September 2010: Kinky

In this show Jonathan Pearson, a PhD student at JBCA, tells us about theoretical cosmology and his work trying to explain the Universe using domain walls and kinky vortons. As always, we have the latest astronomical news, and what you can see in the September night sky in the northern and southern hemispheres.

The News

In the news this month:

• In the history of the universe, the question of which came first, galaxies or supermassive black holes, is somehow reminiscent of the old chicken and egg problem. It has been observed for some time that the mass of the central black hole correlates with properties of the host galaxy, suggesting that their formation may be linked. But did galaxies form first with black holes being created in the centre, or did black holes come first with galaxies forming around them? Black holes are objects with such a strong gravitational field that nothing can escape, not even light. So-called stellar mass black holes are created when massive stars explode, but the supermassive black holes which lie at the heart of galaxies can be many millions of times more massive than the Sun. While we may not be able to see them directly, we can see their effect on surrounding material. Studies of the motions of stars near the centre of the Milky Way show that there must be a massive object at the centre of the Galaxy, the observed stellar velocities can only be explained by an object of some four million solar masses. Our galaxy's central black hole is currently quiet, but others in more distant galaxies are far more active. But, although we know they are there, exactly how they formed is still a puzzle. Models where black hole 'seeds' form from primordial stars cannot explain how billion solar mass black holes form so quickly, before the universe was even one billion years old, and direct formation by gas accretion in the core of a proto-galaxy requires specific conditions that are unlikely to have been the case in reality. Now, in the August 25th issue of the journal Nature, Lucio Mayer and colleagues describe simulations which have provided a possible mechanism. In their model, the collision and merger of early proto-galaxies produces just the right conditions for supermassive black hole formation, without invoking special conditions such as the suppression of star formation. The collisions in their simulations show that such a merger rapidly drives large amounts of gas towards a central unstable disk through a spiral shaped instability that rapidly draws material into the core until it undergoes gravitational collapse, forming a black hole on timescales of 100 million years. As well as providing a suitable formation mechanism, the simulations also reproduce the observational result that smaller dwarf galaxies are too small to form central supermassive black holes, the inflow of material in these smaller mergers is below the threshold required to trigger and support the spiral inflow seen in mergers of more massive systems.

• When stars more than eight times as massive as the Sun run out of fuel at the end of their lives, they explode as so-called core collapse supernovae. The nature of the remnant that is left behind depends on the mass of the original star. Those with initial masses between about eight and twenty-five times that of the Sun are thought to produce compact neutron stars, while those above twenty-five solar masses would result in stellar-mass black holes. But astronomers studying massive stars in the active star forming region Westerlund 1 have discovered an example of a neutron star that formed from a star at least forty times as massive as the Sun, a star massive enough that it would have been expected to form a black hole. The team, led by Simon Clark of the Open University, have been studying stars in Westerlund 1, the closest known super star cluster, located just 16,000 light years away and containing hundreds of massive stars. The cluster was formed in a single starformation event, so the stars all have the same age, making it very useful in starformation studies. As well as massive single stars, the cluster also contains several binary stars, and a magnetar - an unusual type of highly magnetic neutron star. The team studied several binary systems in the cluster, estimating the masses of the stars in each system using the measured orbital velocities (since the heavier a star, the faster other objects will move in orbit around it). The more massive a star is, the sooner it runs out of fuel and explodes so, if all the stars in the cluster are the same age, the star which exploded to create the magnetar must have been more massive than the remaining stars. Using their observations, the astronomers calculated that the magnetar's progenitor must have been more than 40 times as massive as the Sun, raising questions about just how massive a star has to be in order to form a black hole. Such massive stars could form neutron stars instead of black holes, but only if they can lose more than nine tenths of their initial mass before exploding as supernovae. The proposed mechanism for the formation of the magnetar in Westerlund 1 is that the progenitor was initially part of a binary system which has since been disrupted. Such a binary companion would pull material from one star to the other and result in sufficient mass loss for neutron star formation. In their paper, accepted for publication in the journal Astronomy and Astrophysics, the team point out that there are several scenarios for how this mass transfer could occur, and further observations of binary systems in Westerlund 1 should allow the possibilities to be narrowed down.

