APPROACHING GREEN COMET GETS BRIGHTER
Spaceweather.com
A comet that could become visible to the naked eye in August has just exploded in brightness. Comet PANSTARRS (C/2017 S3) brightened 16-fold during the late hours of July 2nd, abruptly increasing in magnitude from +12 to +9. The gas cloud around the comet's nucleus is about 4 arc minutes wide. That means the comet's atmosphere is 260,000 km in diameter, almost twice as wide as the planet Jupiter. These dimensions make it a relatively easy target for garden telescopes. Comet PanSTARRS is falling toward the Sun from the Oort cloud, a vast reservoir of fresh comets in the distant outer solar system. It has never visited the inner planets before, and, as a result, no one can say what will happen when its fragile ices are exposed to solar heat as it approaches the sun in August. Previous estimates of the comet's brightness max out at magnitude +4--that is, barely visible to the unaided eye from dark-sky sites. Additional outbursts could boost its visibility even more. The comet was discovered on Sept. 23, 2017, by the PanSTARRS telescope on the summit of the Haleakala volcano in Maui. PanSTARRS's primary mission is to detect near-Earth asteroids that threaten our planet. In the process,it sweeps up variable stars, supernovas, and comets like this one. With almost a year of data in hand, astronomers have been able to nail down the comet's orbit. Comet PanSTARRS is approaching the sun on a hyperbolic orbit--a narrow open-ended path that will ultimately fling it back to the outer solar system. At perihelion (closest approach to the Sun) on August 15-16, the comet will be inside the orbit of Mercury, blasted by solar radiation at point-blank range.
NEW ORBIT FOR CERES MISSION
NASA
NASA's Dawn spacecraft is manoeuvring to its lowest-ever orbit for a
close-up examination of Ceres, the inner-Solar-System's only dwarf planet.
In early June, Dawn reached its new, final orbit above Ceres. Soon after,
it began collecting images and other scientific data from a vantage point
less than 50 kilometres above the surface of Ceres -- 10 times closer than
it had ever previously been. Dawn will collect gamma-ray and neutron
spectra, which help scientists to understand variations in the chemical
make-up of Ceres' uppermost layer. The very low orbit will also garner
some of Dawn's closest images yet. Engineers mapped out many possible
trajectories before devising a plan that will allow the best scientific
observations. Dawn was launched in 2007 and has been exploring the two
largest bodies in the main asteroid belt, Vesta and Ceres. It entered an
orbit around Ceres in 2015 March.
SURFACE PROPERTIES OF OVER 100 ASTEROIDS REVEALED
NASA
Nearly all asteroids are so far away and so small that the astronomical
community knows them only as moving points of light. The exceptions are
asteroids that have been visited by spacecraft, a small number of large
asteroids resolved by the Hubble Space Telescope or large ground-based
telescopes, or those that have come close enough for radar imaging. When
seen by optical telescopes, these individual sources of reflected sunlight
can provide some very valuable but also very basic information -- for
example, the asteroid's orbit, a rough estimate of its size, sometimes an
approximation of its shape, and perhaps an idea of its physical makeup.
But to learn more about them requires a different type of instrument.
An infrared sensor can, in the right circumstances, not only provide
data on an asteroid's orbit and data that can be used to measure the
object's size accurately, but also its chemical makeup and sometimes even
its surface characteristics. NASA's Near-Earth Object Wide-field Infrared
Survey Explorer, or NEOWISE, spacecraft, in orbit around the Earth, uses
asteroid-hunting thermal sensors that obtain infrared views of asteroids
without the obscuring effects of the Earth's atmosphere. Astronomers have
made an in-depth analysis of more than 100 asteroids that have come under
the temperature-sensing gaze of NEOWISE. That analysis tripled the number
of asteroids which have undergone detailed 'thermophysical' modelling of
asteroid properties that vary with temperature. The results provide a
more accurate glimpse into the surface properties of main-belt asteroids
and also reinforce the capabilities of spaceborne infrared observatories
accurately to assess the sizes of asteroids.
