THE SOCIETY FOR POPULAR ASTRONOMY Electronic News Bulletin No. 480 2018 Nov 18th

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    THE SOCIETY FOR POPULAR ASTRONOMY Electronic News Bulletin No. 480 2018 November 18
    Here is the latest round-up of news from the Society for Popular
    Astronomy. The SPA is arguably Britain's liveliest astronomical
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    NASA/Goddard Space Flight Center

    In 2007 January, scientists saw the first data from the STEREO (Solar and
    Terrestrial Relations Observatory) spacecraft. Instead of the star field
    they expected, a pearly white, feathery smear filled the frame. That bright
    object was not a defect: it was the first satellite image of Comet McNaught
    C/2006 P1), named for astronomer Robert McNaught, who discovered it in 2006
    August. It was one of the brightest comets of the past 50 years. Through-
    out 2007 January, the comet fanned across the Southern Hemisphere's sky, so
    bright that it was visible to the naked eye even during the day. McNaught
    belongs to a rare group of comets, dubbed the Great Comets and known for
    their exceptional brightness. Setting McNaught apart further still from
    its peers, however, was its highly structured tail, composed of many
    distinct dust bands, called striae or striations, that stretched more than
    100 million miles behind the nucleus. One month later, a spacecraft called
    Ulysses encountered the comet's long tail. McNaught was a huge deal when it
    came, because it was so ridiculously bright and beautiful in the sky. It
    had striae — dusty fingers that extended across a great expanse of the sky.
    It was one of the most beautiful comets seen for decades.
    How exactly the tail broke up, scientists do not know. It called to mind
    reports of another storied comet from long ago — the Great Comet of 1744,
    which was said to have fanned out dramatically in six tails over the
    horizon, a phenomenon that astronomers then could not explain. By
    untangling the mystery of McNaught's tail, scientists hoped to learn
    something new about the nature of comets — and solve two cosmic mysteries
    in one. A key difference between studying comets in 1744 and 2007 is, of
    course, our ability now to do so from space. In addition to STEREO's
    serendipitous sighting, another mission, SOHO — the Solar and Heliospheric
    Observatory — made regular observations as McNaught flew by the Sun.
    Researchers hoped that those images might contain their answers.
    Comets are cosmic crumbs of frozen gas, rock and dust left over from the
    formation of the Solar System 4.6 billion years ago — so they may contain
    important clues about Solar System's early history. Those clues may be
    unlocked, as if from a time capsule, every time a comet's orbit brings it
    close to the Sun. Intense heat vaporizes the frozen gases and releases the
    dust within, which streams behind the comet, forming two distinct tails: an
    ion tail carried by the solar wind — the constant flow of charged particles
    from the Sun — and a dust tail. Understanding how dust behaves in the tail
    — how it fragments and clumps together — can teach scientists a great deal
    about similar processes that formed dust into asteroids, moons and even
    planets all those billions of years ago. Appearing as one of the biggest
    and most structurally complex comets in recent history, McNaught was a
    particularly good subject for that type of study. Its brightness and high
    dust production made it much easier to resolve the evolution of fine
    structures in its dust tail. Astronomers noticed weird goings-on in the
    images of those striations, a disruption in the otherwise clean lines. The
    rift seemed to be located at the heliospheric current sheet, a boundary
    where the magnetic orientation, or polarity, of the electrified solar wind
    changes direction. That puzzled scientists, because while they have long
    known that a comet's ion tail is affected by the solar wind, they had never
    seen the solar wind affect dust tails before. Dust in McNaught's tail —
    roughly the size of cigarette smoke — is too heavy, the scientists thought,
    for the solar wind to push it around. On the other hand, an ion tail's
    miniscule, electrically charged ions and electrons easily sail along the
    solar wind. But it was difficult to tell exactly what was going on with
    McNaught's dust, and where, because at roughly 60 miles per second, the
    comet was rapidly travelling in and out of STEREO and SOHO's view. In
    looking for a way to bring it all together to get a complete picture of
    what's happening in the tail they used an image-processing technique that
    compiles all the data from different spacecraft using a simulation of the
    tail, where the location of each tiny speck of dust is mapped by solar
    conditions and physical characteristics like its size and age, or how long
    it had been since it had flown off the head, or coma, of the comet. The end
    result is a map, which layers information from all the images taken at any
    given moment, allowing the dust's movements to be followed. The maps made
    it easier to explain the strange effect that drew attention to the data in
    the first place. Indeed, the current sheet was the culprit behind the
    disruptions in the dust tail, breaking up each striation's smooth, distinct
    lines. For the two days it took the full length of the comet to traverse
    the current sheet, whenever dust encountered the changing magnetic condi-
    tions there, it was jolted out of position, as if crossing some cosmic speed
    bump. That is strong evidence that the dust is electrically charged, and
    that the solar wind is affecting the motion of that dust. Scientists have
    long known that the solar wind affects charged dust; missions like Galileo,
    Cassini and Ulysses watched it move electrically-charged dust through the
    space near Jupiter and Saturn. But it was a surprise for them to see the
    solar wind affect larger dust grains like those in McNaught's tail — about
    100 times bigger than the dust seen ejected from around Jupiter and Saturn
    — because they are that much heavier for the solar wind to push. The work
    sheds light on the nature of striated comet tails from the past and provides
    a lens for studying other comets in the future. But it also opens a new
    line of questioning: what roles did the Sun have in the Solar System's
    formation and early history?


