The SOCIETY for POPULAR ASTRONOMY News Bulletin No. 428 2016 August 28

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    Electronic News Bulletin No. 428 2016 August 28

    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
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    Astronomers have found clear evidence of a planet orbiting the closest
    star to the Sun, Proxima Centauri. The planet, designated Proxima b,
    orbits its cool red parent star every 11 days and has a temperature
    such that liquid water could exist on its surface. The rocky object
    is a little more massive than the Earth and is the closest exo-planet
    to us, and it may also be the closest possible abode for life outside
    the Solar System. Proxima Centauri is too faint to be seen with the
    unaided eye and lies near (and is gravitationally bound) to the much
    brighter double star Alpha Centauri. During the first half of 2016
    Proxima Centauri was regularly observed with the HARPS spectrograph on
    the ESO 3.6-metre telescope at La Silla in Chile and simultaneously
    monitored by other southern telescopes. That was the 'Pale Red Dot'
    campaign, in which a team of astronomers was looking for the tiny
    wobble of the star that would be caused by the gravitational pull of a
    possible orbiting planet. The Pale Red Dot data, when combined with
    earlier observations made at ESO observatories and elsewhere, revealed
    a clear signal of velocity variations of about +/- 5 kilometres per
    hour — about human walking pace. The regular pattern of changing
    radial velocities repeats with a period of 11.2 days. The shifts
    indicate the presence of a planet with a mass at least 1.3 times that
    of the Earth, orbiting only about 7 million kilometres from Proxima.
    Red dwarfs like Proxima Centauri are active stars and can vary in ways
    that could mimic the presence of a planet. To exclude that
    possibility the team also monitored the changing brightness of the
    star very carefully during the campaign. Radial-velocity data taken
    when the star was flaring were excluded from the final analysis.
    Although Proxima b orbits much closer to its star than Mercury does to
    the Sun, the star itself is far fainter than the Sun. As a result,
    Proxima b lies well within the 'habitable zone' around the star and
    has an estimated surface temperature that would allow the presence of
    liquid water. Despite the temperate orbit of Proxima b, the
    conditions on the surface may be strongly affected by the ultraviolet
    and X-ray flares from the star — far more intense than the Earth
    experiences from the Sun. Two separate papers discuss the habit-
    ability of Proxima b, and its climate. They find that the existence
    of liquid water on the planet today cannot be ruled out and, in such
    case, it may be present over the surface of the planet only in the
    sunniest regions, either in an area in the hemisphere of the planet
    facing the star or in a tropical belt. Proxima b's rotation, the
    strong radiation from its star and the formation history of the planet
    makes its climate quite different from that of the Earth, and it is
    unlikely that Proxima b has seasons. This discovery will be the
    beginning of extensive further observations, both with current
    instruments and with the next generation of giant telescopes such as
    the European Extremely Large Telescope. Proxima b will be a prime
    target for the hunt for evidence of life elsewhere in the Universe.
    Indeed, the Alpha Centauri system is also the target of mankind's
    first attempt to travel to another star system, the StarShot project.