Thermophysical modelling is a gold mine for asteroid researchers because it
allows a more comprehensive analysis of the nature of asteroids. Not all
asteroids are suitable for thermophysical modelling because the necessary
raw data sets are not always available. But the team found 122 asteroids
that not only had NEOWISE data, but also detailed models of their rotation
states (how fast an object rotates around its axis, and the orientation of
the axis in space) and multi-faceted models of the asteroids' 3D shapes.
Astronomers now have a better idea of the properties of the surface regolith
and know that small asteroids, as well as fast-rotating ones, have little,
if any, dust covering their surfaces. (Regolith is the term for broken
rocks and dust on the surface.) It could be difficult for fast-rotating
asteroids to retain very fine regolith grains because their low gravity and
high spin rates tend to fling small particles off their surfaces and into
space. Also, it could be that fast-rotating asteroids do not experience
large temperature changes because the Sun's rays are more rapidly
distributed across their surfaces. That would reduce or prevent the thermal
cracking of an asteroid's surface material that could cause the generation
of fine grains of regolith. The team also found that their detailed
calculations for estimated sizes of the asteroids they studied were
consistent with those of the same asteroids calculated by the NEOWISE team
using simpler models. With the asteroids for which they were able to gather
the most information from other sources, their calculations of sizes were
consistent with the radiometrically derived values found by the NEOWISE
team. The discrepancies between the two sets of results were no more
than 10%.
Originally called the Wide-field Infrared Survey Explorer (WISE), the
spacecraft was launched in 2009 December to study galaxies, stars, and
solar- system bodies by imaging the entire sky in infrared light. It was
placed in hibernation in 2011 after its primary astrophysics mission was
completed. In 2013 September it was reactivated, renamed NEOWISE and
assigned a new mission: to assist NASA's efforts to identify and
characterize the population of near-Earth objects. NEOWISE is also
characterizing more distant populations of asteroids and comets to provide
information about their sizes and compositions.
JUNO'S JUPITER MISSION RE-PLANNED
NASA
Plans have been approved for an update to Juno's science operations until
2021 July. They provide for an additional 41 months in orbit around Jupiter
and will enable Juno to achieve its primary science objectives. Juno is in
53-day orbits rather than the 14-day orbits that were initially planned
because of concerns about valves on the spacecraft's fuel system. The
longer orbit means that it will take more time to collect the desired
scientific data. Experts confirmed in April that Juno is on track to
achieve its scientific objectives and is already returning spectacular
results. The Juno spacecraft and all instruments are healthy and operating
normally.
`OUMUAMUA IS A COMET
ESO
`Oumuamua -- the first interstellar object discovered within our Solar
System -- has been the subject of intense scrutiny since its discovery in
2017 October. Now, by combining data from ESO's Very Large Telescope and
other observatories, an international team of astronomers has found that the
object is moving faster than predicted. The measured gain in speed is tiny
and `Oumuamua is still slowing down because of the pull of the Sun -- just
not as fast as predicted by celestial mechanics. The team considered
several possible explanations of the faster-than-predicted speed of this
peculiar interstellar visitor. The most likely one is that `Oumuamua is
venting material from its surface as a result of solar heating -- a
behaviour known as outgassing. The thrust from the ejected material is
thought to provide the small but steady push that is sending `Oumuamua
hurtling out of the Solar System faster than expected -- as of 2018 July
it is travelling at roughly 114,000 kilometres per hour. Such outgassing is
typical behaviour for comets and contradicts the previous classification of
`Oumuamua as an interstellar asteroid. The data show that its boost is
getting smaller the farther away it travels from the Sun, which is typical
for comets. Usually, when comets are warmed by the Sun they eject dust and
gas, which form a cloud of material -- called a coma -- around them, as well
as the characteristic tail. However, the research team could not detect any
visual evidence of outgassing. The team speculated that perhaps small dust
grains such as adorn the surfaces of most comets were eroded away during
`Oumuamua's journey through interstellar space, with only larger grains
remaining. Though a cloud of such larger particles would not be bright
enough to be detected, it would explain the unexpected change to 'Oumuamua's
speed. Not only is `Oumuamua's hypothesized outgassing an unsolved mystery,
but so also is its interstellar origin. The team originally performed the
new observations on `Oumuamua to determine its path, which probably would
have allowed it to trace the object back to its parent star system. The new
result means it will be more challenging to obtain that information.
