In the news this month: discs of dust and galactic winds.
Discs of debris are observable around approximately 15% of stars. Early in the life of a star, the debris disc will be a flat plane filled with ice, rocks and gas that stretches out from the star to the outer edge of the solar system. The speed at which the material moves slows down the further out from the star you travel, and at the very edge everything moves so slowly that things very rarely collide. Collisions within the debris disc are vital for planets to form, and if this does not occur then you are left with a belt around the star containing lots of small comets, astroids and meteors, much like our own Sun's Kuiper belt.
When testing theories of planet formation, debris discs like the Kuiper belt could potentially contain a significant amount of information about the planets that can and cannot be seen around a distant star. For example, as mentioned earlier, debris discs start out filled with ice, rocks and gas but within only ten million years all the gas will disappear due to it either being absorbed by the host star or expelled out of the solar system by powerful winds from the star. The disappearance of the gas is important because it places a limit on how long gas-rich planets like Jupiter have to form, because if they take longer than ten million years there simply would not be any of the gas left to make them. Debris discs can also give us a method of estimating the masses of Jupiter-type planets. As the planets pass near to the debris disc, their enourmous mass causes waves to form, the size of the waves being related to the mass of the passing planet. This is how the mass for a nearby planet orbiting the star Fomalhaut was estimated to be up to three times the mass of Jupiter.
A pair of theoretical exoplanet scientists have been studying the motion of the waves within the debris disc around Fomalhaut, and have found that they are difficult to explain via gravitational interactions with the orbiting Jupiter-like planet. Instead, they have found that some debris discs around other stars still contain a small but significant amount of gas after the first ten million years. No such detection has yet been made for the Fomalhaut debris disc, but if it were present they think that the waves could be explained by interactions between the small amount of gas and the dust, indicating that the involvement of a planet in producing the waves in the disc is minimal. Although this means that the measurement for the mass of the Jupiter-type planet around Fomalhaut may be wrong, in the end it is nothing but a positive for planet formation theories, as whether there is gas or not we have a model which can predict either outcome.
Throughout a galaxy's lifetime, it is constantly being moulded and carved by powerful galactic winds. The age of the stars within a galaxy, its size and even its overall shape, be it a sphere, a disc or anything else, are all defined by the galactic wind which rages in the space between stars. In the early times, the galactic wind is driven by violent ejections of mass from unstable massive stars, then later by supernovae which carve out huge bubbles of hot ionised material, sometimes with such ferocity that the material escapes the galaxy altogether. The other force that drives the galactic wind is found at the core of every galaxy: a supermassive black hole, which consumes stars and launches the left-over gas and dust away at near to the speed of light.
Understanding in detail how galactic winds form and how powerful they are helps us to put together a map of the evolution of galaxies. Currently there are many things not understood; for example, we do not know why nearby galaxies seem to transition sharply from healthy galaxies, which form at least one star per year, into dead galaxies with masses greater than 30 billion Suns that seem to have not formed any stars for billions of years. The answer to problems like this lies in understanding not just the overall picture of the galactic winds within a galaxy, but also the dynamics of individual regions within galaxies. This is a difficult task as it requires observing regions only a couple of hundred light years in size within galaxies tens of millions of light years away.
A team of astronomers working with a range of instruments including the Atacama Large Millimeter/submillimeter Array, the Very Large Array and a number of optical and X-ray telescopes has been working to understand the galactic wind within a nearby galaxy called NGC 253. NGC 253 is a starburst galaxy, meaning it is producing far more stars per year than would typically be expected, and has been observed to contain a substantial galactic wind, driven not by a supermassive black hole or supernovae but primarily by a host of many young, massive stars. The team's analysis focused on a small region near to the centre of NGC 253, and what they observed was a flow of material travelling at hundreds of kilometres per second and carrying away the mass of nine Suns per year. However, the research found that it is highly likely that the majority of the mass carried away will not fully escape the gravitational grip of NGC 253, but will instead become trapped within wispy clouds spread throughout a halo that surrounds the galaxy. The significance of this is that, even if NGC 253 ejects all the material needed to form new stars, which at its current rate it will within a billion years, the mass that becomes trapped in the halo may slowly reincorporate itself into the disc of NGC 253 and start a new round of star formation in the distant future.
Interview with Dan Thornton
Dan Thornton, a PhD student at the Jodrell Bank Centre for Astrophysics and fellow Jodcaster, tells us about Fast Radio Bursts (FRBs). He explains that FRBs are very narrow single pulses of radio waves a few milliseconds in duration, and tells us about the High Time Resolution Universe radio survey on the Parkes telescope, the survey in which they were found. Although there are several theories of their formation, there is not much evidence to constrain their properties - but they are probably cataclysmic events. To date, no optical or gamma-ray counterparts have been found. These FRBs were found at 'cosmologically significant' distances - looking about halfway back to the time of the Big Bang - and Dan goes on to describe how the distances are measured using the dispersion measure. Dan also tells us about the future and follow-up work on these FRBs, looking at identifying counterparts at other frequencies and the work on automatic identification. Further information about FRBs can be found here.
