In the show this time Dr David Moss tells us about the modelling and observing the magnetic fields of spiral galaxies and Dr David Jess talks about the solar corona and its heating processes. Megan brings us the latest astronomy news and we find out about the October night sky from Ian Morison and John Field.
The News
In the news this month:
Seeing the plughole. Most galaxies appear to host a supermassive black hole in the centre, billions of times more massive than a single star. Many of these black holes, like the one in our own Milky Way, are currently quiet, but others are far more active, swallowing vast amounts of gas, dust, planets and stars and generating so much light that they can outshine the rest of their host galaxy. Some of the more active black holes also produce spectacular jets of radiation and high-energy particles which can extend for hundreds of thousands of light years and move at speeds close to that of light. While the large-scale structure of these jets agrees with predictions, the small-scale structures predicted at the black hole's event horizon, where the jet is launched, have yet to be detected. Since black holes are small, and galaxies with such active black holes are located far from the Earth, there is not a single telescope which is capable of making images with enough detail to test these theories. But now a team, led by Shep Doeleman at Haystack Observatory in the US, has used a collection of radio telescopes to peer down into the heart of M87, an elliptical galaxy located 50 million light years away which hosts a black hole some 6 billion times more massive than the Sun. Around active black holes lies a disk of material, known as an accretion disk, which spirals in towards the event horizon, the point beyond which nothing can escape from the intense gravitational field, not even light. This disk rotates, with material within it moving ever closer to the event horizon before finally being pulled in, never to be seen again. Einstein's general theory of relativity, our best description of gravity, says that the mass and spin of a black hole determine the closest possible orbit before matter gets pulled in, and it is this closest orbit where the jet in M87 is launched. By studying the black hole in detail, Einstein's theory can be tested under some of the most extreme conditions in the universe. But understanding what goes on so close to the black hole requires very high resolution observations, so Doeleman's team linked together three telescopes using a technique known as interferometry which allows widely-separated telescopes to be used together as if they were one giant telescope with a diameter equal to the distance between them. Using telescopes in Hawaii, Arizona and California, all operating at a frequency of 230 GHz (a wavelength of 1.3 mm), the astronomers were able to measure the innermost orbit of the accretion disk for the first time. They found it to be only 5.5 times the size of the black hole's event horizon. According to the laws of physics, this size suggests that the accretion disk is spinning in the same direction as the black holeâ the first direct observation to confirm theories of how black holes power jets from the centres of galaxies. The team plans to expand its telescope array, known as the Event Horizon Telescope, adding radio dishes in Chile, Europe, Mexico, Greenland and Antarctica, in order to obtain even more detailed pictures of black holes in the future.
One class of supernova, those known as Type Ia or thermonuclear supernovae, are thought to occur in binary star systems containing a white dwarf (a main-sequence star which has run out of hydrogen and evolved to become a small, dense star with a mass of ~1.4 times that of the Sun) together with a companion that is a red giant, subgiant, main-sequence star or another white dwarf. The white dwarf accumulates material from the companion star, until the density of material on the star's surface becomes extreme enough that a nuclear explosion takes place. If the companion star is a giant, subgiant or main sequence star, the build-up of material is slow and the companion will survive the explosion, remaining behind after the light from the violent destruction of the white dwarf has faded. In the other scenario, where the companion star is also a white dwarf (known as the 'double-degenerate' case), the two white dwarfs would merge before the explosion, leaving no companion behind. Both mechanisms are thought to occur, but which is more common is an open question. There have been several searches for remnant companions before, stars remaining after the explosion in the case where the companion is a giant, subgiant or main sequence star. One possible case where the companion star remained is the supernova observed by Tycho Brahe in 1572, although this result is not certain and has been debated. More recently, observations have restricted surviving companions to small, main-sequence stars, ruling out giant companions. Now, in an article published in Nature during September, a team led by Jonay Gonzalez Hernandez of the Instituto de Astrofisica de Canarias report the result of a search for surviving companions to the progenitor of SN 1006 AD and find no sign of a former companion. Using images and spectra, the team examined all the stars within 4 arcminutes of the position of the supernova. Many of the nearby stars are either too close or too far away from the Earth to be associated with the explosion, while others show no signs of higher levels of heavy chemical elements that are produced in supernova explosions. Together with previous results, this new study suggests that fewer than 20% of type Ia supernovae occur through the single-degenerate channel, that is, through the slow accumulation of mass from a large star. The more common trigger, it seems, is the rapid break-up of a smaller orbiting white dwarf.
