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March 23, 2016 at 5:40 pm #8836Anonymous
The SOCIETY for POPULAR ASTRONOMY Electronic News Bulletin No. 418 2016 March 21
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/
RUSSIAN SATELLITE TO BE BRIGHTEST 'STAR'
The night sky may soon be lit by another man-made 'star'. A team of Russian engineers has reached its fund-raising goals for sending up a satellite that will be so reflective that it will become the brightest object in the night sky, other than the Moon. The satellite will have no other purpose than to glow like a very bright star. While it is sure to be the highlight of romantic late-night trysts the world over, the project has also attracted its share of ire from the scientific community. The gleaming spacecraft could be so bright that it will pollute the night sky, thus making astronomical observations more difficult. The additional light in the sky could have other unforeseen ecological effects as well. Nearly $20,000 was raised on the crowd-funding site Boomstarter to send the so-called 'Mayak' (which translates to 'beacon' in English) satellite into orbit. The tiny space capsule is no larger than a loaf of bread, but once it reaches its desired altitude it will unfurl an impressive pyramid-shaped reflector. It will be in a sun-synchronous orbit 370 miles above the ground, which will ensure that it is always in the sunlight and thus always shining at some vantage point from above the Earth. That educational message might end up being counter-productive, however, if the satellite does cause problems for those who are actually exploring space. Astronomers go to a lot of effort to avoid light pollution in order to get the best views they can, and this satellite could stymie those efforts. Either way, the good news is that the obstruction will only be temporary. It is not clear exactly how long the satellite will remain in stable orbit, but it is unlikely to last for more than several weeks. Of course, if the project is a big hit it could lead to future copycat schemes, though perhaps some regulations would be framed in that case. The satellite is due to be launched in mid-2016.
MERCURY'S 'DARKNESS' EXPLAINED
Scientists have long been puzzled about what makes Mercury's surface so dark. It reflects much less sunlight than the Moon, a body on which surface darkness is controlled by the abundance of iron-rich minerals. Those are known to be rare on Mercury's surface, so what is the darkening agent there? About a year ago, scientists proposed that Mercury's darkness was due to carbon that gradually accumulated from the impact of comets that travelled into the inner Solar System. Now scientists have used data from the MESSENGER mission to confirm that a high abundance of carbon *is* present on Mercury's surface. However, they have also found that, rather than being delivered by comets, the carbon most likely originated deep below the surface, in the form of a now-disrupted and buried ancient graphite-rich crust, some of which was later brought to the surface by impact processes after most of Mercury's current crust had formed. The previous proposal of comets delivering carbon to Mercury was based on modelling and simulation.
MESSENGER obtained its statistically robust data on many orbits on which the spacecraft passed less than 100 km above the surface during its last year of operation. The data used to identify carbon included measurements taken just days before the spacecraft crashed onto Mercury last April. Repeated neutron-spectrometer measurements showed higher amounts of low-energy neutrons, a signature consistent with the presence of elevated carbon, coming from the surface when the spacecraft passed over concentrations of the darkest material. Estimating the amount of carbon present required combining the neutron measurements with other MESSENGER data sets, including X-ray measurements and reflectance spectra. Together, the data indicate that Mercury's surface rocks contain a few per cent by weight of graphitic carbon, much more than other planets. Graphite has the best fit to the reflectance spectra at visible wavelengths, and appears likely from the conditions that produced the material. When Mercury was very young, much of the planet was probably so hot that there was a global 'ocean' of molten magma. From laboratory experiments and modelling, scientists have suggested that as that magma ocean cooled, most minerals that solidified would sink. A notable exception is graphite, which would have been buoyant and floated to form the original crust of Mercury. The finding of abundant carbon on the surface suggests that we may be seeing remnants of Mercury's original ancient crust mixed into the volcanic rocks and impact ejecta that form the surface that we see today.