• Stars form from gas clouds which collapse under gravity, becoming hotter and denser as they collapse until conditions reach the point where nuclear fusion can begin and a star is born. For some time after a star begins to shine, it is surrounded by the remnants of the gas cloud and a disk of dusty debris, visible in infra red observations. However, the stellar wind eventually drives away this material and the disk disappears, so observations of older, more evolved stars would not be expected to show infra red emission above that expected to be generated by the star itself. But such an excess is exactly what is seen in observations of a certain class of evolved close binary star systems. These systems, known as RS CVns, consist of two stars orbiting in close proximity, orbiting each other every few days, close enough to be tidally interacting. In a fraction of these systems, observations show an excess amount of infra red emission that cannot be explained by standard stellar models. The excess observed is however what is normally expected from a debris disk. The catch is that the stars in these systems are old enough that they should have long-since blown away any surrounding debris disk leftover from the starformation process, so where is this emission coming from? A team of astronomers, led by Marco Matranga of the Harvard-Smithsonian Centre for Astrophysics in Massachusetts, carried out a survey of 10 such binary systems and found three that showed evidence of debris disks. The stars in these binaries are separated by just three million kilometres, one fiftieth of the distance between the Earth and the Sun, orbiting each other every few days, always keeping the same face towards each other, much like the Moon always shows the same face to the Earth. They are similar to the Sun, but somewhat younger with ages of between one and a few billion years, and much stronger magnetic fields which drive powerful stellar winds. Previous studies have suggested that as the stars spiral closer together the gravitational variations could lead to the disruption of any surrounding planetary systems, resulting in catastrophic collisions which could create a new debris disk. This new study, published in Astrophysical Journal Letters on August 19th, shows evidence of just such warm dusty debris disks around three binary systems in observations carried out with the Spitzer satellite before it ran out of coolant in May 2009. The astronomers also observed two systems that were reported to have similar infra red excesses in previous IRAS studies, but found no such excess in the new observations. Models of dust grains in such binary systems show that disks created by planetary collisions are likely to dissipate on timescales of between tens and hundreds of years, so the authors suggest that the apparent disappearance of the infra red excess in these two systems could be due to the fact that the disks have dissipated in the years since the previous observations.

• And finally: Since the Moon's orbit around the Earth is not perfectly circular, the distance between the two bodies changes and the apparent size of our nearest neighbour varies. But observations from NASA's Lunar Reconnaissance Orbiter show that the Moon may literally be shrinking. The Moon was much warmer when it formed in the chaotic environment of the young solar system and has cooled slowly over time. It is thought that, as it cooled, the Moon shrank in size during its early history, but recent observations show that this cooling process may have caused more recent tectonic activity. Images from the orbiter have shown features known as lobate scarps, features characteristic of a contraction of the lunar interior. Such features were first discovered near the lunar equator in images from the Apollo missions, but the new data show similar features at much higher latitudes, confirming such scarps are a global phenomenon and making a global contraction the most likely explanation. The images also show scarps which cut through craters, suggesting that the Moon has undergone relatively recent tectonic activity due to ongoing cooling processes, possibly as recently as a hundred million years. Based on the new images, researchers estimate that the distance between the moon's centre and its surface may have shrunk by as much as 100 metres.

Interview

Over the past 10 years or so, astronomers have discovered that most of the energy in the Universe takes forms that we don't understand. There are two different labels for our ignorance; dark matter and dark energy. Dark matter is stuff within galaxies that adds extra gravity to keep them together. Dark energy seems to be making galaxies in the Universe accelerate away from each other like a form of anti-gravity.

Jonathan Pearson describes one of the speculative, theoretical models - kinky vortons - that might explain the cause of dark energy. A kinky vorton is a domain wall; the boundary between two regions of space with different characteristics. A domain wall would be incredibly thin, contain huge amounts of energy and repel matter. Jonathan and Stuart talk about the properties of domain walls and how you might look for them in the Universe.