PLANET 9 MAY BE JUST 'COLLECTIVE GRAVITY'
University of Colorado at Boulder
A team of researchers has offered a new theory for the existence of
planetary oddities like Sedna. That minor planet orbits the Sun at a
distance of 8 billion miles but appears separated from the rest of the
Solar System. One theory for its unusual dynamics is that an as-yet-unseen
ninth planet beyond Neptune may have disturbed the orbits of Sedna and other
detached objects. But the team calculated that the orbits of Sedna and its
ilk may result from such bodies jostling against each other and space
debris in the outer Solar System. Detached objects like Sedna get their
name because they complete enormous, circular orbits that bring them nowhere
close to big planets like Jupiter or Neptune. How they got to the outer
Solar System on their own is an ongoing mystery.
Using computer simulations, scientists calculated that those icy objects
orbit the Sun like the hands of a clock. The orbits of smaller objects,
such as asteroids, however, move faster than the larger ones, such as Sedna.
The orbits of smaller objects pile up to one side of the Sun and crash into
the bigger body, and what happens is those interactions will change its
orbit from an oval shape to a more circular shape. In other words, Sedna's
orbit goes from normal to detached entirely because of such small-scale
interactions. The team's observations also fall in line with research from
2012, which observed that the bigger a detached object gets, the farther
away its orbit becomes from the Sun. The findings may also provide clues
around another phenomenon: the extinction of the dinosaurs. As space debris
interact in the outer Solar System, the orbits of those objects tighten and
widen in a repeating cycle. The cycle could wind up shooting comets toward
the inner Solar System -- including in the direction of the Earth -- on a
predictable time-scale.
GLOBULAR CLUSTERS YOUNGER
University of Warwick
Astronomers say that globular clusters could be up to 4 billion years
younger than previously thought. Newly developed research models show that
they could be as young as 9 billion years old rather than 13 billion. The
discovery calls into question current theories on how galaxies, including
the Milky Way, were formed -- with between 150-180 clusters thought to exist
in the Milky Way alone -- as globular clusters had previously been thought
to be almost as old as the Universe itself. Designed to reconsider the
evolution of stars, the new Binary Population and Spectral Synthesis (BPASS)
models take the details of binary-star evolution within the globular cluster
into account and are used to explore the colours of light from old binary
star populations -- as well as the traces of chemical elements seen in their
spectra. The evolutionary process sees two stars interacting in a binary
system, where one star expands into a giant whilst the gravitational force
of the smaller star strips away the atmosphere, comprising hydrogen and
helium amongst other elements, of the giant. These stars are thought to be
formed at the same time as the globular cluster itself. Through using the
BPASS models and calculating the age of the binary-star systems the
researchers demonstrated that the globular cluster of which they are part
was not as ancient as other models had suggested.
The BPASS models had previously proved effective in exploring the properties
of young stellar populations in environments ranging from our Milky Way all
the way out to the edge of the Universe. Determining ages for stars has
always depended on comparing observations with the models which encapsulate
our understanding of how stars form and evolve. That understanding has
changed over time, and we have been increasingly aware of the effects of
stellar multiplicity -- the interactions between stars and their binary and
tertiary companions. The research's findings point to new avenues of
enquiry into how massive galaxies, and the stars contained within, are
formed. It is important to note that there is still a lot of work to do --
in particular looking at those very 'nearby' systems where astronomers can
resolve individual stars rather than just considering the integrated light
of a cluster -- but this is an interesting and intriguing result. If true,
it changes our picture of the early stages of galaxy evolution and where the
stars that have ended up in today's massive galaxies, such as the Milky Way,
may have formed.