The Night Sky
Ian Morison tells us what we can see in the northern hemisphere night sky during August 2013.
In the evening, the stars Vega, Deneb and Altair, in the constellations of Lyra, Cygnus and Aquila, make up the Summer Triangle in the southern sky. About a third of the way up from Altair towards Vega, the asterism of the Coathanger lies in the dark Cygnus Rift. The Ring Nebula, in Lyra, and the Dumbbell Nebula, in Vulpecula, are nice lanetary nebulae that can be viewed in binoculars in this part of the sky. Pegasus and Andromeda rise as the night moves on, the latter containing the galaxy M31. Cassiopeia and Ursa Major are high overhead, near, as always, to the North Star.
- Jupiter rises at about 03:30 BST (British Summer Time, one hour ahead of Universal Time) at the beginning of the month, shining at magnitude -1.9 and with a disc of 33" across. It is only 20° above the horizon 45 minutes before sunrise. It begins August within 5° of Mars in the sky and gradually moves away during the month. Towards month's end, it rises 3-4 hours before the Sun and lies between the feet of Castor and Pollux, the Gemini Twins. A small telescope reveals the four Galilean moons.
- Saturn lies in Virgo after sunset, 11° away from the bright star Spica in the south-western sky. At magnitude +0.7, it displays a yellowish hue and its disc has a diameter dropping from 16.7 to 16.2" during the month. At the beginning of August, it is 1° away from the star Kappa Virginis, which has a magnitude of +4.2. It moves eastward towards Alpha Libri over the course of the month. Saturn's rings are 17° from the line of sight, with the planet's southern hemisphere visible. A small telescope under good seeing conditions should show Cassini's Division and Saturn's largest moon, Titan, in spite of the relatively low elevation of the system.
- Mars moves from Gemini into Cancer on the 25th. At the beginning of the month, it sits 15° above the north-eastern horizon 45 minutes before sunrise. It has a magnitude of +1.6 and an angular diameter of 4", preventing surface detail from being seen.
- Mercury, appearing in the morning, brightens in magnitude from -0.5 to -0.8 during the month, but its elevation decreases and it is lost from view around the middle of August. Mercury passes 4° below Comet ISON on the 8th, but the comet will be too dim to observe easily; they will appear close together again in late November, when they may be of comparable brightness.
- Venus starts the month 32° east of the setting Sun, but is only 11° above the horizon when it becomes visible. It moves to 39° from the Sun by the end of the month, but its elevation in the evening sky does not improve. Its magnitude changes from -4 to -3.9 during the month, while its angular diameter increases from 12.8 to 14.2" and its phase can be observed using a small telescope. Venus moves from Leo into Virgo, ending August 5° from Spica.
- The Perseid meteor shower peaks on the 12th, and so is best seen after midnight at the beginning of the 12th or 13th. The Moon will not hamper observations this year, and so the debris from Comet Swift-Tuttle will be visible at it blazes through the Earth's atmosphere.
- Venus will lie above a thin, waxing crescent Moon just after sunset on the 9th. You will need a low western horizon and may be able to spot Earthshine - reflected sunlight from the Earth faintly illuminating the dark part of the Moon.
- It is a good time to observe Neptune this month. Coming into opposition on the 27th, when it is furthest from the Sun in the sky and also relatively near to the Earth, it has a magnitude of +7.9 and can be seen using a telescope. It reaches a maximum elevation of 27° when due south. On a dark night, a telescope with an aperture of more than 8" can be used to spot the planet's largest moon, Triton.
- Comet ISON may reach a brightness of magnitude +12 late in the month, bringing it within range for medium-sized telescopes. On the 31st it is about 2° above M44, the Beehive Cluster, in Cancer.
John Field from the Carter Observatory in New Zealand speaks about the southern hemisphere night sky during August 2013.
After sunset, the planets Venus and Saturn, along with the constellations of Virgo and Corvus, can be seen in the west. Venus is a brilliant 'star' near the western horizon, while Saturn is much higher and appears yellow. Small telescopes reveal Saturn's rings and largest moon, Titan, while larger ones show faint banding in the planet's atmosphere and variations in the colour of the rings. Jupiter is in the morning sky, along with Mars and, at the beginning of the month, Mercury. The two brightest stars in the sky after sunset are Spica, in Virgo, and Arcturus, in Boötes the Herdsman. Epsilon Boötis is a lovely double star, consisting of a yellow giant primary star and a bluish-white companion, also known as Pulcherrima - 'most beautiful'.
