October 2011: Dusty

In the show this time, we talk to Dr Greg Sloan about evolved stars and dust. As always, Megan rounds up the latest news and we hear what we can see in the October night sky from Ian Morison and John Field.

The News

In the news this month:

• Our current model of the early universe says that, as it expanded and cooled after the Big Bang, quarks began to coalesce to form protons and neutrons which, when the temperature dropped far enough, began to form simple nuclei. Eventually this material, mainly hydrogen with some helium and trace amounts of lithium, began to clump together, forming the stars and galaxies that we see today. Heavier elements such as carbon, nitrogen and oxygen, in fact pretty much everything that makes up this planet and all the life on it, were created later by processing of this primitive material in stars and supernova explosions. This processing in nuclear fusion reactions produces all the heavier elements that make up the universe. Since less massive stars last longer before running out of fuel, there should be a population of very low-mass stars which have been around since the early days of the universe. Such stars would be small, dim, and have an extremely low proportion of elements heavier than hydrogen and helium and, in a paper published in the journal Nature on September 1st, a team led by Dr Elisabetta Caffau at the University of Heidelberg in Germany has found just such a star in the halo of the Milky Way, but with an unusual chemical make-up.

  The star, located in the constellation of Leo and known as SDSS J102915+172927, has been found to have the lowest amount of elements heavier than helium of all stars yet studied, a quantity known as metallicity. While a few other primitive stars with very low metallicities have been found, the others all have carbon, nitrogen and oxygen in far greater quantities than would be expected for stars from the very first population. It is thought that low-mass stars such as these could only form after the interstellar gas had been enriched by supernova explosions with elements such as carbon and oxygen, since these elements act as a vital cooling agent, reducing the temperature of the gas cloud to the point where gravity can begin to overcome pressure and cause the clumping which eventually leads to stars. This conclusion means that the low abundance of elements including carbon, nitrogen and oxygen in the newly discovered star does not fit with current models of star formation in the early universe.

  A further puzzle with this star is the amount of lithium it contains; its lithium abundance is at least 50 times lower than that predicted by Big Bang nucleosynthesis. The likely explanation is that the stellar material must have experienced temperatures above 2 million K, the temperature required to destroy lithium. While the chemical composition of this star is something of a challenge to current models of early star formation, along with other examples that should be unearthed in planned surveys, it should provide clues which will help in our understanding of the very first stellar population.

• Much later in the universe's history, when previous generations of stars had enriched the interstellar gas clouds with significant quantities of heavy elements, planets began to form. The formation of rocky planets such as the Earth requires substantial amounts of elements such as iron, oxygen, nickel and magnesium, so they could not form until the first generation of stars had died. We currently know of more than 680 extra-solar planets in the Milky Way, with several surveys searching for new examples. One of the latest is Kepler-16b, the first planet confirmed to be orbiting a double star. This so-called circumbinary planet was discovered by the Kepler spacecraft, a probe which stares at the stars in one patch of sky continuously, watching for the tiny dips in brightness caused by planets passing in front of their host stars. Until it was found, the only confirmed planets found by Kepler were orbiting single stars, but since the majority of stars in our galaxy actually exist in multiple systems this discovery increases the likely number of planets in the galaxy.

  The research team, led by Laurance Doyle of the SETI Institute in the USA, used data from the Kepler satellite to find the new planet. The system, Kepler-16, consists of two stars in orbit around each other. The orientation of their orbit is such that, from our point of view on Earth, as the stars orbit each other they periodically eclipse one another, causing a drop in brightness as some of the light is blocked by the transiting star. The data, published in the journal Science during September, also showed that dimming occured when the stars were not transiting each other, implying the existence of a third body in the system. The timing of these eclipse events showed that the additional object was in a wide orbit around both stars, not just orbiting one of them, and the orbital properties and amount of light blocked suggests that the planet has a mass about 1/3 that of Jupiter and a size similar to Saturn. The planet's density is somewhat higher than Saturn's, however, suggesting a higher proportion of heavy elements. Saturn is composed of mainly hydrogen and helium gas, whereas the composition of Kepler-16b is thought to be approximately half hydrogen/helium gas, half heavy elements in the form of ice and/or rock. The fact that the planet repeatedly transits both stars shows that it is orbiting in the same plane as the two stars, strongly suggesting that it formed along with the stars rather than being captured by the stellar system sometime after its formation. While the discovery of the first circumbinary planet vastly increases the total number of planets that are estimated to exist in our galaxy, Kepler-16b itself lies outside of the habitable zone of its parent stellar system, so is unlikely to host life.