    There's a new comet in the morning sky. Discovered just last week by three
    amateur astronomers — one in Arizona and two in Japan — Comet Machholz-
    Fujikawa-Iwamoto (C/2018 V1) has quadrupled in brightness over the past few
    days and is now glowing like a fuzzy 8th-magnitude star in the constellation
    Virgo. The discovery of a comet by amateur astronomers is a rare event
    nowadays because robotic Near-Earth-Object search programmes usually catch
    them first. Comet Machholz-Fujikawa-Iwamoto appears to be a first-time
    visitor to the inner Solar System. It is plunging toward the Sun on a
    nearly-parabolic orbit that will take it just inside the orbit of Mercury.
    Closest approach to the Sun (0.38 AU) is on Dec. 3-4; closest approach to
    Earth (0.67 AU) is Nov. 27. Fresh comets like this one are notoriously
    unpredictable. They can surge in brightness, seeming to promise a
    spectacular display, but suddenly fizzle out as their deposits of ice are
    exhausted by solar heat. So it is uncertain whether the new comet will even
    become a naked-eye object. At the moment it is an easy target for amateur

    University of Arizona

    Astronomers have long believed that many open clusters consist of a single
    generation of stars because, once stars have formed, their radiation blows
    away nearby material needed to make new stars. But in M11 (the 'Wild Duck
    Cluster') — stars of the same brightness appear in different colours,
    suggesting that they are of different ages. Unless scientists had missed
    important clues about stellar evolution, there had to be another explanation
    for the spread of colours in that cluster of about 2,900 stars. Open
    clusters contain thousands of stars that astronomers hypothesize formed
    from the same giant clouds of gas. Those stars come in all sizes, from
    short-lived, giant blue stars dozens of times more massive than our Sun, to
    long-lived low-mass dwarfs that will burn for 10 billion years or longer.
    The brightness and colour of each star change as it grows older, allowing
    scientists to determine its age. Astronomers plot stars' brightness and
    colour in a diagonal line called the main sequence in the Herttzsprung–
    Russell Diagram. The turning point — the point at which a star ages and
    veers off the main sequence — is used to estimate the ages of clusters on
    the basis of the known life expectancy of each star. If the stars leave the
    main sequence at the same point, then they must all be the same age. In
    M11, however, the stars veer off the diagonal at different points. The team
    observed M11 with the MMT to examine the colour spectrum of the stars. They
    used the 'Hectochelle', which can capture detailed spectra of many stars at
    Rotation of a star causes its spectral lines to be broadened. The spectra
    of stars in M11 show that they are spinning at different rates. A rapidly
    rotating star can remain in the main-sequence stage longer than a slowly
    rotating one. A wide range of rotational velocities of stars in a cluster
    results in differences of lifetimes among the stars. Rotational speed is
    like a fountain of youth to a star: the faster it spins, the better it mixes
    hydrogen — the star's fuel — into its core. The more hydrogen the core
    receives, the longer the star lives, causing it to appear redder than
    younger siblings. Stars in the cluster appear of different colours because
    the cloud in which they were born set them in motions that would extend the
    lifetimes of some of them.


