IS THE GREAT RED SPOT UNRAVELLING?
Spaceweather.com
Around the world, amateur astronomers are monitoring a strange phenomenon on
the verge of Jupiter's Great Red Spot (GRS). The giant storm appears to be
unravelling. The plume of gas is enormous, stretching more than 10,000 km
from the central storm to a nearby jet stream that appears to be carrying it
away. Currently such a streamer is peeling off every week or so. The Great
Red Spot is the biggest storm in the Solar System --an anticyclone wider
than Earth with winds blowing 350 mph. Astronomers have been observing it
for hundreds of years. In recent decades, the Great Red Spot has been
shrinking. Once it was wide enough to swallow three Earths; now only one of
our planet could fit inside it. That has led some researchers to wonder if
the GRS could break up or disappear within our lifetimes. Perhaps the
streamers are part of that process. In fact, such unravelling clouds have
been seen before. For instance, the Gemini North adaptive optics telescope
on Mauna Kea saw a lesser but similar streamer in May of 2017. Each
streamer appears to disconnect from the Great Red Spot and dissipate. Then,
after about a week, a new streamer forms and the process repeats. You have
to be lucky to catch it happening. Jupiter spins on its axis every 10 hours
and the GRS is not always visible. A joint effort between many amateurs is
underway to get clear images of the process. Now is a great time to monitor
the action. Jupiter is approaching the Earth for an encounter in 2019 June.
In the weeks ahead, Jupiter will shine four times brighter than Sirius, the
brightest star in the sky, and even small telescopes will reveal its storms,
moons, and cloud belts. You can find Jupiter in the constellation Ophiuchus
in the southern sky at midnight.
SMALL PLANETS SURVIVE DEATH OF THEIR STARS
RAS
Astrophysicists modelled the chances of different planets being destroyed by
tidal forces when their host stars become white dwarfs and have determined
the most significant factors that decide whether they avoid destruction.
Their 'survival guide' for exoplanets could help guide astronomers locate
potential exoplanets around white dwarf stars, as a new generation of even
more powerful telescopes is being developed to search for them. Most stars
like our own Sun will run out of fuel eventually and shrink and become white
dwarfs. Some orbiting bodies that aren't destroyed in the maelstrom caused
when the star blasts away its outer layers will then be subjected to shifts
in tidal forces as the star collapses and becomes super-dense. The
gravitational forces exerted on any orbiting planets would be intense and
would potentially drag them into new orbits, even pushing some further out
in their solar systems. By modelling the effects of a white dwarf's change
in gravity on orbiting rocky bodies, the researchers have determined the
most likely factors that will cause a planet to move within the star's
'destruction radius' -- the distance from the star where an object held
together only by its own gravity will disintegrate by tidal forces. Within
the destruction radius a disc of debris from destroyed planets will form.
Although a planet's survival is dependent on many factors, the models reveal
that the more massive the planet, the more likely that it will be destroyed
through tidal interactions. But destruction is not certain based on the
basis of mass alone and depends partly on viscosity, a measure of resistance
to deformation: low viscosity exo-Earths are easily swallowed even if they
reside at separations within five times the distance between the centre of
the white dwarf and its destruction radius. Saturn's moon Enceladus --
often described as a 'dirty snowball' -- is a good example of a homogeneous
very-low-viscosity planet.
High-viscosity exo-Earths are easily swallowed only if they reside at
distances within twice the separation between the centre of the white dwarf
and its destruction radius. Those planets would be composed entirely of a
dense core of heavier elements, with a similar composition to the 'heavy
metal' planet discovered by another team of astronomers recently. That
planet has avoided engulfment because it is as small as an asteroid.
Distance from the star, like the planet's mass, has a robust correlation
with survival or engulfment. There will always be a safe distance from the
star and that distance depends on many parameters. In general, a rocky
homogeneous planet which resides at a location from the white dwarf which is
beyond about one-third of the distance between Mercury and the Sun is
guaranteed to avoid being swallowed from tidal forces.
SUPER-EARTHS PUSHED CLOSE TO THEIR STARS
Penn State
The galaxy is littered with planetary systems vastly different from ours.
In the solar system, the planet closest to the Sun -- Mercury, with an orbit
of 88 days -- is also the smallest. But the Kepler spacecraft has
discovered thousands of systems full of very large planets -- called
super-Earths -- in very small orbits that zip around their host star several
times every 10 days. Now, researchers may have a better understanding how
such planets formed. A team of astronomers found that as planets form out
of the chaotic churn of gravitational, hydrodynamic -- or, drag -- and
magnetic forces and collisions within the dusty, gaseous protoplanetary disc
that surrounds a star as a planetary system starts to form, the orbits of
these planets eventually get in sync, causing them to slide --
follow-the-leader-style -- toward the star. The team's computer simulations
result in planetary systems with properties that match up with those of
actual planetary systems observed by the Kepler space telescope of solar
systems. Both simulations and observations show large, rocky super-Earths
orbiting very close to their host stars. The simulation is a step toward
understanding why super-Earths gather so close to their host stars, and may
also shed light on why super-Earths are often located so close to their host
star where there doesn't seem to be enough solid material in the proto-
planetary disc to form a planet, let alone a big planet. The computer
simulation shows that, over time, the planets' and disc's gravitational
forces lock the planets into synchronized orbits -- resonance -- with each
other. The planets then begin to migrate in unison, with some moving closer
to the edge of the disc. The combination of the gas disc affecting the
outer planets and the gravitational interactions among the outer and inner
planets can continue to push the inner planets closer to the star, even
interior to the edge of the disc.