• While more and more examples of exoplanets continue to be found, there remain unanswered questions about our own planet and its formation. Certain elements, such as nickel, cobalt and iridium, are classed as siderophile, meaning iron-loving, and are rare on the Earth's surface because of their strong affinity for iron, the element which makes up roughly 88% of the Earth's core. During the Earth's early formation, the heavy iron sank to the core of the planet, taking most of the siderophile elements with it, part of a process known as differentiation (separation into core and mantle). But these elements are not as rare in the planet's crust today as might be expected. One possible explanation for this over-abundance is that these siderophile elements may not be quite so iron-loving at the high temperatures and pressures which existed at the bottom of the early Earth's molten magma ocean. While this explanation works for some such elements, it does not work for all of them.

  Another source of this siderophile material in the crust could be meteorites or asteroids impacting the Earth's surface some time after the initial formation of the planet, after the core had formed but while the solar system was still thick with large asteroids and embryonic planets. While there is plenty of evidence for such a late bombardment, from studies of the lunar surface as well as the Earth's, it is not certain that enough material would have been added in this late veneer to explain the measured abundances. But in a paper published in the journal Nature, a team shows new evidence for just such a process.

  In their paper, published in the journal Nature, Matthias Willbold of the University of Bristol describes the analysis of samples from Isua in Greenland which support the veneer hypothesis. Using new sensitive measurements, Willbold's team were able to determine the content of the isotope tungsten-182 in samples of rock to very high precision. Since tungsten-182 in the early Earth would have mainly ended up in the core, any enrichment found in samples of rock would be evidence for the late veneer theory. Using their new analysis methods, the researchers found that in most samples of rock from elsewhere on the planet, the abundance of tungsten-182 was what would be expected from the early Earth after differentiation, but samples from Isua in Greenland showed an increased level compared to the average for the Earth's mantle. The magnitude of the enrichment is exactly what is predicted by the late veneer model, providing strong evidence for the late bombardment being the cause of an excess, albeit patchy, of siderophile elements in the Earth's crust.

• It is thought that the Earth's growth largely ended with the impact which formed the Moon about 100 million years after the start of the solar system. But new research dating rocks from the Apollo 16 landing site suggests that the Moon is either significantly younger than this, or that the global magma ocean theory may not be the whole story.

  The initial impact of the Earth with a Mars-sized body, which is thought to have created the Moon, is believed to have resulted in a young Moon with a warm magma ocean covering the entire surface. As the Moon cooled it underwent differentiation, heavier elements sank while lighter minerals rose, resulting in a crust of light silicate materials surrounding a denser core. The first silicates to form on the surface were ferroan anorthosites, or FANs, thought to be the oldest lunar rocks. However, attempts to date samples of such rocks from the Apollo missions have given ambiguous results. But a team led by Lars Borg of the Lawrence Livermore National Laboratory in the USA have used improved methods of isotopic dating to determine the age of the Moon more accurately than has been achieved before.

  What they found is that the FANs are significantly younger than previously thought, a mere 4360 million years, implying that the Moon solidified some 200 million years after the formation of the solar system, somewhat later than previous estimates. The team used two isotopic tracers: the ratio of lead-204 to lead-206, and that of samarium-147 to neodymium-143, with both tracers giving the same age. Comparing this new result with the ages of the oldest rocks on the Earth, zircons from deposits found at Jack Hills in Western Australia, suggests that the Moon differentiated some 30 million years after the Earth, implying either that it accreted very slowly, or that it kept enough internal heat to delay the solidification of the surface magma ocean. The alternative explanation is that the sample is not from cooling of a global magma ocean, but instead is the result of a more recent melting event. This has implications for our understanding of the formation of all rocky planets, since the magma ocean theory which is used to explain the observed planetary structures was developed based on these exact rocks.

• And finally. September saw the launch of another mission to the Moon: the Gravity Recovery and Interior Laboratory (GRAIL). Based on a tried and tested concept, the twin spacecraft will orbit the Moon, mapping not its surface but its interior. The two probes are built using the same design as the Gravity Recovery and Climate Experiment (GRACE), another pair of satellites which have been in orbit around the Earth since 2002, mapping the planet's gravity field in exquisite detail. Although carrying several cameras, the GRAIL satellites are not mapping the surface and the returned images are not the point of the mission. The satellites actually only carry one scientific instrument: a high-frequency radio link between the two craft which will be used to measure the distance between them to very high precision as they orbit the Moon at a height of 55 km. When one of the probes passes over a region of higher density, like a mountain, the slight increase in gravitational force will cause it to speed up slightly, altering the separation from its companion. The result of the mission will be the most detailed map of the lunar gravity field ever made, helping scientists understand events such as giant impacts and how the Moon's layers formed as it cooled, results that, in turn, could help in our understanding of the entire inner solar system, including our own planet.