UNEXPECTED CHANGES IN BRIGHT SPOTS ON CERES
Astronomers have found unexpected changes in the bright spots on the 'dwarf planet' (formerly regarded as the largest asteroid) Ceres. Although Ceres appears as little more than a point of light from the Earth, study of its light shows not only the changes expected as it rotates, but also that the spots brighten during the day and also show other variations. Those observations suggest that the material of the spots is volatile and evaporates in the warm glow of sunlight. About 945 km across, Ceres is the largest body in the asteroid belt between Mars and Jupiter, and the only such object classed as a dwarf planet. The Dawn spacecraft has been in orbit around it for more than a year and has mapped its surface in great detail. One of the biggest surprises has been the discovery of very bright spots, only hinted at in earlier work, which reflect far more light than their surroundings. The most prominent spots lie inside the crater Occator, and suggest that Ceres may be a much more active object than most asteroids.
The 'High-Accuracy Radial-velocity Planet Searcher' (HARPS) spectrograph (a more modern and sophisticated version of a type of instrument first developed by the 'moderator' of these Bulletins in the 1960s) measures the Doppler shift of lines in the spectra of stars, in an effort to find planets in orbit around them. Its precision also makes it good for studying objects like Ceres. Data from HARPS have now not only detected the motion of the spots due to the rotation of Ceres about its axis (Ceres' 'day' is just over 9 hours), but also found unexpected additional variations. The team observed Ceres with HARPS for a little over two nights last July and August. It concluded that the observed changes could be due to the presence of volatile substances that evaporate under the action of solar radiation. When the spots inside the Occator crater are on the side illuminated by the Sun they form plumes that reflect sunlight very effectively. The plumes then dissipate quickly, lose reflectivity and produce the observed changes. That effect, however, changes from night to night, giving rise to additional random patterns, on both short and longer time scales. If that interpretation is confirmed, Ceres would seem to be very different from Vesta and the other main-belt asteroids. Despite being relatively isolated, it seems to be internally active. Ceres is known to be rich in water, but it is not clear whether that is related to the bright spots. The energy source that drives the continual leakage of material from the surface is also unknown. Dawn is continuing to study Ceres and the behaviour of its bright spots. Observations from the ground with HARPS and other facilities will of course be able to continue even after the space mission comes to an end.
NEW ANALYSIS OF DISTANT TNO
Trans-Neptunian objects (TNOs) are known as the most pristine types of bodies in the Solar System and orbit the Sun at a greater average distance than that of Neptune. The most famous is Pluto, a dwarf planet discovered over 60 years before any other TNOs. Studying those objects might help us to understand both the formation of our planetary system and the circumstellar material (also known as debris discs) orbiting other stars. Discovered in 2007, (225088) 2007 OR10 is the second-most-distant known TNO to date, following Eris. It is situated at a distance of at least 87 AU and still moving further away up to its aphelion at 100 AU. It has a high orbital eccentricity, and at perihelion it comes nearly as close as Neptune. Ground-based observations show the diameter of 2007 OR10 to be 1535 km. That is notably larger than the value obtained in 2012 and places it as the third-largest dwarf planet after Pluto and Eris. Owing to its large size, 2007 OR10 probably has a shape close to spherical. Infrared thermal spectra taken with the Herschel space observatory indicate the presence of water ice, and a red colour which may be caused by the retention of methane, carbon monoxide and nitrogen on the surface.
VERSATILE INSTRUMENT TO SEARCH FOR KUIPER-BELT OBJECTS
Astronomers at the Palomar Observatory are busy constructing a high-tech instrument that could discover a variety of objects both far away and closer to home. The 'Caltech HIgh-speed Multi-color camERA' (CHIMERA) system is looking for objects in the Kuiper Belt, the band of icy bodies beyond the orbit of Neptune that includes Pluto. It can also detect near-Earth asteroids and exotic types of stars. The Kuiper Belt is a pristine remnant of the formation of the Solar System. By studying it, we may be able to learn something about how the Solar System formed and how it is continuing to evolve. The wide-field camera system allows scientists to monitor thousands of stars simultaneously to see if a Kuiper-Belt object passes in front of any of them. Such an object would diminish a star's light for only a tenth of a second while travelling by, so a camera has to be fast in order to capture it. Each of CHIMERA's cameras will be taking 40 frames per second, allowing it to measure the distinctive diffraction pattern in the wavelength of light to which it is sensitive. That high-speed imaging technique may enable astronomers to find new Kuiper Belt objects far less massive than any other ground-based survey to date. Astronomers are particularly interested in finding Kuiper-Belt objects smaller than 1 kilometre in diameter. Few such objects have been found, and scientists would like to know how common they are, what they are made of and how they collide with other objects. The CHIMERA astronomers estimate that in the first 100 hours of CHIMERA data, they might find dozens of such small, distant objects.
