July 2010

Stateside. In the show this time we have three interviews from the University of Western Ontario in Canada. We hear about magnetic fields in stars from Professor Emeritus John Landstreet, Melissa Battler explains how studying environments on the Earth can help us to understand where life might be in the Universe and Sarah Gallagher tells us about quasars and the role they play in star formation. As ever we have the latest astronomical news, and what you can see in the July night sky in the northern and southern hemispheres.

The News

In the news this month:

• Supernovae are the violent explosions of massive stars, so bright that the events can be seen in distant galaxies. But not all apparent explosions are genuine supernovae. Some fall into the category of supernova impostors, the sudden increase in brightness of a star without the terminal explosion. One such impostor event was the great eruption of eta Carinae, a star which is amongst the most massive known in the Milky Way. Located 7,500 light years away in the constellation of Carina, the star is five million times more luminous than the Sun and an estimated 100 times as massive. Eta Carinae underwent a massive but non-terminal explosion 150 years ago, allowing the close-up study of a supernova impostor. During the eruption, the star lost about ten percent of its mass, throwing off the outer layers in the surrounding nebula. Since then, the star has been enshrouded in a thick cloud of dusty debris and has been losing material at the rate of one Jupiter-mass per year in a strong stellar wind.

  Now, a team of researchers, led by Andrea Mehner at the University of Minnesota, have observed dramatic changes in the star's spectrum. Observations over the last decade have shown an increase in the star's magnitude, but with no major long-term changes in its spectrum, something that might be expected following an event causing a major change in brightness. New observations carried out with the Space Telescope Imaging Spectrograph on the Hubble Space Telescope by Mehner's team show a substantial change in the emission lines, caused by specific elements in the star's atmosphere. According to the team, the sudden rapid decrease in the brightness of the emission lines (dropping to a third of their original strength in just ten years) suggests a decrease in the strength of the stellar wind, possibly signifying a much more rapid return to the pre-explosion state than was previously anticipated. With the wind density decreasing, the nebula should begin to thin and the star itself may become visible to modern telescopes for the first time, possibly within the next decade.

  There are other explanations which may account for the unusual spectroscopic developments, including a change in the latitude dependence of the wind, but the complicated nature of the surrounding nebula and the difficulties constructing accurate models make an accurate assessment problematic.

• After massive stars like Eta Carina explode, the object left behind is thought to be either a neutron star or a black hole, depending on the final mass of the progenitor star. Neutron stars have strong magnetic fields and many behave somewhat like cosmic lighthouses, projecting beams of radio emission into space as they spin. Those whose beams are visible from Earth are known as pulsars. Studying the pulses of radiation as the beams sweep past the Earth can provide valuable information on the physics of these extreme objects, allowing astronomers to probe physics under conditions which are not possible to create in a terrestrial laboratory. While pulsars are known to be extremely accurate natural clocks, their pulse rates are very stable over time, there are however unexplained deviations from the expected spin rate, a phenomenon known as timing noise. Now, a team led by Andrew Lyne at the University of Manchester, have uncovered a mechanism which could explain this noise.

  Over long timescales, the rate at which a pulsar spins (known as the spin down rate) decreases slowly in a predictable way due to the conversion of rotational energy into photons. By studying a large number of pulsars repeatedly over 40 years, the team found that the deviations from the expected spin down rate were actually quasi-periodic on timescales between one and ten years, and that several other pulsar characteristics may be linked to the same phenomenon. One particular pulsar, known as B1931+24, only displays radio pulses intermittently, and long term study showed that it also had two different spin down rates: its spin rate decreased faster when the radio signal was detectable. The team analysed the data on a large sample of pulsars and found a further seventeen which show evidence of quasi-periodic spin down rates, many of which also show variations in the shape of the pulse profile. The authors suggest that the likely explanation is that the pulsar's magnetosphere is switching between two distinct states. Exactly what causes the pulsar to switch between states is not yet known, but if the changes can be accurately modelled then the timing noise can be reduced, and astronomers will find it easier to compensate for errors in pulsar clocks in highly sensitive experiments designed to detect gravitational waves.

• The southern highlands of Mars are thought to be ancient, largely unaltered material from an earlier epoch in the planet's history. Evidence from mineral deposits in the form of hydrated silicates points to a time when the conditions on the surface allowed water to exist as a liquid. Unlike the southern highlands however, the younger northern lowlands are covered by thick lava flows and sediments from past episodes of geological activity, thought to cover older surfaces similar to the much older southern highlands. Now, a team led by John Carter of the University of Paris have found hydrated clay minerals in the northern lowlands that match those in the highland areas of the south. The team used data from instruments on both European Space Agency's Mars Express and NASA's Mars Reconnaissance Orbiter to examine impact craters where asteroids have exposed underlying ancient material up to several kilometres deep. Their results, published in the journal Science, show evidence of hydrated minerals similar to those found in the southern highlands in nine of the ninety one craters they examined, and the authors suggest that this is evidence that the planet had the right conditions for liquid water to exist on a global scale more than four billion years ago.