With the first discoveries of Jupiter-size exoplanets orbiting close to
their host stars, astronomers were inspired to develop multiple models for
how such planets could form, including chaotic interactions in multiple
planet systems, tidal effects and migration through the gas disc. However,
those models did not predict the more recent discoveries of super-Earth-size
planets orbiting so close to their host star. Some astronomers had
suggested that such planets must have formed very near their current
locations. This work demonstrates how short-period super-Earth-size planets
could have formed and migrated to their current locations thanks to the
complex interactions of multiple planet systems. Future research may also
explore why our super-Earthless solar system is different from most other
solar systems. According to the researchers, the best published estimates
suggest that about 30 percent of solar-like stars have some planets closer
to the host star than the Earth is to the Sun. However, they note that
additional planets are could go undetected, especially small planets far
from their star.
STAR FORMATION BURST 2-3 BILLION YEARS AGO
University of Barcelona
A team led by researchers has found, analysing data from the Gaia satellite,
that a severe star-formation burst occurred in the Milky Way about 2 to 3
billion years ago. In that process, more than 50 per cent of the stars that
created the galactic disc may have been born. The results come from the
combination of the distances, colours and magnitude of the stars that were
measured by Gaia with models that predict their distribution in our Galaxy.
Just as a flame fades when there is no gas in the cylinder, the rhythm of
the stellar formation in the Milky Way, fuelled by the gas that was
deposited, should decrease slowly and in a continuous way until it has used
up the existing gas. The results of the study show that, although that was
the process that took place over the first 4 billion years of the disc
formation, a severe star-formation burst, or "stellar baby boom', inverted
that trend. The merging with a satellite galaxy of the Milky Way, which was
rich in gas, could have added new fuel and reactivated the process of
stellar formation. That mechanism would explain the distribution of
distances, ages and masses that are estimated from the data taken from the
European Space Agency Gaia satellite. The time-scale of that star-formation
burst together with the great amount of stellar mass involved in the
process, thousands of millions of solar masses, suggests that the disc of
our Galaxy did not have a steady and paused evolution -- it may have
suffered an external perturbation that began about five US-billion years
ago. Cosmological models predict that our galaxy would have been growing by
merging with other galaxies, a fact that has been stated by other studies
using Gaia data. One of the mergers could be the cause of the severe star-
formation burst that was detected in this study.
UNIVERSE'S FIRST STARS SPEWED POWERFUL JETS
Massachusetts Institute of Technology :
Several hundred million years after the Big Bang, the very first stars
flared into the universe as massively bright accumulations of hydrogen and
helium gas. Within the cores of those first stars, extreme thermonuclear
reactions forged the first heavier elements, including carbon, iron, and
zinc. These first stars were probably immense, short-lived fireballs, and
scientists have assumed that they exploded as similarly spherical
supernovae. But now astronomers have found that those first stars may have
blown apart in a more powerful, asymmetric fashion, spewing forth jets that
were violent enough to eject heavy elements into neighbouring galaxies.
Those elements ultimately served as seeds for the second generation of
stars, some of which can still be observed today. The researchers report a
strong abundance of zinc in HE 1327-2326, an ancient, surviving star that
is among the universe's second generation of stars. They believe that the
star could only have acquired such a large amount of zinc after an
asymmetric explosion of one of the very first stars had enriched its birth
gas cloud. When a star explodes, some proportion of that star gets sucked
into a black hole like a vacuum cleaner. Only when you have some kind of
mechanism, like a jet that can yank out material, can you observe that
material later in a next-generation star. And we believe that's exactly
what could have happened here. This is the first observational evidence
that such an asymmetric supernova took place in the early universe.
HE 1327-2326 was discovered in 2005. At the time, the star was the most
metal-poor one ever observed, meaning that it had extremely low
concentrations of elements heavier than hydrogen and helium -- an indication
that it formed as part of the second generation of stars, at a time when
most of the universe's heavy-element content had yet to be forged. The
first stars were so massive that they had to explode almost immediately.
The smaller stars that formed as the second generation are still available
today, and they preserve the early material left behind by the first stars.
