November 2010: Good Vibrations

In this show we talk to Professor Yvonne Elsworth about solar and stellar vibrations. As always, Megan brings us the latest astronomical news and we hear what can be seen in the November night sky.

The News

• Small lumps of rock hit the Moon quite regularly, but in 2009 two artificial projectiles impacted on the lunar surface in an experiment designed to search for water in the permanent shadows of a crater near the lunar south pole. Hints of subsurface water on the Moon had already been found in 1999 when NASA's Lunar Prospector spacecraft detected signatures of concentrated hydrogen, the "H" in "H2O", near the lunar poles. The Lunar CRater Observation and Sensing Satellite, or LCROSS, was a low-cost mission launched together with the Lunar Reconnaissance Orbiter in June 2009. The mission consisted of the Centaur upper stage of the Atlas-V launch vehicle, and a shepherding spacecraft equipped with various cameras and sensors. Moving at a speed of 1.5 miles per second, the Centaur stage impacted the lunar surface on October 9th 2009, sending up a plume of material from the permanently shadowed floor of the crater Cabeus. The LCROSS spacecraft observed the impact before flying through the plume to impact the surface some four minutes later. In the October 22nd issue of Science magazine, several teams working on data from the impact publish their findings.

  Cabeus crater was chosen for the experiment as it contains an area which is permanently in shadow, due to its location close to the lunar south pole. The low temperatures, combined with the movement of soil (regolith) by micrometeorite impacts (known as "impact gardening") which buries accumulated material, makes such craters ideal places to search for volatiles - chemicals which are solid only at very low temperatures.

  Previous results from a neutron spectrometer aboard the Lunar Prospector spacecraft suggested that ice could make up between half and one percent of the soil near the lunar poles, and further results from a neutron detector on the Lunar Reconnaissance Orbiter showed a strong hydrogen signal, originally thought to be from water ice. But observations of the LCROSS plume, made with another instrument (LAMP, an ultraviolet spectrograph) on-board LRO, showed that as much of the hydrogen signal comes from molecular hydrogen as it does from water. Water is thought to have accumulated from cometary impacts, distributing water across the lunar surface in the ejecta, but it is far from certain where the molecular hydrogen originated.

  The results from the nine instruments on-board the LCROSS shepherding spacecraft, reported in Science on October 22nd, show signatures of numerous different chemicals, including water vapour, water ice and hydroxyl radicals, a common result of the breaking up of water molecules. Using the spectra obtained, the LCROSS team calculated that the maximum amount of water vapour and ice visible in the field of view of the instruments was 155 kilograms. By estimating the amount of material that was excavated by the Centaur impact and became observable by reaching sunlight, they calculated that the concentration of water in the lunar regolith at the impact site was 5.6 percent. They also found that the observed abundances of other volatile compounds, such as ammonia, sulphur dioxide and carbon monoxide, were far higher than the abundances found in comets, suggesting that molecule formation may be going on in these shadowed regions on the surfaces of cold dust grains.

• The most distant object in the known universe is a highly luminous gamma ray burst, a single explosion discovered near maximum light, at a redshift of 8.2, a time when the universe was only 630 million years old, less than 5 percent of its current age. The most distant known galaxy lies at a redshift of 6.96, the light we see now left the galaxy just 750 million years after the Big Bang. However, both these records have now been broken by a galaxy discovered by the Hubble Space Telescope which has a redshift of 8.56 and an estimated distance of 87 Gpc, making it the most distant object currently known.

  First seen in the Hubble Ultra Deep Field, the deepest single image ever taken in near-infrared light, the galaxy (known as UDFy-38135539) was initially classified as a candidate high redshift object based on its colours. Now, a team led by Matthew Lehnert at the Observatoire de Paris in France, has used spectroscopic observations to confirm that the object is the most distant galaxy so far detected.

  Since the universe is expanding, the further away an object is, the faster it appears to be moving away from us. This results in a shift in wavelength of the light emitted from an object (known as redshift) with the size of the shift relating to the distance between us and the object. (This is similar to the shift in pitch you hear when a police car travels past at high speed.) This effect allows distances to be calculated by measuring the shift in spectral lines from known chemicals. Lehnert's team used a sensitive spectrograph on the Very Large Telescope located in Chile to observe the spectrum of this galaxy and found an emission line which is likely to be caused by hydrogen shifted to redder wavelengths by the relative motion between the galaxy and us.

