jupiter and Saturn -- where ammonia is observed above the clouds, but no
hydrogen sulphide.  Those differences in atmospheric composition shed light
on questions about the planets' formation and history.  The Gemini data,
obtained with the Near-Infrared Integral Field Spectrometer (NIFS), sampled
reflected sunlight from a region immediately above the main visible cloud
layer in Uranus' atmosphere.  While the lines astronomers were trying to
detect were just barely there, they were detected unambiguously.

University of Sydney
Astronomers have revealed the 'DNA' of more than 340,000 stars in the Milky
Way, which should help them find the siblings of the Sun, now scattered
across the sky.  That is a major announcement from an ambitious Galactic
Archaeology survey, called GALAH, launched in late 2013 as part of a quest
to uncover the formulation and evolution of galaxies.  When complete, GALAH
will have investigated more than a million stars.  The GALAH survey used the
HERMES spectrograph at the 3.9-metre Anglo-Australian Telescope near Coona-
barabran, to collect spectra for the 340,000 stars.  The 'DNA' collected
traces the ancestry of stars, showing astronomers how the Universe went from
having only hydrogen and helium -- just after the Big Bang -- to being
filled today with all the elements we have here on Earth that are necessary
for life.  The team explained that the Sun, like all stars, was born in a
group or cluster of thousands of stars.  Every star in that cluster would
have the same chemical composition, or DNA; the clusters are quickly pulled
apart by our Milky Way Galaxy and are now scattered across the sky.  The
GALAH team's aim is to make DNA matches between stars to find their long-
lost siblings.  For each star, its DNA is the amount it contains of each of
nearly two dozen chemical elements such as oxygen, aluminium, and iron.

Astronomers collect the DNA of a star by spectroscopy.  The team has trained
a computer code named The Cannon to recognize patterns in the spectra of a
subset of stars that have been analyzed very carefully, and then use The
Cannon's machine-learning algorithms to determine the amount of each element
for all of the 340,000 stars.  The Cannon is named for Annie Jump Cannon, a
pioneering American astronomer who classified the spectra of around 340,000
stars by eye over several decades a century ago.  The GALAH survey's data
release was timed to coincide with the huge release of data from the
European Gaia satellite, which has mapped more than 1600 million stars in
the Milky Way -- making it by far the biggest and most accurate atlas of the
night sky to date.  In combination with velocities from GALAH, Gaia data
will give not just the positions and distances of the stars, but also their
motions within the Galaxy.


Circumbinary planets are planets that orbit two stars instead of just one.
Dozens of such planets have so far been discovered, but working out whether
they may be habitable or not can be difficult.  Moving around two stars
instead of just one can lead to large changes in a planet's orbit, which
means that in many cases it is either ejected from the system entirely, or
it crashes violently into one of the two stars.  Traditional approaches to
calculating which of those occurs for a given planet get significantly more
complicated as soon as the extra star is thrown into the mix.  When
astronomers simulated millions of possible planets with different orbits
using traditional methods, they found that some planets that were being
predicted as stable that were clearly not, and vice versa.  Planets need to
survive for (US)-billions of years in order for life to evolve, so finding
out whether orbits are stable or not is an important question for
habitability.  The new work shows how machine learning can make accurate
predictions even if the standard approach -- based on Newton's laws of
gravity and motion -- breaks down.  Classification with numerous complex,
inter-connected parameters is the perfect problem for machine learning.
After creating ten million hypothetical circumbinary planets with different
orbits, and simulating each one to test for stability, that huge training
set was fed into the deep-learning network.  Within just a few hours, the
network was able to out-perform the accuracy of the standard approach. 
More circumbinary planets are likely to be discovered by NASA's Transiting
Exoplanet Survey Satellite (TESS) mission.

