November 2012: Gigapixel

In the show this time, Dr Martin Stringer talks to Liz about the principles of supernova driven winds. Ian Morison and John Field tell us what not to miss in the November night sky and Megan gives us a round up of the latest astronomy news.

The News

In the news this month:

• Our nearest neighbour in space, the Moon has long fascinated mankind, but how exactly did it form? The current model says that another body smaller than our own planet hit the young proto-Earth with the Moon forming from the debris, but the details of this model are not well-defined. October saw the publication of, not one, but three papers on the origins of the Moon.

  Every rocky body in the solar system has a distinct chemical fingerprint which distinguishes it, allowing us to identify meteorites which land on the Earth as coming from, say, Mars or the Moon. But the Moon's chemical make-up is startlingly similar to that of the Earth. If it formed in an impact between the Earth and another object, then it's composition should be different, incorporating the chemical fingerprint of the impactor as well.

  Two of the new studies, published in the journal Science, describe different models of the actual impact. They take different approaches to the problem, but both find solutions which produce a Moon-sized satellite with an Earth-like composition. One study, by Matija Cuk and Sarah Stewart of Harvard University, show that a giant impact onto a very fast-spinning young Earth (with a day of between just two and three hours) could result in a debris disk made mostly of material ejected from the Earth's mantle. The other study, by Robin Canup of the Southwest Research Institute in Boulder, Colorado, finds that if the impactor was of a similar size to that of the proto-Earth, then the resulting planet and moon would both contain material from the impactor, resulting in them both having a similar composition today.

  A third study, published in the journal Nature on the same day, backs up the impact model of the Moon's formation by studying one particular chemical in lunar rocks. Chemical elements can exist as different isotopes of different masses. Geological processes such as volcanic activity affect certain isotopes more than others, whilst leaving the relative abundances of some elements virtually unchanged. A cataclysmic event such as that which created the Moon, whatever the size of the impactor, would have altered the isotopic content of so-called volatile elements, those which can exist as a vapour above temperatures of around 1000 degrees. One such volatile element for which volcanic activity has little effect is zinc. A team led by Randal Paniello at Washington University in Missouri, studied the isotopic content of zinc in lunar rocks, compared it with that of the Earth and found that the Moon is depleted in lighter isotopes of zinc relative to the Earth, Mars and primitive meteorites. In the impact scenario, the collision formed a cloud of dust and gas made up of the mantle of the Earth, and possibly part of the impacting body. Since lighter isotopes can move faster, they can more easily escape the cloud before the Moon finally condensed, leaving just the heavier isotopes behind and explaining the chemical composition of the Moon we see today.

  While these studies provide valuable new insights into the nature of our nearest neighbour, the question of the Moon's formation is not yet fully answered.

• The team of astronomers responsible for finding the first-known planet around a star other than the Sun have now discovered the closest exoplanet to date. Published in the journal Nature on October 17th, their latest discovery is also the smallest known planet orbiting a Sun-like star.

  This new discovery has been made using the HARPS instrument, a high-resolution, high-precision spectrograph located at the 3.6-metre telescope at ESO's La Silla Observatory in Chile. Using the Doppler effect, the same effect which makes the sound of a siren change in pitch as an ambulance races past, HARPS can measure the tiny changes in the colour of light from a star as it is gradually pulled out of position by the gravitational force of an orbiting planet. Larger planets have stronger gravitational fields, which in turn cause more significant effects on their host stars, so small planets similar to the Earth are hard to detect because their gravitational influence is much smaller. The effect also depends on how close the planet is to a star; the closer a planet is, the shorter its orbit and the greater the effect on the speed of its host star. This means that, using this radial velocity technique, the easiest planets to find are large gas giants orbiting very close to their stars. Indeed, most exoplanets found so far using this technique have been of this type. Because of the high precision of the HARPS instrument, however, it is capable of finding much smaller planets in larger orbits.