September saw 600 of the world's leading planetary scientists gathering in Madrid for the European Planetary Science Conference. Presentations covered everything from planets, moons, comets and asteroids in our own Solar System, to exoplanets orbiting other stars, and even astrobiology and astrochemistry. As well as early results from the Curiosity rover, currently making its way across Gale crater on Mars, one presentation focussed on sub-surface liquid water on Jupiter's moon Europa. Europa's icy surface has been known about since the Voyager spacecraft visited the Jovian system in the 1970s. In the 1990s, more detailed images obtained by the Galileo probe showed much more detail and it became clear that the moon's surface is continually changing as the ice cracks and moves. It is thought that the moon is made mainly of rock and iron, with a water shell sitting between the rocky interior and the solid ice surface, but the question of just how thick this liquid layer is, and how close it is to the surface, is one that is much debated. The heat maintaining the sub-surface liquid comes as a by-product of the continual gravitational tugging from Jupiter, the same mechanism responsible for the high levels of volcanic activity on another of Jupiter's moons, Io.
At the European Planetary Science Congress, Klara Kalousova from the University of Nantes and Charles University in Prague, presented research suggesting that water does not stay in a liquid state near Europa's surface for longer than a few tens of thousands of years â the blink of an eye in geological terms. By mathematically modelling mixtures of liquid water and solid ice under different conditions she found that, due to factors such as density and viscosity differences, pockets of liquid water close to the surface would migrate rapidly downwards through partially molten ice and eventually reach the subsurface ocean, so any future missions to explore Europa's ocean may need to dig very deep.
And finally, the Hubble Space Telescope has taken many spectacular pictures since it was launched in 1990, including the Hubble Deep Field, a long exposure of a totally unremarkable patch of sky in the constellation of Ursa Major. This image covered such a small patch of sky that it contained only a handful of foreground stars from the Milky Way, but it also contained thousands of distant galaxies, some at extremely large distances such that we are observing them as they were when the Universe was a tiny fraction of its current age. Some years later, the Hubble Ultra Deep Field was created, a million-second exposure covering another small patch of sky about one tenth the diameter of the full Moon. This time in the constellation of Fornax, this image contains some 10,000 galaxies down to a magnitude of 30. Now, an even deeper image has been made. Observed in part of the Hubble Ultra Deep Field, the eXtreme Deep Field is the product of 2000 individual images and a total exposure time of two million seconds. Despite its smaller size on the sky, the XDF contains about 5,500 galaxies, some of them are so distant that the light we detect now left them when the universe was a mere 450 million years old. Many of the galaxies in this image are seen at a time when they were still forming, accumulating material from their surroundings and forming stars at a prodigious rate. The new image will be used to study how galaxies have formed and evolved over the history of the universe.
Interview with Dr David Moss
Dr. David Moss talks to us about modelling the magnetic filed of spiral galaxies. He explains how they observe the magnetic field using radio telescopes and how these observations match with their mathematical codes. He talks about the dynamo theory, the theory used to explain the formation and amplification of the magnetic field in spiral galaxies.
Interview with Dr David Jess
Dr. David Jess talks to us about the Sun and its various layers , focusing in particular on heating processes in the corona. He also tells us about solar telescopes and the challenges associated with them.
The Night Sky
Northern Hemisphere
Ian Morison tells us what we can see in the northern hemisphere night sky during October 2012.