NASA/Goddard Space Flight Center

Seventeen years ago, astronomers witnessed a supernova go off 40 million
light-years away in the galaxy called NGC 7424, located in the southern
constellation Grus.  Now, in the fading afterglow of that explosion, the
Hubble Space Telescope has captured the first image of a surviving companion
to a supernova.  It is the most compelling evidence that some supernovae
originate in double-star systems.  We know that the majority of massive
stars are in binary pairs.  Many of those pairs will interact and transfer
gas from one star to the other when their orbits bring them close together.
The companion to the supernova's progenitor star was no innocent bystander
to the explosion.  It siphoned off almost all of the hydrogen from the
doomed star's stellar envelope, the region that transports energy from the
star's core to its atmosphere.  Millions of years before the primary star
went supernova, the companion's thievery created an instability in the
primary star, causing it episodically to blow off a cocoon and shells of
hydrogen gas.  The supernova, called SN 2001ig, is categorized as a Type
IIb stripped-envelope supernova.  That type of supernova is unusual because
most, but not all, of the hydrogen is gone prior to the explosion.  How
stripped-envelope supernovae lose that outer envelope is not entirely
clear.  They were originally thought to come from single stars with very
fast winds that pushed off the outer envelopes.  The problem was that when
astronomers started looking for the primary stars from which supernovae
were spawned, they couldn't find them for many stripped-envelope supernovae.
That was especially bizarre, because astronomers expected that they would be
the most massive and the brightest progenitor stars.  Also, the sheer number
of stripped-envelope supernovae is greater than predicted.  That fact led
scientists to theorize that many of the primary stars were in lower-mass
binary systems, and they set out to prove it.

Looking for a binary companion after a supernova explosion is no easy task.
First, it has to be at a relatively close distance to the Earth for Hubble
to see such a faint star.  SN 2001ig and its companion are about at that
limit.  Within that distance range, not many supernovae go off.  Even more
importantly, astronomers have to know the exact position through very
precise measurements.  In 2002, shortly after SN 2001ig exploded, scientists
pinpointed the precise location of the supernova with the European Southern
Observatory's Very Large Telescope (VLT) in Chile.  In 2004, they then
followed up with the Gemini South Observatory in Chile.  That observation
first hinted at the presence of a surviving binary companion.  Knowing the
exact coordinates, astronomers were able to focus Hubble on that location
12 years later, as the supernova's glow faded.  With Hubble's ultraviolet
capability, they were able to find and photograph the surviving companion.
Before the supernova explosion, the orbit of the two stars around each
other took about a year.  When the primary star exploded, it had far less
impact on the surviving companion than might be thought.  Imagine an avocado
pit -- representing the dense core of the companion star -- embedded in a
gelatin dessert -- representing the star's gaseous envelope.  As a shock
wave passes through, the gelatin might temporarily stretch and wobble, but
the avocado pit would remain intact.  In 2014, astronomers used Hubble to
detect the companion of another Type IIb supernova, SN 1993J.  However, they
captured a spectrum, not an image.  The case of SN 2001ig is the first time
a surviving companion has been photographed.  Perhaps as many as half of all
stripped-envelope supernovae have companions -- the other half lose their
outer envelopes via stellar winds.  The next endeavour is to look at
completely-stripped-envelope supernovae, as opposed to SN 2001ig and SN
1993J, which were only about 90 per cent stripped.  Completely-stripped-
envelope supernovae don't have much shock interaction with gas in the
surrounding stellar environment, since their outer envelopes were lost long
before the explosion.  Without shock interaction, they fade much more
quickly.  That means that the team will probably have to wait only two or
three years to look for surviving companions.