The Night Sky

Northern Hemisphere

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

The nights are drawing in. Overhead in the south after sunset are Cygnus the Swan, Lyra the Lyre and Aquila the Eagle. Their respective brightest stars, Deneb, Vega and Altair make up the Summer Triangle. A third of the way up from Altair to Vega, the dark patch of sky known as the Cygnus Rift can be seen through binoculars. It is a dust cloud obscuring the starlight beyond, and contains the asterism Brocchi’s Cluster, often called the Coathanger. The constellatiof Pegasus, the Winged Horse, is low and inverted in the south-east, near to our neighbouring giant galaxy, M31, located in the Andromeda constellation and bearing the same name. The galaxy can be found by curving two stars up and left of the top left corner of the Square of Pegasus, which is the star Alpha Andromedae, then moving two stars to the right. It appears as a hazy glow in binoculars or, in a dark sky, to the unaided eye. The Andromeda galaxy is 2.5 million light years away, and may be around 20% more massive than our Milky Way. Andromeda and the Milky Way are the two largest galaxies in the Local Group. The constellations of Cassiopeia and Perseus rise in the east, beneath the band of the Milky Way. The Perseus Double Cluster lies between them, visible to the naked eye, distinguishable with binoculars and full of stars through a telescope.

The Planets

• Jupiter, at magnitude -2.9, rises in the east around 21:00 British Summer Time (BST; one hour ahead of Universal Time) and remains visible for much of the night. It rises earlier each night, appearing around 19:00 BST by the end of the month. It reaches opposition on the 21st, appearing due south around 01:00 BST on the 22nd. It still lacks its south equatorial belt, one of the dark bands normally visible on either side of its equator. Meanwhile, the Great Red Spot is darker than usual.
• Saturn can only just be seen in the first week of the month, low in the west and to the right of Venus. After this, it disappears for two months. It passes behind the Sun on the 30th.
• Mercury passes behind the Sun on the 3rd, a location called inferior conjunction. It becomes visible again high in the sky later in the month, the best time to see it before dawn this year.
• Mars is at magnitude +1.5, very low in the south-west after sunset. Its angular size is about 4”, making it unresolvable with a normal telescope.
• Venus is very close to Mars in the sky.

Highlights

• Mercury presents a good pre-dawn apparition on the 19th, at magnitude -0.3. Never more than 18° away from the Sun, it is often lost in sunlight. It is best seen at greatest elongation, when the ecliptic is at its greatest angle to the horizon, which occurs around dawn in autumn and sunset in spring. This month, it sits about 15° above the horizon for some time each morning.
• M15 is a globular cluster in Pegasus, imaged by schools with the Faulkes Telescope. Following the line of the last two stars of the neck and head of Pegasus up and to the left of the constellation Delphinus, it can be seen as a fuzzy glow with binoculars. A telescope reveals the individual stars, the brightest of which is at magnitude +12.6. Over 13 billion years old, it is one of the most ancient globular clusters known, and lies 33,000 light years from Earth, within our galaxy. It has a total magnitude of +6.4, corresponding to a luminosity of 360,000 times that of the Sun, and may contain a black hole.
• Mars and Venus come within a few degrees of each other in early September, either side of the star Spica in Virgo, low in the west after sunset. The thin crescent Moon is just below Spica on the 10th, and above and left of Venus on the 11th.
• Sinus Iridum, the Bay of Rainbows, is a curved inlet of Oceanus Procellarum on the Moon. It is best seen when the terminator crosses the bay, as the tops of the mountains are in sunlight and resemble a dinosaur’s backbone. Virtual Moon Atlas states that this will happen on the evening of the 18th.
• Jupiter comes within a degree of Uranus as both reach opposition on the 21st. Jupiter is at its largest angular size since 1963, and will not appear bigger until 2022. This is because it is at its closest to the Sun, while Earth is at it furthest from the Sun, so the two planets are near their minimum separation. Now is the best time to see the details of the Jovian system.