  This is an exciting discovery because it is the first galaxy discovered in the so-called epoch of reionisation, the period in the history of the universe where the neutral material between the newly formed galaxies was being ionised - the light from young, hot stars stripped electrons from hydrogen atoms. The authors used the measured light from the galaxy to calculate the size of the region of surrounding gas which the galaxy should have been able to ionise on its own and found that, in order to explain the size of the ionised bubble which is consistent with the observations, there must be other sources of radiation. One suggestion is that dwarf galaxies clustering around larger, more easily observed galaxies, may be responsible for this additional radiation, but there are other explanations.

  While observations such as these are difficult with current ground-based telescopes due to the faint nature of these distant sources, the planned next generation of larger and more sensitive ground- and space-based instruments should make observations of such sources much easier.

• Left over from the supernova explosions of massive stars, neutron stars are incredibly dense and compact objects, but very little is known about their internal structure. Pulsars, spinning neutron stars with powerful jets of radio emission which act something like cosmic lighthouses, are useful probes of extreme physics such as General Relativity and forms of matter so dense that investigating them in laboratories on the Earth is extremely difficult.

  Various models of the internal structure of a neutron star have been proposed, including various exotic forms of matter, but determining which is closest to reality requires knowledge of the distribution of masses and radii, measurements which require careful observations and depend to some extent on the exact models and assumptions used.

  Reported in Nature on 28th October, a team of astronomers using the Green Bank radio telescope in the US have discovered a pulsar with a mass twice that of the Sun. These new results have implications not only for our understanding of neutron stars and their formation, but also for our understanding of nuclear physics and matter at very high densities, and suggest that many of the theoretical models of neutron star structure can now be ruled out.

  Led by Paul Demorest of the National Radio Astronomy Observatory in the US, the astronomers observed the binary millisecond pulsar J1614-2230, a pulsar orbiting a white dwarf in a binary system which lies almost edge-on to our line of sight. This geometry was vital, allowing them to make use of an effect known as the Shapiro delay, an effect of general relativity. This is the delay of a signal caused as it moves through the gravitational field of the white dwarf companion - a delay which is a maximum when the pulsar lies on the far side of its orbit relative to the Earth. This effect allowed the mass of both the neutron star and its white dwarf companion to be measured precisely.

  The neutron star was expected to have a mass of about 1.5 times that of the Sun, but they calculated a mass of 1.97 solar masses for the neutron star, the highest mass to be accurately measured for such an object. Combining this mass measurement with predictions based on various different physical models allows several scenarios to be ruled out, including several exotic states of matter containing subatomic particles such as hyperons or kaons.

  The discovery also has implications for other astronomical events. One class of gamma ray burst is thought to be the result of colliding neutron stars. The fact that neutron stars have now been shown to be this massive makes this a viable mechanism for these events.

• And finally: October saw the first public data release from the Herschel-ATLAS project, the largest project awarded open time on the infra red satellite. Covering an area of 16-square degrees on the sky (more than 60 times the area of the full moon), the field contains more than 6000 galaxies imaged in five infra red bands (or colours) between 100 and 500 microns. The data were obtained as part of Herschel's Science Demonstration Phase in late 2009 and the ATLAS team have been working hard to process the raw images and produce catalogues of the objects detected. With so many galaxies in the field, the new data should provide much useful information to astronomers studying the evolution of galaxies over the history of the universe.

Interview with Professor Yvonne Elsworth

Professor Yvonne Elsworth is the head of the Octave research group at the University of Birmingham. Her research is in the fields of helioseismology and astroseismology, studying the oscillations of the Sun and other stars.

Acoustic waves travelling through the interior of the Sun will disturb the solar material and be seen as oscillations of the solar surface. Professor Elsworth uses the BiSON telescopes spread across the world to obtain continuous observations of the Sun. These observations allow the solar interior to be probed.

Professor Elsworth is also involved in astroseismology research, studying the oscillations of other stars. This is done by using data from space-based missions such as CoRoT and Kepler. Since recording this interview, the first results from the Kepler data have been released. You can read more about this on Universe Today and even listen to a Red Giant Symphony!

The Night Sky

Northern Hemisphere

Ian Morison tells us what we can see in the northern hemisphere night sky during November 2010.