    Solving a decades-old mystery, astronomers have discovered an extremely hot
    magnetosphere around a white dwarf, a remnant of a star like our Sun. White
    dwarfs are the final stage in the lives of stars like our Sun. At the end
    of their lives, those stars eject their outer atmospheres, leaving behind a
    hot, compact and dense core that cools over billions of years. The
    temperature on their surfaces is typically around 100,000 degrees C. Some
    white dwarfs, though, challenge scientists, as they show evidence for highly
    ionized metals. In astronomy, 'metals' means every element heavier than
    helium, and high ionization here means that all but one of the outer
    electrons usually in their atoms have been stripped away. That process
    needs a temperature of 1 million degrees C, far hotter than the surfaces of
    even the hottest white-dwarf stars. The team used the 3.5-m Calar Alto
    telescope in Spain to discover and observe a white dwarf in the direction of
    the constellation of Triangulum, catalogued as GALEXJ014636.8+323615,
    located 1200 light-years away. Spectra of the white dwarf revealed the
    signatures of highly-ionized metals. Intriguingly, they varied over a
    period of six hours — the same time it takes for the white dwarf to rotate.
    The team concluded that the magnetic field around the star — the magneto-
    sphere — traps material flowing from its surface. Shocks within the
    magnetosphere heat the material dramatically, stripping almost all the
    electrons from the metal atoms. The axis of the magnetic field of the white
    dwarf is tilted with respect to the rotational axis. That means that the
    amount of shock-heated material we see varies as the star rotates. More and
    more such stars have been found, without there being any clue as to where
    the highly-ionized metals come from, but now the shock-heated magnetosphere
    model finally explains their origin. Magnetospheres are found around other
    types of stars, but this is the first report of one around a white dwarf.
    The discovery might have far-reaching consequences. Ignoring their
    magnetospheres could mean measurements of other basic properties of white
    dwarfs are wrong, such as their temperatures and masses. The team now plans
    to model them in detail and to extend the research by studying more of them.


    ESO's GRAVITY instrument has added further evidence to the long-standing
    assumption that a supermassive black hole lurks in the centre of the Milky
    Way. New observations show clumps of gas swirling around at about 30% of
    the speed of light on a circular orbit just outside its event horizon — the
    first time material has been observed orbiting close to the point of no
    return, and the most detailed observations yet of material orbiting so close
    to a black hole. The GRAVITY instrument on the Very Large Telescope (VLT)
    Interferometer has been used by scientists to observe flares of infrared
    radiation coming from the accretion disc around Sagittarius A*, the massive
    object at the heart of the Milky Way. The observed flares provide long-
    awaited confirmation that the object in the centre of our galaxy is, as has
    long been assumed, a supermassive black hole. The flares originate from
    material orbiting very close to the black hole's event horizon, making these
    observations the most detailed ones yet of material orbiting so close to a
    black hole. While some matter in the accretion disc — the belt of gas
    orbiting Sagittarius A* at relativistic speeds — can orbit the black hole
    safely, anything that gets too close is doomed to be pulled beyond the event
    horizon. The closest point to a black hole that material can orbit without
    being irresistibly drawn inwards by the immense mass is known as the
    innermost stable orbit, and it is from there that the observed flares
    originate. Those measurements were only possible thanks to international
    collaboration and state-of-the-art instrumentation. The GRAVITY instrument
    which made this work possible combines the light from four telescopes of
    the VLT to create a virtual super-telescope 130 metres in diameter, and
    has already been used to probe the nature of Sagittarius A*.
    Earlier this year, GRAVITY and SINFONI, another instrument on the VLT,
    allowed the same team accurately to measure the close fly-by of the star S2
    as it passed through the extreme gravitational field near Sagittarius A*,
    and for the first time revealed the effects predicted by Einstein's general
    relativity in such an extreme environment. During S2's close fly-by, strong
    infrared emission was also observed. That emission, from highly energetic
    electrons very close to the black hole, was visible as three prominent
    bright flares, and exactly matches theoretical predictions for hot spots
    orbiting close to a black hole of four million solar masses. The flares are
    thought to originate from magnetic interactions in the very hot gas orbiting
    very close to Sagittarius A*.