Southern Hemisphere

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

Jupiter returns to the evening sky this month, rising in the east after sunset. Named after the King of the Greek gods, the largest of the Solar System’s planets takes 12 years to orbit the Sun, passing through one zodiacal house roughly every Earth year in our sky. In Māori, it is called Pareārau or Kōpū-nui. Galileo observed Jupiter’s disc and four largest moons in the 17th century, the moons ranging from 3000 to 5000 km in diameter. Io is the nearest of these to Jupiter, and is the most volcanically active body in the Solar System due to gravitational friction from the objects around it. Europa is the smoothest object in the Solar System, probably containing water under an ice layer many kilometres thick, and may be capable of supporting life. Ganymede and Callisto are the outermost of the four main moons, which are among a total of 63 known to orbit Jupiter. Galileo also observed bands of cloud on the planet, one of which contains the Great Red Spot, a storm 2.5 times the diameter of the Earth that has be seen continuously for 200 years. Jupiter, at 318 times the mass of the Earth, outweighs all the other planets in the Solar System together. Venus, the Evening Star, appears in the west after sunset. Mars, fainter, sits below. The star Vega shines on the northern horizon, while the Milky Way spans the sky from north to south. The orange star Antares, the heart of the constellation Scorpius, is overhead to the west. The Scorpion’s tail, or hook of Māui to the Māori, curls towards the zenith, while the Southern Cross and its pointers lie in the south-west. Beyond Scorpius’ tail is Sagittarius, often named the Teapot after the shape of its brightest stars. Sagittarius, the Archer, is said to be firing an arrow at Scorpius in revenge for its killing of Orion the Hunter. Aquila, the Eagle, is north along the Milky Way. Its brightest star, Altair, referred to as Poutū-te-rangi by Māori astronomers, is the twelfth-brightest in our sky and one of the closest at 16 light years distant. Imaging reveals that this star spins rapidly enough to make it noticeably oblate. Altair, Vega in Lyra and Deneb in Cygnus form what is known in the southern hemisphere as the Winter Triangle, which is the Summer Triangle to those in the northern hemisphere. Canopus, the second-brightest star in the sky, is low in the south. The navigator of Spartan King Menelaus in Greek mythology, to the Māori it is Atutahi, chief of the heavens. It appears as a circumpolar star from New Zealand, and was once called Alpha Argos, part of the constellation Argo, the great ship of Jason in Greek mythology. This constellation has since been divided into three, and Canopus is known as Eta Carinae, the brightest star in Carina, the ship’s keel. The Hipparcos satellite measured Canopus to be 310 light years from Earth, with a mass 8.5 times that of our Sun and outshining it by a factor of 15,000. Carina contains a number of star clusters. One of these, IC 2602, known as the Southern Pleiades, is a degree across and surrounds the 3rd magnitude star Theta Carinae. Binoculars reveal its many stars. Nearby, NGC 3532 is visible to the naked eye as a haze near the Eta Carinae Nebula. A favourite of John Herschel, it contains 150 stars and covers one degree of sky, twice that of the full Moon. With a telescope, a number of small lines and orange stars can be seen. NGC 2516, another open cluster, can be seen by eye on a moonless night. Its scattered groups of stars can be seen through binoculars or a telescope, and three bright orange stars stand out within it.

Highlights

• Sunspots are reappearing on the Sun as it emerges from solar minimum. A number of coronal mass ejections occurred during August, sending out charged particles into the Solar System. These can cause the Aurora Australis to be seen, although they must be bright to reach the northern latitudes of most land in the southern hemisphere. The chances of seeing these Southern Lights will increase with solar activity.

Odds and Ends

NASA are asking members of the public to pick the wake-up songs for the final space shuttle missions. You can choose from a list of 40 previous wake-up songs for STS-133 and write and submit your own song for STS-134

Roy explains how pulsars have been used to weight the planets in our Solar System.

The discovery of multiple exoplanets around a star have been announced by two teams. The ESO HARPS instrument has detected 5 Neptune-like planets around the star HD 10180 and has found evidence that there may be a further 2 planets in this system, one of which would be 1.4 times the mass of the Earth. The NASA Kepler mission has announced the discovery of two Saturn-like planets around the star Kepler-9, with the possibility of a third planet 1.5 times the size of the Earth.

The Big Bear Solar Observatory has released the most detailed visible light image of a sunspot.

The Jodcast team went on a somewhat crazy mission during August, visiting all 7 of the telescopes in the eMerlin array in a day. The trip was filmed for a future Jodcast video, but short clips from the day are up on Youtube.

On the 26th August, the hashtag #astromovies was born on Twitter. A couple of people blogged about it and Dr Paul Woods from JBCA has compiled a full list. Some of our favourites include Gone with the Solar Wind, Lord of the Ring Nebula and Jod-Castaway.

In reponse to the Ask an Astronomer question on black holes in the August 2010 Extra show, listener EarthUnit has posted a link on the forum to a lecture on black holes by Professor Alex Filippenko.

Finally, the School of Atmospheric Science at the University of Manchester have started up their own podcast called The Barometer.

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