The Summer Triangle of Deneb in Cygnus, Vega in Lyra and Altair in Aquila sets in the west during the early part of the night. The Square of Pegasus is upside-down in the south at this time of year. Above and left is Andromeda, the constellation containing our nearest neighbouring spiral galaxy, which is called by the same name or by the reference M31. The ‘W’ of Cassopeia is overhead, while Perseus lies in the east. The Double Cluster is situated between them, two open clusters visible as a fuzzy glow with binoculars or the unaided eye and resolvable using a telescope. The winter constellations are beginning to appear, with Taurus the Bull visible after dusk. The bright star Aldebaran is the bull's eye, while the more distant Hyades Cluster makes up his face. The Pleiades Cluster, or Seven Sisters, is up and to the right of this, and many more than seven stars can be made out with binoculars or a telescope. Orion follows close behind Taurus, rising in the south-east. Its stars have varied colours, with red Betelgeuse at the top left and blue Rigel at the bottom right, both giants of more than ten times the mass of our Sun.

The Planets

• Jupiter is the brightest object in the night sky besides the Moon in the early part of November, appearing in the south or south-east in the evening at magnitude -2.8. It has passed opposition and so is now receding from us, becoming very slightly smaller and fainter during the month. With a diameter of 45”, details such as the Great Red Spot and the current absence of the South Equatorial Belt can be observed. Jupiter’s four largest satellites, the Galilean Moons, can be seen through binoculars. Io, the nearest to the planet, has a molten interior and numerous volcanoes due to tidal friction from its host. Slightly further out, Europa has an icy crust, cracked by tidal forces and possibly covering an ocean of water which might even sustain life.
• Saturn becomes visible before dawn as the month progresses, having passed behind the Sun (superior conjunction) at the end of September. It is brighter than it has been during the last year, at magnitude +0.9, because the angle of its rings to our line of sight is now increasing, reaching 9° from edge-on.
• Mercury is very low in the south-west after sunset, and only readily visible for southern observers in the later part of the month.
• Mars is similarly low in the sky as the ecliptic is at a shallow angle to the horizon at sunset.
• Venus passed in front of the Sun (inferior conjunction) at the end of October, but reappears just before dawn early this month. It rises quickly because the ecliptic makes a wide angle with the horizon at sunrise at this time of year. Its brightness reaches a peak at magnitude -4.9 in the middle of the month. Venus always appear roughly the same brightness from Earth, because its surface is more fully illuminated by the Sun when it is further away from us.

Highlights

• The Leonid meteor shower occurs this month, peaking around the 17th-18th as we pass through the debris left by Comet Tempel-Tuttle. The peak of the shower will be late on the 17th, but the Moon will largely obscure the meteors this year. The best viewing may happen when it is setting, at about 04:00 UT/GMT (Universal Time/Greenwich Mean Time) on the 18th. The radiant, in the constellation of Leo, will be fairly high in the east at this time.
• On the 8th, southerly observers will be able to see the thin crescent Moon alongside Mars low in the south-west just after sunset.
• Towards the end of the month, Saturn and Venus will be close together in the pre-dawn sky, low in the south-east and not far from the Moon. Saturn will be near the double star Gamma Virginis, also called Porrima, which is at magnitude +2.7, and above Alpha Virginis, better known as Spica, a star of magnitude +1.0. Venus will be below, and to the left of, Spica.
• You can explore the Moon’s Maria (basalt planes, or, literally, ‘seas’) around the 21st. Some of these dark regions form the ‘Man in the Moon’, while smaller ones can be investigated with binoculars or a telescope. The basalt has been kicked up from under the Lunar surface by massive asteroid impacts throughout the Moon’s history.
• The Andromeda Galaxy is high in the south in the early part of the night throughout the month. To find it, move one star to the left of the top-left star in the Square of Pegasus. Take a slight right from that direction to the next star, then turn sharp right to a faint star and go the same distance again in the same direction to reach another faint star and a fuzzy glow. This is the core of the galaxy, and can be seen with the naked eye in a dark sky. Binoculars reveal a wealth of detail. It can also be found by following the direction in which the ‘V’ of the right-hand half of Cassiopeia points. If you then return to the star where you turned sharp right, and continue the same distance again in this reverse direction, you may see the Triangulum Galaxy, M33, the third-largest galaxy in the Local Group after Andromeda and the Milky Way. Its orientation, almost face-on to us, makes its light less concentrated than that from Andromeda.