Another scientific focus for CHIMERA is near-Earth asteroids, which the instrument should be able to detect even if they are only about 10 metres across. Astronomers predict that by using CHIMERA on the Hale telescope at Palomar, they could find several near-Earth objects per night of observation. Transient or pulsing objects such as binary-star systems, pulsing white dwarfs and brown dwarfs can also be seen with CHIMERA. CHIMERA uses detectors called electron-multiplying charged-coupled devices (EMCCDs), making for an extremely high-sensitivity, low-noise camera system. One of the EMCCDs is sensitive to near-infrared light, while the other operates at green and blue wavelengths, and the combination constitutes a robust system for scanning perturbations in starlight. The detectors run at -100Â°C in order to reduce noise. Not only can scientists image over a wide field, but in other modes they can also image objects rotating hundreds of times per second.
TWO NEW EXOTIC PULSARS
Currently, there are 25 confirmed pulsars in 47 Tucanae, a globular cluster that contains at least half a million stars — the second-most-populous cluster in our Galaxy. All the pulsars were discovered by reprocessing archival observations from the Parkes radio telescope, part of the 'Australia Telescope National Facility' (ATNF) network of radio telescopes. Two of the newly discovered pulsars, named PSRs J0024-7204aa and J0024-7204ab, are millisecond pulsars — rapidly rotating neutron stars — with a high spin frequency, much higher than those of the Galactic population.
J0024-7204aa has a pulse frequency higher than that of any other pulsar currently known in the cluster and ranks 12th amongst all currently known pulsars. Astronomers think that millisecond pulsars are related to low-mass X-ray binary systems. The high-energy X-rays from such systems are emitted by the collision between the accretion disc of matter of a companion star and particles racing away from the pulsar at tremendous speeds. However, J0024-7204aa is unlikely to be in a fast binary system as there is no evidence that is accelerating.
The second pulsar is weaker and fainter and only occasionally seen in the data. It is situated in the central region of 47 Tucanae, and has a proper motion consistent with the proper motion of the cluster. One of the cherished goals of modern pulsar astronomy is to discover a system with a millisecond pulsar in orbit around a black hole. Evolutionary models suggest that such a system is likely to form only in globular clusters where there are high rates of stellar interaction. Such a system, if it exists, is considered an authentic 'holy grail' for testing Einstein's theory of gravity. The two new discoveries add to the growing known population of pulsars in globular clusters. Such pulsars are weak and hard to detect. However, in the near future, much more sensitive telescopes, such as MeerKAT and the Square-Kilometre Array, will be operating and will be able to make regular observations of such pulsars.
RED FLASHES FROM BLACK-HOLE OUTBURST
Violent red flashes, lasting just fractions of a second, have been observed during one of the brightest black-hole outbursts in recent years. Last June, a black hole called V404 Cygni underwent dramatic brightening for about two weeks, as it devoured material that it had stripped off an orbiting companion star. V404 Cygni, which is about 7,800 light-years away, was the first black hole to be definitely identified in our Galaxy and can appear extremely bright when it is actively devouring material. The astronomers associated the red colour with fast-moving jets of matter that were ejected from close to the hole. Those observations provide new insights into the formation of such jets and extreme black-hole phenomena. The very high speed tells us that the region where the red light is being emitted must be very compact. Piecing together clues about the colour, speed, and the power of the flashes, astronomers conclude that the light is being emitted from the base of the black-hole jet. The origin of such jets is still unknown, although strong magnetic fields are suspected to play a role. Furthermore, the red flashes were found to be strongest at the peak of the black hole's feeding frenzy. Researchers speculate that when the black hole was being rapidly fed by its companion star, it reacted violently by spewing out some of the material as a fast-moving jet. The duration of the flashing episodes could be related to the switching on and off of the jet, seen for the first time in detail.