DISC OF MILKY WAY IS BIGGER THAN HAS BEEN THOUGHT
Instituto de Astrofisica de Canarias (IAC)
Spiral galaxies such as the Milky Way have discs, which in comparison with
their diameters are really thin, in which the major fraction of their stars
are found. The discs are limited in size, so beyond a certain radius there
are very few stars found. In our Galaxy we were not aware that there are
stars in the disc at distances from the centre more than twice that of the
Sun. That means that our own star has apparently been orbiting at about
half the Galactic radius. However now we know that there are stars quite a
bit further out, at more than three times this distance, and it is probable
that some stars are at more than four times the distance of the Sun from the
Galactic centre. In broad terms we can think of galaxies like the Milky Way
as being composed of a rotating disc, which includes spiral arms, and a
halo, spherical in shape, which surrounds it. This piece of research has
compared the abundances of metals (heavy elements) in the stars of the
Galactic plane with those of the halo, to find that there is a mixture of
disc and halo stars out to the large distances indicated. The researchers
came to those conclusions after making a statistical analysis of survey data
from APOGEE and LAMOST, two projects which obtain spectra of stars to
extract information about their velocities and their chemical compositions.
Using the metallicities of the stars in the catalogues from the spectral
atlases of APOGEE and LAMOST, and with the distances at which the objects
are situated, astronomers have shown that there is an appreciable fraction
of stars with higher metallicity, characteristic of disc stars, further out
than the previously assumed limit on the radius of the Galaxy disc.
MASSIVE STARS IN STARBURST GALAXIES
ESO
Probing the distant Universe, a team of scientists used the Atacama Large
Millimetre/submillimetre Array (ALMA) to investigate the proportion of
massive stars in four distant gas-rich starburst galaxies. Those galaxies
are seen as they were when the Universe was much younger than it is now, so
the infant galaxies are unlikely to have undergone many previous episodes of
star formation, which might otherwise have confused the results. The team
developed a new technique -- analogous to radiocarbon dating -- to measure
the abundances of different types of carbon monoxide in four very distant,
dust-shrouded starburst galaxies. They observed the ratio of two types of
carbon monoxide containing different isotopes. Carbon and oxygen isotopes
have different origins. Oxygen-18 is produced more in massive stars, and
carbon-13 is produced more in low- to intermediate-mass stars. Thanks to
the new technique the team was able to observe through the dust in those
galaxies and assess for the first time the masses of their stars. The mass
of a star is the most important factor determining how it will evolve.
Massive stars shine brilliantly and have short lives, and less massive ones,
such as the Sun, shine more modestly for billions of years. Knowing the
proportions of stars of different masses that are formed in galaxies
therefore underpins astronomers' understanding of the formation and
evolution of galaxies throughout the history of the Universe. Consequently,
it gives us crucial insights about the chemical elements available to form
new stars and planets and, ultimately, the number of seed black holes that
may coalesce to form the supermassive black holes that we see in the centres
of many galaxies.
The team found that the ratio of 18O to 13C was about 10 times higher in
starburst galaxies in the early Universe than it is in galaxies such as the
Milky Way, meaning that there is a much higher proportion of massive stars
within the starburst galaxies. The ALMA finding is consistent with another
discovery in the local Universe. A team from the University of Oxford made
spectroscopic measurements with ESO's VLT of 800 stars in the gigantic
star-forming region 30 Doradus in the Large Magellanic Cloud in order to
investigate the overall distribution of stellar ages and initial masses. It
found around 30% more stars with masses more than 30 times that of the Sun
than expected, and about 70% more than expected above 60 solar masses. The
results challenge the previously predicted 150-solar-mass limit for the
maximum birth mass of stars and even suggest that stars could have birth
masses up to 300 solar masses. The findings lead us to question our under-
standing of cosmic history. Astronomers building models of the Universe
must now go back to the drawing board, with yet more sophistication
required.