• And finally: The first global map of gravitational anomalies from the European Space Agency's GOCE satellite has been released. The Gravity field and steady-state Ocean Current Explorer satellite, or GOCE, was launched in March 2009 and has been accurately mapping the Earth's gravitational field for the last twelve months. Based on just two months of data, the first map shows the Earth's geoid in the highest resolution yet obtained. The geoid is essentially the shape of an imaginary ocean covering the Earth's surface, affected only by the Earth's gravity without the effects of tides or currents. The shape of the geoid will help in measuring ocean circulation and sea level changes caused by global warming, as well as in many other fields of earth science and oceanography. The satellite is continuing to orbit at just 254.9 km, the lowest sustained orbit by any earth observation satellite, and each two-month chunk of data will improve the accuracy of the geoid model.

Interviews

Dave is currently on an "Astro Tour" of North America, visiting science centres and planetariums across the continent. In May, he visited the University of Western Ontario in Canada and managed to get several interviews with their staff and students, three of which are included in this epiosde

Professor Emeritus John Landstreet tells us about his work studying magnetism in stars.

Melissa Battler explains how her work studying the geology of certain environments on Earth can teach us about where life might exist in the Universe.

Dr Sarah Gallagher tells us about quasar winds.

The Night Sky

Northern Hemisphere

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

The nights are gradually beginning to lengthen again, making stargazing a little easier. Arcturus, high in the south in the constellation of Boötes, is bright enough to be mistaken for a planet. To the lower left is the arclet of stars known as the Corona Borealis - the Northern Crown. Up and to the left of that is Hercules, below which lies the mostly empty constellation Ophiuchus, through which the Milky Way passes. The Sun also moves through Ophiuchus, yet it is not one zodiacal houses. Below it, observers at more southern latitudes can see the red star Antares in Scorpius and, to the left, the `teapot' of Saggitarius. Following the tea from the spout leads to the open clusters M6 and M7, while above the lid of the teapot is the Lagoon Nebula, M8. The star Vega is bright in the constellation of Lyra, the Lyre; nearby and lower down, Deneb resides in Cygnus, the Swan; Altair rises about an hour after sunset in Aquila, the Eagle. These three stars complete the Summer Triangle, within which lies a rich region for observations with binoculars or telescope.

The Planets

• Jupiter rises soon after midnight in late July, and shines high in the dawn sky at magnitude -2.7. One of its dark bands, the south equatorial belt, has recently disappeared, while the nearby Great Red Spot has become darker and more prominent than it has been in several years.
• Saturn is visible to the south-west of Virgo and Leo just before sunset. It's magnitude is +1.1, with an angular diameter of 17" and rings out to 38". The plane of the rings is now about 3 degrees from edge-on to our line of sight and increasing. A number of its moons, the brightest of which is Titan, are visible using a telescope.
• Mercury is an evening object, appearing about 6 degrees above the horizon in the west-north-west, half an hour after sunset. It is visible at magnitude -0.5 in the middle of the month, dropping to +0.1 by the end. Its angular size increases from about 5 to 7" over the same period. On the 27th it passes 0.3 degrees south of the star Regulus in Leo, the Lion.
• Mars is at magnitude +1.4, decreasing through the month. It appears in the south-west evening sky to the lower left of the star Denebola in Leo's tail, between Regulus in Leo and Spica in Virgo, the Virgin. Its angular size is 5".
• Venus dominates as the Evening Star at a magnitude of about -4.1 throughout the month. It is approaching the Earth as its illuminated surface area decreases, keeping its brightness almost constant.