Southern Hemisphere

John Field from the Carter Observatory in New Zealand speaks about the southern night sky during November 2010.

Summer is approaching, and the nights are getting shorter. Canis Major, Orion and Taurus rise in the east after sunset, all upside-down. They formed a hunting scene in the eyes of the Ancient Greeks and Romans. Sirius, the brightest star in the sky and part of Canis Major, is one of the nearest to us, at 8.6 light-years’ distance. It has a faint white dwarf companion, Sirius B, referred to as the Pup, which is the remnant of a now-dead star. The open star cluster M41, as numbered in the Messier Catalogue, lies just to the south. It can be seen as a haze with the unaided eye, while binoculars reveal individual stars in its population of around 100, including red giants. The Messier Catalogue was first published by Charles Messier in the 18th century as a list of night-sky objects which could be mistaken for comets. In the northern hemisphere, where all these objects are visible, astronomical societies often attempt to observe the whole catalogue in a one-night ‘Messier marathon’. In 1995, Sir Patrick Moore produced a similar compendium, the Caldwell Catalogue, containing objects from both hemispheres. M42, the Orion Nebula, is the outer edge of a molecular gas cloud about 1200 light-years from us and is visible as a haze above the three stars of Orion’s Belt. Stars are forming there, some of which can be seen to blow the edge of the cloud outwards. The nebula appears through binoculars as a bright knot with ‘wings’ on either side, illuminated by intense radiation from the star Theta Orionis, which is one of four stars in the central knot known collectively as the Trapezium. These can be seen using a small telescope. To the Māori, Orion’s Belt is known as Tautoru, and represents a tree branch that is part of a bird snare. Māori astronomers arranged stars into different constellations depending on the time of year. For native people on the east coast of New Zealand, the Pleiades and Hyades Clusters and Orion’s Belt and Sword form Te Waka o Tama-rereti, a canoe, which rises from the sea. The Pleiades are the prow, the Hyades the sail, the Belt the stern and the Sword the sternpost. In Māori mythology, Tama-rereti sailed across the heavens in this canoe and seeded the sky with Na Fetu, the stars, leaving in his wake Te Ikaroa, the Milky Way. The Southern Cross is low in the south at this time of year, but the bright star Achernar is high in the sky, and the Magellanic Clouds can easily be seen.

Highlights

• The Taurid and Leonid meteor showers occur at the beginning and in the middle of the month respectively, events that were once thought to result from terrestrial gases igniting in the Earth’satmosphere. Meteors are actually caused by Solar System debris burning up as it enters the atmosphere. Some are random, appearing at any time and in any part of the sky. Others come in showers - bunches of meteors emanating from specific parts of the sky, or radiants, and at certain times of year. The frequency of meteors in a shower builds to a peak and then fades away. The Leonids originate from the Sickle, an asterism within the constellation of Leo, and are associated with Comet Tempel-Tuttle. They are low in the sky for southern observers, and more visible further north. This year a near-full Moon will also reduce visibility. However, the number and brightness of meteors varies from year to year, so good observations may still be possible before dawn.
• Comet Hartley 2 is visible with binoculars or telescopes throughout the month. It was brightest at the end of October, but low to the north in the morning sky. It moves higher in the sky this month and passes into Canis Major in the first week of November, appearing close to its brightest star, Procyon, on the 6th. The comet becomes fainter during the month as it moves away from the Sun.

The Planets

• Venus and Saturn rise in the east in the early morning, not far from the Leonids’ radiant, and appear higher in the sky as the month progresses.
• Mercury and Mars are low in the west in the evening.
• Jupiter is in the north in the evening sky, well placed for viewing through binoculars or a telescope.
• Uranus is near to Jupiter in the sky, but gradually moving away. Both are becoming fainter as they move away from Earth, having passed opposition, the time when they appeared on the opposite side of the sky to the Sun.

Odds and Ends

Two spacecraft from the NASA THEMIS mission have now been brought back to life as the ARTEMIS mission.

At the time of recording, the space shuttle Discovery was due to be launched at the beginning of November. However, the launch has now been pushed back to the end of the month due to various problems. As well as carrying six human astronauts, Discovery will be taking the robot astronaut Robotnaut 2 up to the International Space Station. November 2 also marks the 10th anniversary of people living permanently on the ISS. At the time of the anniversary, the ISS will have completed 57,361 orbits of the Earth, traveling about 1.5 billion miles.

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