  The Alpha Centauri system is just 4.3 light years from our solar system, and actually consists of three stars. The brightest, Alpha Centauri A, and its fainter companion, Alpha Centauri B, are both similar to the Sun and are in a close orbit, while their much fainter and redder companion, known as Proxima Centauri, orbits at a greater distance. The system has been searched for planets in the past, but none were found until now since other instruments lack the precision needed to find such small planets. This new planet has a minimum mass similar to that of the Earth and is in orbit around Alpha Centauri B, a star which is slightly cooler and smaller than the Sun. The small mass of this planet highlights the precision of HARPS - the pull it exerts on Alpha Centauri B is just 50 centimetres per second.

  Despite the similarities with our own planet, this newly-discovered world is unlikely to host life as we know it since it orbits its parent star at just 4% of the distance between the Earth and the Sun, about six million kilometres (compared to the Earth's distance from the Sun at 150 million kilometres!), completing an orbit in just three and a quarter Earth-days.

• And finally, NASA's newest set of X-ray eyes in the sky, the Nuclear Spectroscopic Telescope Array (NuSTAR), has taken its first look at the giant black hole at the centre of our galaxy, and caught it in the middle of a flare.

  Launched on June 13, NuSTAR is the first telescope capable of producing focused images of the highest-energy X-rays. For two days the telescope teamed up with other observatories to look at Sagittarius A*, the name given to the compact radio source located at the very centre of the Milky Way. Participating telescopes included the Chandra X-ray Observatory, which looks at lower-energy X-ray light; and the W.M. Keck Observatory atop Mauna Kea in Hawaii, which took infrared images.

  Observations show a massive black hole lies at the centre of the Milky Way. When black holes consume fuel - whether a star, a gas cloud or, as recent Chandra observations have suggested, even asteroids - they erupt with extra energy. In the case of NuSTAR, its state-of-the-art telescope is picking up X-rays emitted by matter very close to the black hole in a region where particles are accelerated at speeds close to that of light, and heated to temperatures of around 100 million degrees Celsius.

  Compared to supermassive black holes at the centres of other galaxies, Sgr A* is relatively quiet. Active black holes tend to swallow stars and other fuel from around them, but Sgr A* is thought only to nibble or not eat at all, a process that is not fully understood. These new images from NuSTAR, released during October, combined with the simultaneous observations taken at other wavelengths, will help understand the physics of how black holes snack and grow in size.

Interview with Dr Martin Stringer

Dr Martin Stringer from the l'Observatoire de Paris talks about supernova driven winds. He explains how to calculate the energy produced by these winds and what affect this energy has on the galaxies in which they occur. He also describes and compares the analytical and numerical approaches he and his collaborators have used to simulate these winds. They find that while analytical analysis will give the general properties of all of the supernova driven winds in a galaxy, numerical simulations are sensitive enough to resolve individual supernovae.

The Night Sky

Northern Hemisphere

Ian Morison tells us what we can see in the northern hemisphere night sky during November 2012.

The summer triangle of Deneb in Cygnus, Vega in Lyra and Altair in Aquila is still visible in the west after sunset. Pegasus, the Winged Horse, is upside-down and high in the south. The number of stars visible within the four stars making up the Square of Pegasus is a good test of the darkness of the night sky, with more than 5 being reasonable. Pisces is below Pegasus, while Andromeda is up to the left, and the Andromeda and Triangulum galaxies can be located in this region by 'star-hopping'. Cassiopeia is almost overhead, while Perseus is lower to the east. The Double Cluster is a nice feature to spot along the Milky Way between these two constellations, and can be seen using binoculars or a small telescope. Algol, the Demon Star, is an occulting binary star in Perseus whose brightness varies every few days as a result of its eclipses. Auriga, also along the Milky Way, rises in the east and contains the bright star Capella, as well as a number of open star clusters which can be seen through binoculars. One of our nearest open clusters, the Pleiades, is in Taurus, as is the very nearest, the Hyades. Aldebaran, the brightest star in Taurus, appears to be in this cluster but is actually much closer to us. The planet Jupiter is also moving across the cluster this month. Orion rises later in the evening, with the three stars of his Belt pointing down towards Sirius, the brightest star in the night sky. The Orion Nebula is a fuzzy glow beneath the Belt, and is a region in which stars are being formed. Gemini rises just to the north of Orion, with its bright stars Castor and Pollux, the Twins.