This star has just a sprinkle of elements heavier than hydrogen and helium,
so we know that it must have formed as part of the second generation of
stars. In May of 2016, the team was able to observe the star which orbits
just 5,000 light years away. The researchers used an instrument aboard the
Hubble Space Telescope, the Cosmic Origins Spectrograph, to measure the
minute abundances of various elements in the star. The spectrograph is
designed to pick up faint ultraviolet light. Some of the wavelengths are
absorbed by certain elements, such as zinc. The researchers made a list of
heavy elements that they suspected might be within such an ancient star,
that they planned to look for in the UV data, including silicon, iron,
phosophorus, and zinc. The team found that, no matter how it measured it,
there was a really strong abundance of zinc. The researchers ran over
10,000 simulations of supernovae, each with different explosion energies,
configurations, and other parameters. They found that while most of the
spherical supernova simulations were able to produce a secondary star with
the elemental compositions the researchers observed in HE 1327-2326, none of
them reproduced the zinc signal. As it turns out, the only simulation that
could explain the star's makeup, including its high abundance of zinc, was
one of an aspherical, jet-ejecting supernova of a first star. Such a
supernova would have been extremely explosive, with a power equivalent to
about a nonillion times (that's 1 with 30 zeroes after it) that of a
hydrogen bomb. The team's results may shift scientists' understanding of
reionization, a pivotal period during which the gas in the universe morphed
from being completely neutral, to ionized -- a state that made it possible
for galaxies to take shape. Those first supernovae could also have been
powerful enough to shoot heavy elements into neighbouring 'virgin galaxies'
that had yet to form any stars of their own. Once you have some heavy
elements in a hydrogen and helium gas, you have a much easier time forming
stars, especially little ones. The working hypothesis is, maybe second
generation stars of that kind formed in those polluted virgin systems, and
not in the same system as the supernova explosion itself, which is always
what we had assumed, without thinking in any other way. So this is opening
up a new channel for early star formation.
HOW ANCIENT GALAXIES LIT UP THE UNIVERSE
RAS
The Spitzer Space Telescope has revealed that some of the Universe's
earliest galaxies were brighter than expected. The excess light is a
by-product of the galaxies releasing incredibly high amounts of ionizing
radiation. The finding offers clues to the cause of the Epoch of
Reionization, a major cosmic event that transformed the Universe from being
mostly opaque to the brilliant starscape seen today. Researchers report on
observations of some of the first galaxies to form in the Universe, less
than 1 billion years after the Big Bang (or a little more than 13 billion
years ago). The data show that in a few specific wavelengths of infrared
light, the galaxies are considerably brighter than scientists anticipated.
The study is the first to confirm that phenomenon for a large sampling of
galaxies from that period, showing that those were not special cases of
excessive brightness, but that even average galaxies present at that time
were much brighter in those wavelengths than galaxies we see today. No one
knows for sure when the first stars in our Universe burst into life. But
evidence suggests that between about 100 million and 200 million years after
the Big Bang, the Universe was filled mostly with neutral hydrogen gas that
had perhaps just begun to coalesce into stars, which then began to form the
first galaxies. By about 1 billion years after the big bang, the Universe
had become a sparkling firmament. Something else had changed, too:
electrons of the omnipresent neutral hydrogen gas had been stripped away in
a process known as ionization. The Epoch of Reionization -- the changeover
from a universe full of neutral hydrogen to one filled with ionized hydrogen
-- is well documented.
Before that Universe-wide transformation, long-wavelength forms of light,
such as radio waves and visible light, traversed the universe more or less
unencumbered. But shorter wavelengths of light -- including ultraviolet
light, X-rays and gamma rays -- were stopped short by neutral hydrogen
atoms. Those collisions would strip the neutral hydrogen atoms of their
electrons, ionizing them. But what could possibly have produced enough
ionizing radiation to affect all the hydrogen in the Universe? Was it
individual stars? Giant galaxies? If either were the culprit, those early
cosmic colonisers would have been different from most modern stars and
galaxies, which typically do not release high amounts of ionizing radiation.
Then again, perhaps something else entirely caused the event, such as
quasars -- galaxies with incredibly bright centres powered by huge amounts
of material orbiting supermassive black holes.
To peer back in time to the era just before the Epoch of Reionization ended,
Spitzer stared at two regions of the sky for more than 200 hours each,
allowing it to collect light that had travelled for more than 13 billion
years to reach us. As some of the longest observations ever carried out by
Spitzer, they were part of an observing campaign called GREATS, short for
GOODS Re-ionization Era wide-Area Treasury from Spitzer. OODS (itself an
acronym: Great Observatories Origins Deep Survey) is another campaign that
performed the first observations of some GREATS targets. The study also used
archival data from the NASA / ESA Hubble Space Telescope. Using these
ultra-deep observations by Spitzer, the team of astronomers observed 135
distant galaxies and found that they were all particularly bright in two
specific wavelengths of infrared light produced by ionizing radiation
interacting with hydrogen and oxygen gases within the galaxies. This
implies that these galaxies were dominated by young, massive stars composed
mostly of hydrogen and helium. They contain very small amounts of 'heavy'
elements (like nitrogen, carbon and oxygen) compared to stars found in
average modern galaxies. These stars were not the first stars to form in
the Universe (those would have been composed of hydrogen and helium only)
but were still members of a very early generation of stars. The Epoch of
Reionization wasn't an instantaneous event, so while the new results are not
enough to close the book on that cosmic event, they do provide new details
about how the Universe evolved at that time and how the transition played
out.
Topic: Late May Astronomy Bulletin (Read 1095 times)