The constellations of Cygnus, Lyra and Aquila can still be seen low in the west after sunset, hosting the Summer Triangle of the stars Deneb, Vega and Altair. The dark Cygnus Rift is about one third of the way from Altair towards Vega and contains Brocchi's Cluster, whose brighter stars form the asterism known as the Coathanger. Albireo, the head of Cygnus the Swan, is up and to the left of Brocchi's Cluster. A telescope reveals that it is a double star, made up of golden and blue companions. The Great Square of Pegasus is in the south in the evening. Starting at its top-left star, Alpheratz, move one star to the left, then move to another star slightly to the right, followed by a sharp right turn to the next star and then continue on the same distance again, and you will come to the Andromeda Galaxy, M31. It is visible to unaided eye on a clear, moonless night, but can be seen more easily using binoculars. In a dark sky, binoculars can also pick out the Triangulum Galaxy, M33. To find it, move back through the last two stars that you followed on the way to Andromeda and carry on the same distance again, then look for a faint patch of light. The constellation of Taurus rises in the east later in the evening, and the planet Jupiter is between the horns of the Bull at the beginning of the month. Orion rises later into the night, signalling the onset of the longer nights of autumn and winter. Cassiopeia is overhead, and if you follow the Milky Way down from here towards Perseus, you will find a pair of star clusters called the Double Cluster.
The Planets
- Jupiter rises around 22:00 BST (British Summer Time, one hour ahead of Universal Time) at the beginning of the month and around 20:00 BST at the end. It is best seen before dawn, when it reaches 60° elevation in the southern sky. It is in Taurus and has begun retrograde (westward) motion which will continue until February, when it will be back near the Hyades Cluster where it was to be found a few weeks ago. Its angular size increases over the month from 43 to 47", allowing surface details and moons to be seen.
- Saturn passes behind the Sun (conjunction) on the 25th, so it is not easy to spot this month. In the first week of October, however, it can be found low in the west-south-west half an hour after sunset, although you will probably need binoculars to distinguish it from the Sun's fading glow as it has a magnitude of +0.7.
- Mercury reaches elongation (its furthest point from the Sun in the sky) on the 26th, but is difficult to see as it only reaches 3° above the horizon half an hour after sunset.
- Mars still lingers over the south-western horizon just after sunset. It starts the month in Libra at magnitude +1.2, but moves quickly into Scorpius. It passes just 2° below the double star Beta Scorpii on the night of the 10th-11th. The two main components of Beta Scorpii, which is in the Scorpion's tail, are separated by 13.5" and can be distinguished using a telescope. Each of them actually consists of three stars, making the system a sextuplet. With a larger telescope, you can split one of the stars of the double into two objects of 3.9" separation.
- Venus dominates the pre-dawn sky with a magnitude of around -4.1 at the start of October. Moving through Leo, it is just 8' from the bright star Regulus at about 08:00 BST on the 3rd, so close that the glare of the planet may obscure the star. When they are slightly further apart, a colour contrast may be observed between them. The angular size of Venus drops from 16 to 13" during the month, while its illumination increases from 71 to 80%, so the planet's brightness declines only slightly to magnitude -4.0.
Highlights
- The 10th-magnitude comet C/2011 F1 (LINEAR) is close to the globular cluster M5 from the 4th to the 7th. The comet comes within 2° of the cluster in the constellation of Serpens, and can be seen with a small telescope. You may even be able to make out its small tail, pointing eastwards.
- An occultation of a star of magnitude +4.6 by the Moon ends at around 03:45 BST on the 7th. The star, Chi2 Orionis, will be obscured by the waning gibbous Moon and will then reappear.
- The Draconid meteor shower peaks on the 8th as the Earth crosses the orbit of the comet Giacobini-Zinner. The radiant, from which the comets appear to come, is in the north-west in the constellation of Draco, and you will benefit from a low north-western horizon after sunset if you are trying to see them. You may see more meteors by looking slightly away from the radiant, perhaps upwards to Cassiopeia. The Moon, rising in the east at this time, should not pose too much of a problem to the visibility of the meteors.