The Planets

• Jupiter rises around 19:00 UT (Universal Time) or 20:00 BST (British Summer Time, 1 hour ahead of UT) at the beginning of the month and is in the sky at twilight by the end. It reaches 60° elevation in the south, helping us to see it with less atmospheric interference this year and next. It is 7° up and to the left of the star Aldebaran at the start of November, moving above it in its retrograde (westward) motion during the month. Lots of surface detail, and the Galilean moons, are visible in a small telescope as its angular size is around 48".
• Saturn passed conjunction (moving behind the Sun) on the 25th of October, and still cannot be seen at the beginning of November as it is only 6° from the Sun in the sky. By the end of the month, however, it is 32° from the Sun and rises 3 hours before the Sun. Its rings are opening out and are now 18° from the line of sight, helping you to pick out Cassini's Division if you have a small telescope, and its largest moon, Titan, is also visible.
• Mercury passes inferior conjunction (moving in front of the Sun) on the 17th, preventing us seeing it until near the end of the month, when it is nicely visible to the lower left of Saturn and Venus at magnitude -0.3.
• Mars is still visible, low in the west after sunset, starting the month in the constellation of Ophiuchus and moving into Sagittarius on the 27th. It has a magnitude of +1.2 and an angular size of 4", making it difficult to see any surface detail.
• Venus rises three hours before the Sun, blazing at magnitude -4. Its angular size decreases over the month, but its illumination increases from 81 to 88%, so it dims only slightly, to magnitude -3.9.

Highlights

• It is a great time to observe Jupiter this month and for the rest of the year. It is high in the south in Taurus. Its North Equatorial Belt has broadened, and the Great Red Spot can be seen against the South Equatorial Belt. At the end of November, you can see a complete rotation of Jupiter during night hours.
• Mars passes below a thin crescent Moon after sunset on the 16th, in the south-western sky.
• The planet Neptune can be found in Aquarius using binoculars from around the 10th to the 15th, when the Moon is new and does not drown out its faint blue light.
• The Leonid meteor shower peaks on the 16th and 17th, just after new Moon. The meteors originate from dust cast off by Comet Tempel-Tuttle, and you may see 20 meteors per hour radiating from the constellation of Leo after midnight.
• Venus and Saturn are 0.8° apart just before dawn on the 27th, with Venus much the brighter of the two.
• Jupiter is near the full Moon after sunset on the 28th
• Mercury is near to its greatest elongation (furthest point from the Sun in the sky) and appears with Saturn and Venus in the east before dawn on the 30th.

Southern Hemisphere

John Field from the Carter Observatory in New Zealand speaks about the southern hemisphere night sky during November 2012.

The winter constellations of Scorpius and Sagittarius are disappearing in the twilight sky, heralding the approach of summer. Early in the month, the planet Mercury is below the star Antares in the west, but by the middle of the month it sets with the Sun and can no longer be seen. Mars is higher up in Ophiuchus, the Serpent Bearer, a constellation which is sometimes called 'the Coffin' due to the shape made by its brightest stars. The globular clusters M10 and M12 lie within 3° of each other in Ophiuchus. With respective magnitudes of +6.4 and +7.6, they are visible in binoculars, although spotting M12 requires a dark sky.