    Carnegie Institution for Science

    Brown dwarfs are smaller than stars, but more massive than giant
    planets, so they provide a natural link between stellar and planetary
    science. However, they also show incredible variation when it comes
    to size, temperature, and chemistry, which makes them difficult to
    understand, too. Astronomers have surveyed various properties of 152
    suspected young brown dwarfs in order to categorize their diversity,
    and found that atmospheric properties may be behind many of their
    differences, a discovery that may apply to planets outside the Solar
    System as well. Scientists are very interested in brown dwarfs, which
    hold promise for explaining not just planetary evolution, but also
    stellar formation. Such objects are more difficult to observe than
    more-massive and brighter stars, but they vastly outnumber stars like
    the Sun. They represent the smallest and lightest objects that can
    form like stars do in the Galaxy, so they are an important 'book-end'
    in astronomy. For the moment, data on brown dwarfs can be used as
    stand-ins for contemplating extra-solar worlds that we hope to study
    with future instruments like the Webb Space Telescope. Brown dwarfs
    are far easier to study than planets, because they are not overwhelmed
    by the brightness of a host star. But the tremendous diversity we see
    in the properties of the brown-dwarf population means that there is
    still much about them that remains unknown or poorly understood.
    Brown dwarfs are too small to sustain the hydrogen-fusion process that
    fuels stars, so after formation they slowly cool and contract over
    time and their surface gravity increases. That means that their
    temperatures can range from nearly as hot as a star to as cool as a
    planet, which is thought to influence their atmospheric conditions,
    too. Their masses also range between those of stars and of giant
    planets, and they demonstrate great diversity in age and chemical
    composition. By quantifying the observable properties of so many
    young brown-dwarf candidates, researchers were able to show that those
    objects have vast ranges of colour and spectral features. Identifying
    the cause of those ranges was at the heart of the team's work. By
    locating the birth-places of many of the brown dwarfs, it was able to
    eliminate age and chemical composition differences as the underlying
    reasons for the great variation. That left atmospheric conditions —
    meaning weather phenomena or differences in cloud composition and
    structure — as the primary suspect for what drives the extreme
    differences between objects of similar origin. All of the brown-dwarf
    birth-places identified in this work are regions that also host exo-
    planets, so these same findings hold for giant planets orbiting nearby
    stars. The team considers the young brown dwarfs to be siblings of
    giant exo-planets. As close family members, they can probably be used
    to investigate how the planetary ageing process works.


    Astronomers have identified a young star, located almost 11,000 light-
    years away, which could help us understand how the most massive stars
    in the Universe are formed. The young star, already more than 30
    times the mass of our Sun, is still in the process of gathering
    material from its parent molecular cloud, and will be even more
    massive when it finally reaches adulthood. The researchers, led by a
    team at the University of Cambridge, have identified a key stage in
    the birth of a very massive star, and found that such stars form in a
    similar way to much smaller stars like our Sun — from a rotating disc
    of gas and dust. In our Galaxy, massive young stars — those with
    masses at least eight times that of the Sun — are much more difficult
    to study than smaller stars. That is because they live fast and die
    young, making them rare among the 100,000 million stars in the Milky
    Way, and on average, they are much further away. An average star like
    our Sun is formed over a few million years, whereas massive stars are
    formed orders of magnitude more quickly — in around 100,000 years.
    Massive stars also burn through their fuel much more quickly, so they
    have shorter overall life-spans, making them harder to catch when they
    are infants. The proto-star now identified resides in an infrared
    dark cloud — a very cold and dense region which makes an ideal
    stellar nursery. However, that rich star-forming region is difficult
    to observe with conventional telescopes, since the young stars are
    surrounded by a thick, opaque cloud of gas and dust. But by using the
    Submillimeter Array (SMA) in Hawaii and the Very Large Array (VLA) in
    New Mexico, both of which use relatively long wavelengths of light,
    the researchers were able to 'see' through the cloud and into the
    stellar nursery itself.

    By measuring the amount of radiation emitted by cold dust near the
    star, and from the spectra of various different molecules in the gas,
    the researchers were able to determine the presence of a Keplerian
    disc — one which rotates more quickly at its centre than at its edge.
    It is exciting to find such a disc around a massive young star,
    because it suggests that massive stars form in a similar way to lower-
    mass stars, like our Sun. From their observations, the team found the
    mass of the proto-star to be over 30 times that of the Sun. In
    addition, the disc surrounding the young star was also calculated to
    be relatively massive, between two and three times the mass of the
    Sun. Calculations suggest that the disc could in fact be hiding even
    more mass under layers of gas and dust. The disc may even be so
    massive that it can break up under its own gravity, forming a series
    of less-massive companion proto-stars. The next step for the
    researchers will be to observe the region with the Atacama Large
    Millimetre Array (ALMA), located in Chile. That powerful instrument
    may allow any potential companions to be seen, and shed further light
    on this intriguing young heavyweight in our Galaxy.