- Mars is near to a thin crescent Moon on the 18th, and is also just above the star Antares, the brightest star in Scorpius. This can be seen about an hour after sunset, low in the south-west. You may also spot Earthshine - the reflection of sunlight from the Earth onto the dark part of the Moon.
- The Orionid meteor shower peaks on 21st, with the radiant being in Orion. The Moon may hamper observations slightly, but not too much. These meteors come from Comet Halley, and are among the fastest that arrive on Earth, entering the atmosphere at around 40 kilometres per second.
Southern Hemisphere
John Field from the Carter Observatory in New Zealand speaks about the southern hemisphere night sky during October 2012.
The constellations of Scorpius and Sagittarius are in the western sky after sunset. Under a clear, dark sky, they may be accompanied by a triangular glow called the zodiacal light, which is sunlight reflecting off dust in the plane of the Solar System. This is a good time of year to spot it, because the Milky Way is at right angles to the ecliptic and so does not interfere. It will be most visible around the time of new Moon on the 16th. The planet Saturn and the star Spica are low in the west, and will soon be lost in the twilight sky as the nights shorten. Mars, moving away from the Earth, is higher up in the sky towards the star Antares, and the two share a reddish appearance. Neptune and Uranus are in the evening sky as well, but are difficult to pick out, even with binoculars, because they are so faint. The bright planet Jupiter rises later in the night. Binoculars or a small telescope reveal its four largest moons as they change position from night to night, while larger telescopes show atmospheric features on the planet, such as its light and dark bands.
Canopus, the brightest star currently in the evening sky, climbs in the south as the evening progresses. It twinkles in colour and brightness, especially when near the horizon. To Māori people, this star is known as Atutahi, the high chief of the heavens. The only star brighter than Canopus is Sirius, which rises in the south-east after midnight. Once it has risen, the three brightest night-time stars are visible as Alpha Centauri is also up. Alpha Centauri is one of two pointer stars that indicate the constellation of Crux, the Southern Cross. It appears as a single, yellowish star to the unaided eye, but a telescope splits it into a binary system. Crux is in the south-west after sunset. It reaches near to the horizon around midnight, but never sets in the skies over New Zealand. Travelling along the Milky Way from Crux, we come to a haze called the Carina Nebula, which is a vast star-forming region. In the north-eastern sky, the Andromeda Galaxy can be spotted, rising around midnight near the Great Square of Pegasus. A dark sky and a low northern horizon are needed to spot it from Wellington.
Odds and Ends
We mentioned in the May 2012 Jodcast that NASA astronauts would undergo cave training with ESA in an exchange with NASA after ESA astronauts visited an underwater training facility off the coast of Florida. They've recently completed the cave training expedition, but the ESA astronauts have been blogging about it. The expedition involved astronauts from the USA, Russia, China and Japan, as well as from ESA, learning to cooperate as a multicultural team. In a recent post they have fun with long-exposure photography underground.
New comet discovered C/2012 S1. Predicted that it could be brighter than the full moon by November 2013.
The Mars Rover Curiosity has imaged what scientist believe are dried streambeds on the surface of Mars. Rounded pebbles and conglomorates similar to those seen on Earth suggest that water once flowed on the Martian Surface. See: Universe Today and BBC News for more information.
Show Credits
News: Megan Argo Interview: Dr David Moss and Liz Guzman Interview: Dr David Jess and Melanie Gendre Night sky: Ian Morison and John Field Presenters: Adam Avison, Leo Huckvale and Stuart Harper Editors: Christina Smith, George Bendo, Claire Bretherton, Liz Guzman and Mark Purver Segment Voice: Cormac Purcell Website: Dan Thornton and Stuart Lowe Producer: Dan Thornton Cover art: An Ultra-Violet view of the Sun. CREDIT: NASA
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