    Johns Hopkins University

    Astronomers have found what could be one of the Universe's oldest stars, a
    body almost entirely made of materials spewed from the Big Bang. The
    discovery of this approximately 13.5-billion-year-old tiny star means that
    more stars with very low mass and very low metal content are likely to be
    out there — perhaps even some of the Universe's very first stars. The star
    is unusual because unlike other stars with very low metal content, it is
    part of the Milky Way's 'thin disc' — the part of the Galaxy in which our
    own Sun resides. And because this star is so old, researchers say it is
    possible that our galactic neighbourhood is at least 3 billion years older
    than previously thought. The Universe's first stars would have consisted
    entirely of elements like hydrogen, helium, and small amounts of lithium.
    Those stars then produced elements heavier than helium in their cores and
    seeded the Universe with them when they exploded as supernovae. The next
    generation of stars formed from clouds of material laced with those metals,
    incorporating them into their makeup. The metal content, or metallicity,
    of stars in the Universe increased as the cycle of star birth and death
    continued. The newly discovered star's extremely low metallicity indicates
    that, in a cosmic family tree, it could be as little as one generation
    removed from the Big Bang. Indeed, it is the new record holder for the star
    with the smallest complement of heavy elements — it has about the same
    heavy-element content as the planet Mercury. In contrast, our Sun is many
    generations down the line and has a mass of heavy elements equal to 14
    Jupiters. Astronomers have found around 30 ancient 'ultra-metal-poor'
    stars with the approximate mass of the Sun. The star the team found,
    however, is only 14 percent the mass of the Sun.
    The star is part of a two-star system orbiting around a common point. The
    team found the tiny, almost invisibly faint, secondary star after another
    group of astronomers discovered the much brighter primary star. That team
    measured the primary's composition by studying a high-resolution optical
    spectrum, and found it to have extremely low metallicity. The existence of
    the companion star turned out to be the big discovery. The team was able to
    infer its mass by studying the slight variation that it induces in the
    radial velocity of the primary star. As recently as the late 1990s,
    researchers believed that only massive stars could have formed in the
    earliest stages of the Universe — and that they could never be observed
    because they burn through their fuel and die so quickly. But as astro-
    nomical simulations became more sophisticated, they began to hint that, in
    certain situations, a star from that time period but with particularly low
    mass could still exist. Unlike huge stars, low-mass ones can live for
    exceedingly long times. Red dwarf stars, for instance, with a fraction of
    the mass of the Sun, are thought to live for billions of years. The
    discovery of the new ultra-metal-poor star, named 2MASS J18082002-5104378 B, opens the possibility of observing even older stars.

    University of Groningen

    Some ten billion years ago, the Milky Way merged with a large galaxy. The
    stars from that partner, named Gaia-Enceladus, make up most of the Milky
    Way's halo and also shaped its thick disc, giving it its inflated form.
    Large galaxies like our Milky Way are the result of mergers of smaller
    galaxies. An outstanding question is whether a galaxy like the Milky Way is
    the product of many small mergers or of a few large ones. Researchers have
    looked for 'fossils' in our Milky Way which might offer some hints as to its
    evolution. The research uses the chemical composition, the position and the
    trajectory of stars in the halo to deduce their history and thereby to
    identify the mergers which created the early Milky Way. The second data
    release from the Gaia satellite mission last April provided data on around
    1.7 billion stars, and they has been used to look for traces of mergers in
    the halo. Astronomers expected stars from fused satellites in the halo.
    What they did not expect to find was that most halo stars actually have a
    shared origin in one very large merger. The chemical signature of many halo
    stars was clearly different from that of the 'native' Milky Way stars. They
    are a fairly homogeneous group, which indicates that they share a common
    origin. In plots of both trajectory and chemical signature, the 'invaders'
    stood out clearly.
    The youngest stars from Gaia-Enceladus are actually younger than the native
    Milky Way stars in what is now the thick-disc region. That means that the
    progenitor of the thick disc was already present when the fusion happened,
    and Gaia-Enceladus, because of its large size, shook it and puffed it up.
    In a previous paper, the team had already described a huge 'blob' of stars
    sharing a common origin. Now, it shows that stars from the blob in the halo
    are the debris from the merging of the Milky Way with a galaxy which was
    slightly more massive than the Small Magellanic Cloud, some ten billion
    years ago. That galaxy is called Gaia-Enceladus, after the giant Enceladus
    who in Greek mythology was born of Gaia (the Earth goddess) and Uranus (the
    Sky god).