    NASA/Goddard Space Flight Center

    The Hubble Space Telescope has found two tiny dwarf galaxies that have
    wandered from a vast cosmic wilderness into a nearby galaxy cluster.
    After being quiescent for billions of years, they are ready to begin a
    firestorm of star birth. Studying those and other similar galaxies can
    provide further clues to dwarf-galaxy formation and evolution. The Hubble
    observations suggest that the galaxies, called Pisces A and B, are late
    bloomers because they have spent most of their existence in the Local
    Void, a region of the Universe sparsely populated with galaxies. The
    Local Void is roughly 150 million light-years across. Under the steady pull
    of gravity from the galactic cluster, the dwarf galaxies have at last entered
    a crowded region that is denser in intergalactic gas. In that gas-rich
    environment, star birth may have been triggered by gas raining down on
    the galaxies as they plough through the denser region. Another idea
    is that the duo may have encountered a gaseous filament, which
    compresses gas in the galaxies and stokes star birth. The objects are
    at the edge of a nearby filament of dense gas. Each galaxy contains
    only about 10 million stars. Dwarf galaxies are the building blocks
    from which larger galaxies were formed in the early Universe.
    Inhabiting a sparse desert of largely empty space for most of the
    Universe's history, the two galaxies avoided that busy construction
    period. They may have spent most of their history in the void. If
    that is so, the void environment would have slowed their evolution.
    Evidence for the galaxies' void address is that their hydrogen content
    is somewhat high relative to that of similar galaxies. In the past,
    galaxies contained higher concentrations of hydrogen. But the two
    galaxies of interest here seem to retain that more primitive
    composition, rather than the enriched composition of contemporary
    galaxies, owing to a less vigorous history of star formation. The
    galaxies also are quite compact relative to the typical star-forming
    galaxies in our own neighborhood.
    The dwarf galaxies are small and faint, so finding them is extremely
    difficult. Astronomers found them by using radio telescopes in a
    unique survey to measure the hydrogen content in the Milky Way. The
    observations found thousands of small blobs of dense hydrogen gas.
    Most of them are gas clouds within our own Galaxy, but astronomers
    identified 30 to 50 of the blobs as possible external galaxies. The
    researchers used the WIYN telescope in Arizona to study 15 of the most
    promising candidates in visible light. On the basis of those
    observations, the research team selected the two that are the most
    likely candidates to be nearby galaxies and analyzed them with
    Hubble's 'Advanced Camera for Surveys'. Hubble confirmed that both of
    them, Pisces A and B, are dwarf galaxies. The Hubble telescope is
    aptly suited to study nearby, dim dwarf galaxies because it can
    resolve individual stars and help astronomers estimate the galaxies'
    distances. Distance is important for determining a galaxy's
    brightness, and, in these Hubble observations, for calculating how far
    away the galaxies are from nearby voids. Pisces A is about 19 million
    light-years from the Earth and Pisces B roughly 30 million light-years
    away. On the basis of the galaxies' locations, an analysis of the
    stars' colours allowed the astronomers to trace the star-formation
    history of both galaxies. Each galaxy contains about 20 to 30 bright
    blue stars, a sign that they are very young, less than 100 million
    years old. The team estimates that less than 100 million years ago,
    the galaxies doubled their star-formation rate. Eventually, the star
    formation may slow down again if the galaxies become satellites of a
    much larger galaxy.


    NASA is preparing to launch its first mission to return a sample of an
    asteroid to Earth. The mission will help scientists investigate how
    planets formed and how life began, as well as improve our under-
    standing of asteroids that could collide with the Earth. The Origins,
    Spectral Interpretation, Resource Identification, Security-Regolith
    Explorer (OSIRIS-REx) spacecraft will travel to the near-Earth
    asteroid Bennu and bring a sample back for study. The craft is
    scheduled for launch on September 8 from Cape Canaveral in Florida and
    should reach its asteroid target in 2018. After a careful survey of
    Bennu to characterize the asteroid and to locate the most promising
    sample sites, OSIRIS-REx will collect between 60 and 2000 grams of
    surface material with its robotic arm and return the sample to Earth
    via a detachable capsule in 2023.

    Bulletin compiled by Clive Down
    (c) 2016 The Society for Popular Astronomy
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