August sees the Milky Way stretching from east to west across the evening sky, and it contains most of the brightest stars in our night sky. In the north, Vega is to the left of the Milky Way, while Canopus is to the south. Alpha Centauri and Beta Centauri are in the south-west after sunset and point towards the Southern Cross, the smallest of the 88 official constellations. The winter constellation of Scorpius sits high overhead, with the orange star Antares marking the Scorpion's heart and a curve of stars forming its body. Following Scorpius is Sagittarius the Archer, another zodiacal constellation.
Away from the Milky Way, the density of stars drops off rapidly. About halfway above the southern horizon in the evening is Tucana the Toucan. Alpha Tucanae has a magnitude of +2.8, while Beta Tucanae is a loosely bound group of six stars and Kappa Tucanae is a system of two pairs of binary stars. Tucana contains the dwarf galaxy known as the Small Magellanic Cloud (SMC) and the second-brightest globular cluster in our sky, 47 Tucanae. The SMC appears as a cloudy smudge to the unaided eye, and contains several hundred million stars some 200,000 light-years away. With binoculars or a small telescope, star clusters can be seen in and around the SMC. The fuzzy blob of 47 Tucanae sits alongside the SMC, and is a globular cluster of millions of stars within our own galaxy. A stunning sight through binoculars or a telescope, professional telescopes have spent long hours examining 47 Tucanae, but have so far found no planets orbiting its stars. This may be due to the low metallicity of the stars, but the absence of gas giants is still puzzling. The brightest globular cluster in the sky, Omega Centauri, is also well placed for viewing, appearing to the naked eye as a fuzzy star in the north. Over the night, the Southern Cross and the Pointer Stars turn about a point called the South Celestial Pole. This point would appear directly overhead if you were at the South Pole, and all the stars in the southern sky appear to circle around it as the Earth rotates.
- A number of bright aurorae have recently been visible from the lower North Island and the South Island of New Zealand, including four observed in Wellington. These can now be forecast, and may contain over the coming month.
- Late next month, Comet ISON will reappear in the morning sky, and may begin to give an indication of how bright it will get later in the year.
Odds and Ends
The Atacama Large Millimeter/submillimeter Array (ALMA) announced in the middle of July that astronomers had used the telescope to detect carbon monoxide (CO) ice around TW Hydrae, a protostellar object at a distance of 175 light-years from Earth. This is the first time that astronomers have seen a "snow line" around another star. The astronomers did not actually find the ice by looking at the CO itself because CO can be found in gas form around many stars and in the interstellar medium. Instead, they determined that ice was present by detecting diazenylium (N2H+), an ion that is easily destroyed when mixed with CO gas but that can exist in solid CO. This result is very important for understanding how planets form around other stars and for understanding how organic chemicals can be created in protostellar discs.
Scientists at the Albert Einstein Institute in Postsdam have been carrying out numerical simulations on neutron star mergers and, for the first time, have seen instabilities which would lead to the formation of giant magnetic fields and some of the most energetic explosions that the Universe has ever seen. When two neutron stars in a binary system merge, they collapse into a black hole and the one-second-long gamma-ray burst they emit as a result of this contains the same amount of energy as our galaxy emits in a year. It is now believed that such high energy levels can be achieved via "magetorotational instabilities" - in other words, turbulence within the colliding plasmas of the neutron stars which would result in enormous magnetic fields, thus fuelling these highly energetic gamma-ray bursts.
A 24-hour observation of Comet ISON using NASA's infra-red Spitzer Telescope has revealed gas and dust flowing from the icy body in a coma 300,000 kilometres long. 1,000 tonnes of what is probably carbon dioxide, along with 50,000 tonnes of dust grains, is being ejected per day as the comet approaches the Sun. ISON will shortly get close enough to melt water ice, and will make a closest approach to the Sun of 1.16 million kilometres on the 28th of November - an encounter which it may or may not survive. Observers will have to wait some time longer to find out whether the comet will become bright enough to be seen with the naked eye from Earth.
|Interview:||Christina Smith and Dan Thornton|
|Night sky:||Ian Morison and John Field|
|Presenters:||George Bendo, Mel Irfan and Mark Purver|
|Editors:||Adam Avison, George Bendo, Mark Purver and Christina Smith|
|Segment Voice:||Mike Peel|
|Website:||Mark Purver and Stuart Lowe|
|Cover art:||Composite image of the Parkes Radio Telescope and a radio map of the Milky Way, with an artist's depiction of a Fast Radio Burst coming from outside our Galaxy. CREDIT: CSIRO/Harvard/Swinburne Astronomy Productions|
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