Highlights

• The thin crescent Moon will be just to right of the Pleiades cluster in the pre-dawn sky of the 8th. Earthshine - sunlight reflected off the Earth's clouds - should be visible on the shadowed part of the Moon's surface. The brighter stars of the Pleiades will be discernible with binoculars.
• Looking with binoculars up the right side of the keystone - the four stars in a trapezium at the heart of Hercules - will lead to the globular cluster M13, the brightest globular cluster in the northern sky. Globular clusters are small, spherical concentrations of stars in the galactic halo which surrounds the centre of the Milky Way. They are as old as the galaxy, and each appears as a fuzzy ball which contains roughly a million stars. Harlow Shapley plotted the positions of globular clusters, finding them to be spherically distributed about the galactic centre. From this, he inferred the distance from the Sun to the centre of the Milky Way to be 27,000 light years.
• Saturn, Mars and Venus line up on the evening of the 15th, above and to the left of the Moon.
• On the 20th, the Moon's terminator (edge of illumination) will be close to two of its craters, Tycho and Copernicus, allowing them to be seen clearly through binoculars. Tycho is 85 km wide and 5 km deep, lying in the Southern Lunar Highlands near the bottom of the Moon. It is thought to have been formed 108 million years ago by a fragment of one of the Baptistina family of asteroids, another of which may have created the Earth's Chicxulub crater and wiped out the dinosaurs 65 million years ago. At full Moon, radial debris lines emanating from Tycho are visible. Copernicus is around 800 million years old and lies higher than Tycho, in the eastern part of Oceanus Procolarum, beyond the curving Appenine Mountains. It is 93 km wide and 4 km deep, with terraced, rather than sheer, sides.
• The largest object in the Asteroid Belt, the dwarf planet Ceres, can be seen in the lower part of Ophiuchus. Although normally washed out by the light from the Milky Way, in July it passes in front a cloud of dust and gas called Barnard 78. At magnitude +7, it can be viewed with binoculars below the star Theta Ophiuchi.
• Mars and Saturn are close together on the 31st, above and to the left of Venus.
• Uranus can easily be found to the right of Jupiter with binoculars during July, and, at magnitude +5.8, may be just visible to the naked eye under very dark skies. Two objects of similar brightness will be visible near one another, the right-hand one being Uranus. Its turquoise disc can be seen through a small telescope. Jupiter will move noticeably further to the left of Uranus during the month.

Southern Hemisphere

John Field from the Carter Observatory in New Zealand tells us some of the highlights that can be seen in the southern night sky during July 2010.

The smallest constellation, Crux - the Southern Cross - is high to the south in the winter sky, with its two pointer stars to the left. Crux was known to the ancient Greeks and was considered part of the larger constellation Centaurus, but precession of the equinoxes changed Earth's orientation and pushed it south over the centuries. It is Te Punga - the anchor - to the Māori, and represents a possum in a tree to Australian Aborigines. The brightest star in Crux, Alpha Crucis, is actually a double star system, resolvable with a telescope. The second-brightest star, Beta Crucis, is also in a binary system. The Jewel Box, a hazy patch of light nearby, can be resolved into a beautiful open cluster using binoculars or a telescope. John Herschel made its first telescopic observation. The southern celestial pole is located near the halfway point of a line pointing upwards from Crux towards the bright star Achernar. The Magellanic Clouds, two more hazy patches about three quarters of the way along the line, can be seen unaided in a dark sky. The Large Magellanic Cloud (LMC) lies about 160,000 light years from us and is around 14,000 light years in length; the Small Magellanic Cloud (SMC) is about 200,000 light years away and 7,000 light years long. These dwarf galaxies interact with each other and orbit the Milky Way. The LMC has a barred spiral structure, with its own star clusters and emission nebulae, the brightest of which is called the Tarantula. This is distinguishable on the left side of the LMC using a telescope, and has an estimated mass of 450,000 Solar Masses. It is the most active known star-forming region in the Local Group of galaxies, and may one day become a globular cluster. If it were as close to us as is the Orion Nebula in our own galaxy, it would be bright enough to cast visible shadows on the Earth at night. At the centre of the Tarantula Nebula is a compact star cluster, R136, containing many giant stars. The brightest supernova since the invention of the telescope, Supernova 1987A, was observed near this region by astronomers in New Zealand and Chile in 1987.

The Planets

• Venus, Mars and Saturn are high in the north.
• Venus will be close to Regulus on the 10th.
• Mars moves from Leo to Virgo during the month, and approaches Saturn in the night sky.
• Jupiter is visible in the early morning, but rises at 10pm by the end of the month.

Highlights

• The Milky Way stretches along the southern horizon before sunrise. From southern Australia's Blue Mountains, the Milky Way forms a spectacular ring around the horizon, revealing the structure of our galaxy.

Odds and Ends

As mentioned several times during the show, Dave is on tour across North America during the summer, visiting science centres and planetariums. You can follow his adventures on his AstroTour2010 blog

In May 2010, the Japan Aerospace Exploration Agency (JAXA) launched the IKAROS solar sail into space. The solar sail was successfully expanded on June 10 2010 and JAXA is now monitoring IKAROS and running tests to evaluate its performance.

The 100th mission to the International Space Station, Soyuz TMA-19, launched on June 15 2010 carrying three members of the 24th long duration ISS mission.

There is a movie on the Jodrell Bank facebook group showing a time lapse recording of the Lovell Telescope taken on the morning of the summer solstice.

Show Credits