On the opposite side of the sky, Taurus, Orion and Canis Major are rising in the east during the evening, and these summer constellations will become visible earlier as the month progresses. Sirius, Canopus and Alpha Centauri, the three brightest stars in the night sky, skirt the southern horizon. Sirius, to the east, marks the top of Canis Major, the Large Dog, which is inverted in the southern hemisphere sky. Taurus and Orion lie to the its north-east. The face of Taurus is outlined by the V-shaped asterism known as the Hyades Cluster. In Greek mythology, the Hyades were five daughters of Atlas whose half-sisters were the Pleiades, and so the Pleiades star cluster is nearby. The orange star Aldebaran - 'the Follower' in Arabic, in reference to the way it follows the Pleiades across the sky - is the brightest star among the Hyades, but is not actually part of the cluster as it lies much closer to us. The Horns of Taurus extend down towards the northern horizon, and the planet Jupiter sits between them, rising high in the sky by midnight. Jupiter will be at its closest point to Earth around the time of its opposition (furthest point from the Sun in the sky) in December, so it will be at its brightest in this part of the year. Binoculars and small telescopes reveal its four brightest moons, which change position relative to the planet from night to night. M1, the Crab Nebula, is near the fainter of the two horn tips, and is an expanding cloud of gas and dust created when a star exploded in a supernova - an event recorded in the year 1054. Visible as a dim haze through binoculars, the Crab Nebula shows shape and detail to telescopes of over 100mm in aperture size. The Pleiades Cluster, by constrast, is a group of blue stars which are significant to many cultures, being variously named the Seven Sisters, Matariki and Subaru in different parts of the World. At around 100 million years old, its stars are comparatively youthful. The planets pass through or near to the Pleiades as it lies along the ecliptic.

East of Taurus is Orion, upside-down, with three aligned stars marking his Belt and a fainter line of stars forming his Sword. A faint haze in the Sword is visible to the naked eye and is called the Great Nebula in Orion, a vast star-forming region. Binoculars and small telescopes reveal some detail, while larger telescopes show tendrils of gas. The Belt and Sword make up the asterism known in the southern hemisphere as the Pot or the Saucepan, while the Māori call the Belt 'Tautoru'. Further south, Canopus is beside the Milky Way, with Alpha Centauri further along still. Alpha and Beta Centauri points towards Crux, the Southern Cross, a diamond-shaped constellation which appears in the south-west after sunset. With a declination of -60°, Crux never sets over New Zealand, but dips towards the southern horizon before ascending again in the south-east after midnight. Nearby are the similarly shaped asterisms of the Diamond Cross and the False Cross.

Highlights

• Australia, New Zealand and most of the Pacific islands see a solar eclipse on the 14th, which is total in parts of northern Australia. Cairns and Port Douglas experience just over 2 minutes of totality around 06:39 EST (Eastern Standard Time, 10 hours ahead of Universal Time), when the Sun's corona may be visible, as well as bright stars and planets. In New Zealand, the time of maximum eclipse is around 10:36 NZDT (New Zealand Daylight Time, 13 hours ahead of Universal Time); North Cape sees 91% of the Sun covered, while in Invercargill the proportion is 58%. Never look directly at the Sun, even when it is partially eclipsed.
• There is a lunar eclipse on the 29th as the full Moon passes through the Earth's shadow. The eclipse is penumbral, meaning that the Moon has part of the Sun blocked from its view but is never entirely in the Earth's shadow. The point of greatest eclipse occurs at around 03:31 NZDT.

Odds and Ends

With solar maximum approaching next year, you can check the three-day aurora forecast and find out whether the northern lights are likely to be visible in your area. The information is based on observations of solar activity and the strength of the solar wind at the Earth. Viewers looking for the southern lights can get updates from the Australian government's Ionospheric Prediction Service.

Astrobiologists have found that an exoplanet's capacity to sustain microbial life could be evaluated using its colour. By filtering the recieved light into different colour bands and plotting the colour intensities against each other, it would be possible to distinguish planets with surfaces favourable to 'extremophile' bacteria such as rock, snow or water. Planets flagged in this way could then be followed up with precise spectroscopic measurements.

The European Southern Observatory (ESO) has released one of the biggest-ever astronomical images, weighing in at a whopping 9 gigapixels. The image was created by stitching together thousands of smaller images taken in the infrared by the VISTA telescope at the Paranal observatory in Chile. Click here to access the original, zoomable image on the ESO website.

Researchers at the Centre for Astrophysics in Cambridge Massachusetts have discovered spiral arms at the centre of the elliptical galaxy Centaurus A, which is thought to have swallowed a spiral galaxy about 300million years ago. Although Centaurus A is the first elliptical galaxy found to have spiral arms, the advent of more sensitive technologies, such as the Atacama Large Millimetre Array (ALMA), may result in the discovery of many more.

Show Credits