Osaka University

Astronomers have found a new fundamental law that stipulates the evolution
of galaxy clusters.  Galaxy clusters are the largest celestial bodies in the
Universe.  However, it has been difficult to measure their sizes and masses
accurately because they mainly consist of dark matter that we cannot observe
directly.  One way to observe the dark matter indirectly is to use the
gravitational lensing effect based on Einstein's theory of relativity.
Light rays from a galaxy behind a cluster are pulled by the gravity of the
cluster as they pass through it, and their paths are bent.  That is exactly
the same effect as a lens, focussing the light of the distant galaxy and
distorting its shape.  If we can measure the distortion of the shape for
many background galaxies, we can reveal the gravitational field of the
cluster, and as a result, we can accurately estimate its size and mass.
Combining such estimates with gas-temperature data from the Chandra X-ray
satellite, the research group statistically examined the latest data and
found that they conform to a simple law involving only the size, mass, and
gas temperature of clusters.  Moreover, by making use of computer simula-
tions, they showed that clusters have grown over 4 to 8 US-billion years
according to the law.  Theoretically, the law means that those gigantic
clusters are still in adolescence, growing by drawing in a large amount of
surrounding material with their strong gravity.  The law is so simple that
we can use it to calibrate cluster-mass-observable relationships, which are
a key ingredient for studying the cosmological laws of the Universe.


The ALMA and APEX telescopes have looked deep into space -- back to the
time when the Universe was a tenth of its current age -- and witnessed the
beginnings of gargantuan cosmic pileups: the impending collisions of young,
starburst galaxies.  Astronomers thought that those events occurred around
three billion years after the Big Bang, so they were surprised when the new
observations revealed them happening when the Universe was only half that
age.  Those ancient systems of galaxies are thought to be building the most
massive structures in the known Universe: galaxy clusters.  Using the
Atacama Large Millimetre/submillimetre Array (ALMA) and the Atacama
Pathfinder Experiment (APEX), scientists have uncovered startlingly dense
concentrations of galaxies that are poised to merge, forming the cores of
what will eventually become colossal galaxy clusters.  Looking 90% of the
way across the observable Universe, the scientists observed a galaxy
proto-cluster named SPT2349-56.  The light from that object began travelling
towards us when the Universe was only about a tenth of its current age.  The
individual galaxies in that dense cosmic pileup are starburst galaxies, and
the concentration of vigorous star formation in such a compact region makes
it by far the most active region ever observed in the young Universe.
Thousands of stars are born there every year, compared to just one in our
own Milky Way.  Astronomers discovered a similar mega-merger formed by ten
dusty star-forming galaxies, nicknamed a 'dusty red core' because of its
very red colour, by combining observations from ALMA and the APEX.  The
lifetime of dusty starbursts is thought to be relatively short, because
they consume their gas at an extraordinary rate.  At any given time, in any
corner  of the Universe, such galaxies are usually in the minority.  So,
finding numerous dusty starbursts shining at the same time like this is
very puzzling, and something that we still need to understand.

Those forming galaxy clusters were first observed as faint smudges of light
by the South Pole Telescope and the Herschel Space Observatory.  Subsequent
ALMA and APEX observations showed that they had unusual structure and
confirmed that their light originated much earlier than expected, 'only'
1.5 billion years after the Big Bang.  The new high-resolution ALMA
observations finally revealed that the two faint glows are not single
objects, but are actually composed of fourteen and ten individual massive
galaxies respectively, each within a radius comparable to the distance
between the Milky Way and the neighbouring Magellanic Clouds.  Those
discoveries by ALMA are only the tip of the iceberg.  Additional observ-
ations with the APEX telescope show that the real number of star-forming
galaxies is likely to be even three times higher.  Ongoing observations
with the MUSE instrument on ESO's VLT are also identifying additional
galaxies.  Current theoretical and computer models suggest that proto-
clusters as massive as these should have taken much longer to evolve.  By
using data from ALMA, the researchers are able to study cluster formation
less than 1.5 billion years after the Big Bang.  How that assembly of
galaxies got so big so fast is a mystery.  It was not built up gradually
over billions of years, as astronomers might expect.  The discovery
provides a great opportunity to study how massive galaxies came together
to build enormous galaxy clusters.

Winner BBC Quiz of the Year 2015, 2016 and yet again in 2017.

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