Interview with Dr Greg Sloan

Dr Greg Sloan studies the thick dust shells of ejected mass surrounding dying stars. He uses infrared spectra to investigate star mass loss and to understand how the accumulated dust can actively take part in the mass loss of its dying parent star. He has examined galaxies within our 'local' area, which, rather than being similar to our own Milky Way, are in fact closer in chemistry to the primordial Universe. In this interview, he discusses the difference between the Milky Way and our neighbouring galaxies, and we find out what astrophysicists can learn from dust shells and hear about stars dropping out of the visible night sky! Dr Sloan also talks about the data he accumulated from the Spitzer Space Telescope and the possibilities of using ALMA (a topical telescope at the moment) and the Planck satellite for his research.

The Night Sky

Northern Hemisphere

Ian Morison tells us what we can see in the northern hemisphere night sky during October 2011.

The Cygnus Rift, a dark band of dust in the Milky Way, can be seen cutting through the Summer Triangle made up of the stars Deneb, Vega and Altair. Brocchi's Cluster lies between Altair and Vega. In a dark sky, the constellation of Delphinus, the Dolphin, can be seen below and left of Cygnus the Swan, with the tiny constellation of Sagitta, the Arrow, above it. The planetary nebula known as the Dumbbell Nebula can be seen through binoculars or a small telescope above the tip of the Arrow. The Square of Pegasus is to the east, from which the Andromeda Galaxy can be located with binoculars or, if it is very dark, with the naked eye. The Triangulum Galaxy can also found nearby using binoculars. Further east, Taurus rises late in the evening, containing the Pleiades Cluster. The Double Cluster can be seen with the naked eye in Perseus, below the 'w' of Cassiopeia which sits almost overhead. Two distinct star clusters can be made out with binoculars. Orion the Hunter rises after midnight BST (British Summer Time, one hour ahead of Greenwich Mean Time), allowing the Orion Nebula to be seen below his famous Belt.

The Planets

• Jupiter is visible all night by month's end, reaching opposition (the opposite side of the Earth to the Sun) on the 29th. At a maximum elevation of 47°, it is higher than it has been for several years, reducing atmospheric scintillation and improving the resolution with which detail can be discerned through a telescope. It also has an angular diameter of 50", which is near its largest possible apparent size, because it was recently at perihelion (closest approach to the Sun) as well as this month coming to opposition and therefore close to the Earth. The Equatorial Belts can be seen through a telescope, as can the Great Red Spot at certain times.
• Saturn and Mercury are not visible. Saturn passes behind the Sun (superior conjunction) on the 18th, and will reappear before dawn in about a month. Mercury reached superior conjunction on the 28th of September and so is also washed out by the Sun's glare.
• Mars moves from near the Beehive Cluster in Cancer down into Leo during the month, rising around 1am BST. With an angular size of 5.2", surface details can only be seen through a telescope under good seeing conditions. Itt will reach an angular size of over 10" early in the new year, allowing its features to be resolved much more clearly.
• Venus is still near the far side of the Sun from the Earth's viewpoint, so its disc is 94% illuminated but it appears relatively small at 10" across. It is briefly visible very low in the south-west just after sunset due to its bright magnitude of -3.9, but you may require binoculars to spot it (remember never to point binoculars or telescopes anywhere near the Sun).

Highlights

• The peak of the Draconid meteor shower is visible around the constellation of Draco, low in the north-west from around 8pm to 10pm GMT on the 8th. It is caused by dust particles from the tail of comet Giacobini-Zinner entering the Earth's atmosphere. As the dust is not yet evenly distributed around the comet's orbit, the shower is more intense approximately every 13 years, when the comet is closest to the Earth. The intensity is unpredictable, and this year's shower may be spectacular.
• The Orionid meteor shower peaks on the 21st and is visible late at night for a few nights either side of this, around the constellation of Orion. The dust causing the shower is believed to come from comet Halley. The Moon may hamper meteor sightings, but some should still be visible in the south before dawn.
• Uranus and Neptune can be observed without the glare of the Moon from the 24th to 28th, the former in Pisces and the latter in Aquarius.