Owing to the unpredictable nature and rarity of bright black-hole outbursts, astronomers have very little time to react. For example, the last previous eruption of V404 Cygni was in 1989. V404 Cygni was exceptionally bright last June and provided an excellent observational opportunity. In fact, it was one of the brightest black-hole outbursts in recent years; most outbursts are far dimmer, making them difficult to study. Each flash in last year's outburt of V404 Cyg was very intense, equivalent to the power output of about 1,000 Suns. Some of the flashes were shorter than 1/40th of a second — much less than the duration of a typical blink of an eye. Such observations require novel technology, so astronomers used the ULTRACAM fast camera on the William Herschel Telescope in the Canary Islands. The 2015 event has greatly motivated astronomers to coordinate worldwide efforts to observe future outbursts. Their short durations, and strong emissions across the entire electromagnetic spectrum, require close communication, sharing of data, and collaborative efforts amongst astronomers.
REPEATING COSMIC-RAY BURSTS FROM SAME SKY LOCATION
Astronomers have for the first time detected repeating short bursts of radio waves from an enigmatic source that is probably located well beyond the edge of our Milky Way galaxy. The findings indicate that such 'fast radio bursts' come from an extremely powerful object which occasionally produces multiple bursts in under a minute. All previously detected fast radio bursts (FRBs) have appeared to be one-off events. Because of that, most theories about their origin have involved cataclysmic incidents that destroy their source — a star exploding in a supernova, for example, or a neutron star collapsing into a black hole. The new finding, however, shows that at least some FRBs have other origins. FRBs, which last just a few milliseconds, have puzzled scientists since they were first reported nearly a decade ago. Despite extensive follow-up efforts, until now astronomers have searched in vain for repeat bursts. That changed last November, when a scientist was sifting through results from observations made with the world's largest radio telescope, at Arecibo in Puerto Rico. The new data showed ten bursts with properties similar to those of an FRB detected in 2012. The finding suggests that the bursts must have come from a very unusual object, such as a rotating neutron star having unprecedented power that enables the emission of extremely bright pulses. It is also possible that the finding represents the first discovery of a sub-class of the cosmic fast-radio-burst population.
Scientists believe that those and other radio bursts originate from distant galaxies, on the basis of an effect known as plasma dispersion. Pulses that travel through the cosmos are distinguished from man-made interference by the influence of interstellar electrons, which cause radio waves to travel more slowly at lower radio frequencies. The ten newly discovered bursts, like the one detected in 2012, have three times the maximum dispersion measure that would be expected from a source within the Milky Way. Intriguingly, the most likely implication of the new Arecibo finding — that the repeating FRB originates from a very young extragalactic neutron star — is at odds with the results of a study published last week in Nature by another research team. That paper suggested that FRBs are related to cataclysmic events, such as short gamma-ray bursts, which cannot generate repeat events. However, the apparent conflict between the studies could be resolved, if it turns out that there are at least two kinds of FRB sources. In future research, the team hopes to identify the galaxy where the radio bursts originated. To do so, it will need to detect bursts with radio telescopes having far more resolving power than Arecibo's 305-m dish. By interferometry with widely-spread radio-telescope arrays, the astronomers may be able to achieve the needed resolution. Once researchers have precisely localized the repeater's position on the sky, they will be able to compare observations from optical and X-ray telescopes and see if there is a galaxy there. Finding the host galaxy of the source is critical to understanding its properties.