    After nine years in deep space collecting data that indicate our sky to be
    filled with billions of hidden planets — more planets even than stars —
    the Kepler space telescope has run out of fuel needed for further science
    operations. NASA has decided to retire the spacecraft within its current,
    safe orbit, away from the Earth. Kepler leaves a legacy of more than 2,600
    planet discoveries from outside the Solar System, many of which could be
    promising places for life. Kepler has opened our eyes to the diversity of
    planets that exist in our Galaxy. The most recent analysis of Kepler's
    discoveries concludes that 20 to 50 per cent of the stars visible in the
    night sky are likely to have small, possibly rocky, planets similar in size
    to the Earth and located within the habitable zones of their parent stars.
    That that means they are located at distances from their parent stars where
    liquid water — a vital ingredient to life as we know it — might pool on
    the planetary surface. The most common size of planet Kepler found does not
    exist in the Solar System — between the sizes of the Earth and Neptune —
    and we have much to learn about such planets. Kepler also found that Nature
    often produces jam-packed planetary systems, in some cases with so many
    planets orbiting close to their parent stars that our own Solar System
    looks sparse by comparison. Launched on 2009 March 6, the Kepler space
    telescope combined cutting-edge techniques in measuring stellar brightness
    with the largest digital camera outfitted for outer-space observations at
    that time. Originally positioned to stare continuously at 150,000 stars in
    one star-studded patch of the sky in the constellation Cygnus, Kepler took
    the first survey of planets in our galaxy and became the agency's first
    mission to detect Earth-size planets in the habitable zones of their stars.
    Four years into the mission, after the primary mission objectives had been
    met, mechanical failures temporarily halted observations. The mission team
    was able to devise a fix, switching the spacecraft's field of view roughly
    every three months. That enabled an extended mission for the spacecraft,
    dubbed K2, which lasted as long as the first mission and raised Kepler's
    count of surveyed stars up to more than 500,000. The observation of so many
    stars has allowed scientists to understand better their behaviours and
    properties, which is critical information in studying the planets that orbit
    them. New research into stars with Kepler data also is furthering other
    areas of astronomy, such as the history of our Milky Way Galaxy and the
    beginning stages of supernovae. The data from the extended mission were
    also made available to the public and scientific community immediately,
    allowing discoveries to be made at an incredible pace and setting a high bar
    for other missions. Scientists are expected to spend a decade or more in
    search of new discoveries in the treasure trove of data Kepler is providing.


    The Dawn spacecraft has gone silent, ending a historic mission that studied
    time capsules from the Solar System's earliest chapter. Dawn has missed
    scheduled communications sessions with NASA's Deep Space Network, and
    mission managers concluded that the spacecraft finally ran out of hydrazine,
    the fuel that enables the spacecraft to control its pointing. Dawn can no
    longer keep its antennae trained on the Earth to communicate with mission
    control or turn its solar panels to the Sun to recharge. The spacecraft was
    launched 11 years ago to visit the two largest objects in the main asteroid
    belt. Currently, it is in orbit around the dwarf planet Ceres, where it
    will remain for decades. Propelled by ion engines, the spacecraft achieved
    many firsts along the way. In 2011, when Dawn arrived at Vesta, the second
    largest asteroid in the main belt, the spacecraft became the first to orbit
    a body in the region between Mars and Jupiter.

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

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