Southern Hemisphere

John Field from the Carter Observatory in New Zealand speaks about the southern hemisphere night sky during October 2011.

The constellations of Scorpius and Sagittarius lie in the west. The planets Mercury and Venus are below them, appearing close together at sunset at the end of the month. In a dark sky, the Zodiacal Light may be visible after twilight as a faint, broad column of light. It is the reflection of sunlight off meteoric dust in the plane of the Solar System, hence its presence in the zodiacal constellations through which the ecliptic plane passes. Jupiter rises in the east after sunset and is high in the northern sky by midnight. The planet's four largest moons can be seen through binoculars or a telescope, and move from night to night, sometimes disappearing behind or in front of their host. Canopus is the brightest star in the evening sky this month, climbing in the south as the evening progresses. It is later outshone by the brightest of all night-time stars in our sky, Sirius, which rises in the east after midnight. While Sirius is much closer to us than Canopus, Canopus is intrinsically far more luminous, shining some 13,000 more intensely than our Sun. The third-brightest star in the sky, Alpha Centauri, will also be visible, near the Milky Way. The constellation of Crux, the Southern Cross, skirts the southern horizon from west to east during the night but does not set, while the four stars forming the Great Square of Pegasus rise and move across the northern sky from east to west before setting again. The triple star Epsilon Pegasi is not in the Square but is the brightest star of the Pegasus constellation, and consists of a yellow supergiant at magnitude +2.9 (visible with the naked eye), a blue companion at magnitude +8 (visible using binoculars) and a further companion at magnitude +11 (visible through a telescope). Nearby is the globular cluster M15 at magnitude +6, with a bright centre and fainter rays scattering from its core. A larger telescope reveals a planetary nebula within the cluster. The constellation of Cetus, the Whale, is near to Aquarius, the Water Carrier, and Eridanus, the River. It is near to the edge of the ecliptic plane and planets sometimes move briefly through it. The asteroid Vesta was discovered there in 1807. Alpha Ceti, or Menkar (the Nose), is a red giant star of magnitude +2.5. Binoculars show an unconnected blue star of magnitude +5.6 nearby. Beta Ceti, or Deneb Kaitos (the Whale's Tail), is the constellation's brightest star, a yellow giant of magnitude +2.0. Gamma Ceti, or Kaffaljidhma, is a double star comprising individual stars of magnitudes +3.7 and +6.4. Omicron Ceti, or Mira, is a red star whose variable brightness was recognised by the Dutch astronomer David Fabricius in 1596. It swells and contracts with a period of between 320 and 370 days, giving it a magnitude range of +3 to +9. To the naked eye, this makes it appear and disappear over the course of a year. The flare star UV Ceti is a red dwarf star which undergoes sudden increases in brightness lasting only a few minutes, taking it from magnitude +13 to +7. Tau Ceti, at 11 light-years away, is the closest Sun-like star to our Solar System. It has a debris disc which may one day coalesce into planets.

Odds and Ends

The first part of the Chinese space station Tiangong 1 was launched on September 29.

NASA announced the beginning of development of the Space Launch System, an advanced heavy-lift launch vehicle that will provide an entirely new national capability for human exploration beyond Earth's orbit. The project will take advantage of technologies developed from the Space Shuttle Program and the Constellation Program. The first developmental flight, or mission, is targeted for the end of 2017.

October 4 - 10 is World Space Week.

At the end of September, the aurora was spotted in the south of England (photos tweeted by @scienceoxford). A spectacular display of the southern lights (aurora australis) was also snapped by astronauts on board the ISS.

New observations by NASA's Wide-field Infrared Survey Explorer (WISE) exonerate the family of asteroids some believed was responsible for the demise of the dinosaurs. The age of this asteroid family, remnants of a collision between giant asteroid Baptistina with another object in the main belt between Mars and Jupiter, was originally estimated to be about 160 million years ago. The debris of this collision would then have been reaching Earth about 80 million years ago, around the time dinosaurs disappeared. This age estimate, which was obtained by looking at the light reflected by the asteroid, has been revised by WISE, which uses direct infrared observation of the asteroids to determine their age. According to WISE, the original Baptistina asteroid actually broke up closer to 80 million years ago, meaning that the debris did not reach Earth in time to be the culprit in the extinction of dinosaurs.

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