THE SOCIETY FOR POPULAR ASTRONOMY Electronic News Bulletin No. 471 2018 July 8

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    THE SOCIETY FOR POPULAR ASTRONOMY Electronic News Bulletin No. 471 2018 July 8

    Here is the latest round-up of news from the Society for Popular
    Astronomy. The SPA is arguably Britain's liveliest astronomical
    society, with members all over the world. We accept subscription
    payments online at our secure site and can take credit and debit
    cards. You can join or renew via a secure server or just see how
    much we have to offer by visiting http://www.popastro.com/

    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.
    Bulletin compiled by Clive Down (c) 2018 The Society for Popular Astronomy
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