EINSTEIN TESTED OUTSIDE MILKY WAY
ESO
Astronomers using the MUSE instrument on ESO's Very Large Telescope in
Chile, and the Hubble Space Telescope, have made the most precise test yet
of Einstein's general theory of relativity outside the Milky Way. The
nearby galaxy ESO 325-G004 acts as a strong gravitational lens, distorting
light from a distant galaxy behind it to create an Einstein ring around its
centre. By comparing the mass of ESO 325-G004 with the curvature of space
around it, the astronomers found that gravity on those astronomical
length-scales behaves as predicted by general relativity. That rules out
some alternative theories of gravity. The Muse team first determined the
mass of ESO 325-G004 by measuring how fast its stars were moving. But the
team was also able to measure another aspect of gravity. Using the Hubble
Telescope, they observed an Einstein ring resulting from light from a
distant galaxy being distorted by the intervening ESO 325-G004. Observing
the ring allowed the astronomers to measure how light, and therefore
space-time, is being distorted by the huge mass of the intervening galaxy.
Einstein's general theory of relativity predicts that objects deform
space-time around them, causing any light that passes by to be deflected.
That results in the phenomenon known as gravitational lensing. That effect
is only noticeable for very massive objects. A few hundred strong gravita-
tional lenses are known, but most are too distant for precise measurements
of their mass. However, the galaxy ESO 325-G004 is one of the closest
lenses, at 'only' 450 million light-years from us. Astronomers know the
mass of the foreground galaxy from MUSE and measured the amount of gravi-
tational lensing seen from Hubble. They then compared those two ways to
measure the strength of gravity -- and the result was just what general
relativity predicts, with an uncertainty of only 9%. This is the most
precise test of general relativity outside the Milky Way to date -- and
this is using just one galaxy!
General relativity has been tested with exquisite accuracy on Solar-System
scales, and the motions of stars around the black hole at the centre of the
Milky Way are under detailed study, but previously there had been no precise
tests on larger astronomical scales. Testing the long-range properties of
gravity is vital to validate our current cosmological model. The findings
may have important implications for models of gravity alternative to general
relativity. The alternative theories predict that the effects of gravity
on the curvature of space-time are 'scale-dependent'. That means that
gravity should behave differently across astronomical length-scales from the
way it behaves on the smaller scales of the Solar System. The team found
that that is unlikely to be true unless the differences occur only on
length scales larger than 6000 light-years.
UNIVERSE'S MISSING ORDINARY MATTER
University of Colorado at Boulder
Researchers have helped to find the last reservoir of ordinary matter hiding
in the Universe. Ordinary matter, or 'baryons', make up all physical
objects in existence, from stars to the cores of black holes. But until now,
astrophysicists had only been able to locate about two-thirds of the matter
that theorists predict was created by the Big Bang. In the new research, an
international team pinned down the missing third, finding it in the space
between galaxies. That lost matter exists as filaments of oxygen gas at
temperatures of around 1 million degrees Celsius. The finding is a major
step for astrophysics. This is one of the key pillars of testing the Big
Bang theory: figuring out the baryon census of hydrogen and helium and
everything else in the periodic table. Researchers have a good idea of
where to find most of the ordinary matter in the Universe -- not to be
confused with dark matter, which scientists have yet to locate: about 10%
sits in galaxies, and close to 60% is in the diffuse clouds of gas that lie
between galaxies. In 2012, astronomers suggested that the missing 30% of
baryons were likely to exist in a web-like pattern in space called the
warm-hot intergalactic medium (WHIM).
To search for missing atoms in that region between galaxies, the team
pointed a series of satellites at a quasar called 1ES 1553 -- a black hole
at the centre of a galaxy that is consuming and spitting out huge quantities
of gas. Scientists can glean a lot of information by recording how the
radiation from a quasar passes through space, a bit like a sailor seeing a
lighthouse through fog. First, the researchers used the Cosmic Origins
Spectrograph on the Hubble Telescope to get an idea of where they might find
the missing baryons. Next, they homed in on those baryons using ESA's X-ray
Multi-Mirror Mission (XMM-Newton) satellite. The team found the signature
of highly-ionized oxygen gas lying between the quasar and our Solar System
-- and at a high enough density, when extrapolated to the entire Universe,
to account for the last 30% of ordinary matter. The team suspects that
galaxies and quasars blew that gas out into deep space over billions of
years.
Topic: Early July Astronomy Bulletin (Read 1165 times)