HUBBLE BREAKS COSMIC DISTANCE RECORD
ESA/Hubble Information Centre
By pushing the Hubble space telescope to the limit, astronomers have broken the record for the most remote galaxy ever observed. GN-z11 existed just 400 million years after the Big Bang, and provides new insight into the first generation of galaxies. This is the first time that the distance of an object so far away has been measured from its spectrum, which makes the measurement extremely reliable. Although extremely faint, the galaxy is unusually bright in relation to its distance from the Earth. The distance measurement of GN-z11 provides additional strong evidence that other unusually bright galaxies found in earlier Hubble images really *are* at extraordinary distances, showing that we are closing in on the first galaxies that formed in the Universe. Previously, astronomers had estimated GN-z11's distance by analyzing its colour in images taken with both Hubble and the Spitzer space telescope. Now, for the first time for a galaxy at such an extreme distance, the team has used Hubble's Wide-Field Camera 3 (WFC3) to measure GN-z11's red-shift spectroscopically. That puts GN-z11 at a distance that was thought to be reachable only with the upcoming James Webb space telescope.
The most distant previously measured galaxy, EGSY8p7, has a redshift of 8.68. Now, the team has found GN-z11's redshift to be 11.1, which corresponds to 400 million years after the Big Bang. The previous record-holder was seen in the middle of the epoch when starlight from primordial galaxies was beginning to heat and clear a fog of cold hydrogen gas. That transitional period is known as the re-ionization era. GN-z11 is observed 150 million years earlier, near the very beginning of that transition in the evolution of the Universe. The combination of observations taken by Hubble and Spitzer revealed that the infant galaxy is 25 times smaller than the Milky Way and has just one per cent of our Galaxy's mass in stars. However, the number of stars in the newborn GN-z11 is growing fast: the galaxy is seen to have been forming stars at a rate about 20 times greater than the Milky Way does today. It is that high star-formation rate that makes the remote galaxy bright enough for Hubble to see and to perform detailed observations. However, the discovery also raises many new questions, as the existence of such a bright and large galaxy is not predicted by theory. It seems amazing that a galaxy so massive existed only 200 to 300 million years after the very first stars started to form. It must have taken really fast growth, producing stars at a huge rate, to have formed a galaxy of a thousand million solar masses so soon.
EXOMARS ON ITS WAY
European Space Agency
The first of two joint ESA-Roscosmos missions has begun its seven-month journey to Mars, where it will address unsolved problems of the planet's atmosphere that could indicate present-day 'geological' — or even biological — activity. The Trace Gas Orbiter (TGO) and the Schiaparelli entry, descent and landing demonstrator lifted off on a Proton-M rocket operated by Russia's Roscosmos from Baikonur, Kazakhstan. The Trace Gas Orbiter and Schiaparelli will travel to Mars together before separating on October 16 at a distance of 900000 km from the planet. Then, on October 19, Schiaparelli will enter the Martian atmosphere, descending to the surface in just under six minutes. It is intended to demonstrate key entry, descent, and landing technologies for future missions, and will conduct a number of environmental studies during its short mission on the surface. For example, it will obtain the first measurements of electric fields on the surface of Mars that, combined with measurements of the concentration of atmospheric dust, will provide new insights into the role of electric forces on dust lifting — the trigger for dust storms. Meanwhile, on the same day, TGO will enter an elliptical four-day orbit around Mars, taking it from about 300 km at its nearest to around 96,000 km at its furthest point. After a year of complex 'aerobraking' manoeuvres during which the spacecraft will use the planet's atmosphere to lower the orbit slowly to a circular 400 km, its scientific mission to analyze rare gases in the atmosphere will begin. Of particular interest is methane, which on Earth points to active geological or biological processes. One of the mission's key goals is to follow up on the methane detection made by the Mars Express in 2004 to understand the processes at play in its generation and destruction, with a thousand times the accuracy of previous measurements. TGO will also image features on the Martian surface that may be related to trace-gas sources such as volcanoes. In addition, it will be able to detect buried water-ice deposits, which, along with locations identified as sources of the trace gases, could influence the choice of landing sites for future missions. The orbiter will also act as a data relay for the second ExoMars mission, comprising a rover and stationary surface platform, which is scheduled for launch in 2018, arriving in early 2019.
Bulletin compiled by Clive Down
(c) 2016 the Society for Popular Astronomy
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