September 2011: Diamond

In the show this time, we talk to Dr Martin Bureau about galaxies and we find out about an exciting new discovery in the pulsar world. As always, Megan rounds up the latest news and we hear what we can see in the September night sky from Ian Morison and John Field.

The News

In the news this month:

• August 24th saw the discovery of one of the closest supernovae of its kind in recent years. Located in the , the explosion was reported by the Palomar Transient Factory (PTF), a survey of the sky which aims to detect and catalogue transients using two telescopes at the Palomar Observatory in California. Catalogued by the PTF collaboration as PTF11kly, the position of the supernova was distributed rapidly through the Astronomers Telegrams, allowing follow-up by other astronomers using a variety of telescopes across the electromagnetic spectrum.

  With transient events like supernovae, such rapid follow-up with other telescopes is extremely useful in trying to understand the physics of what happens in the explosion. Comparatively little is known about the first few hours to days of supernova evolution since they are often discovered days (and sometimes weeks) after the initial explosion.

  In the case of PTF11kly, also catalogued as SN2011fe, the event was spotted very early in its evolution, as the brightness was still increasing, and spectroscopic observations by the Liverpool Telescope in the Canary Islands quickly showed that this particular event was of the class known as type Ia supernovae. This kind of event is thought to be caused by a thermonuclear explosion on the surface of a white dwarf star in a binary system, although there are variations in the theoretical models which detailed observations could help to resolve. This particular supernova is the closest example of a type Ia event in almost forty years. This is significant because it is this type of supernova which is used to measure the expansion of the universe, so having a good understanding of the underlying physics of these explosions is essential for cosmological studies.

  Since it was discovered so early in its evolution, PTF11kly should continue to brighten over the next few days before it begins to fade. At discovery, the object had a magnitude of 17 but was brightening rapidly. Located in M101, the Pinwheel galaxy in Ursa Major, it is estimated that the supernova could become bright enough to spot with binoculars or a small telescope. In contrast, observations carried out with the Very Large Array, a collection of 27 radio telescopes located in New Mexico, show no radio emission from this supernova. This is not surprising, as so far no type Ia supernova has ever been spotted by radio telescopes, despite numerous searches.

  Telescopes of various types will continue to monitor this supernova as it evolves over the following months, and astronomers will use the data collected to test various aspects of the physics and chemistry of supernova models.

• Antimatter is often thought of as something that is only created in particle accelerators (or that only exists in science fiction movies), but it is actually present in small quantities throughout the universe. Now, a team of researchers have detected the presence of naturally occuring antimatter right here in the neighbourhood of the Earth. This population of antiparticles originates from cosmic ray interactions in the Earth's upper atmosphere where they are subsequently trapped in the planet's magnetosphere.

  Anti-protons can be produced in a number of ways, through cosmic rays interacting with the interstellar medium, the natural decay processes of some types of particles from our own atmosphere, or in cosmic ray air showers from high energy particles impacting on the atmosphere, although most of the antiparticles would annihilate with their normal counterparts fairly quickly, especially at lower altitudes where the density of the atmosphere is higher.

  The existence of anti-protons around the Earth was predicted many years ago, but predictions differ, and experiments on board both Salyut-7 and the Mir space station only succeeded in placing upper limits on their abundance. The anti-protons found by the PAMELA satellite (Payload for Antimatter Matter Exploration and Light-nuclei Astrophysics) are located in the Earth's Van Allen belts, doughnut-shaped regions defined by the magnetic field of the Earth. The magnetic fields trap charged particles, resulting in regions with a relatively high density of positively charged protons, and others with a high density of anti-protons.

  Launched from the Baikonur Cosmodrome in 2006, PAMELA is designed to detect cosmic particles with energies between tens of mega electron Volts and hundreds of giga electron Volts. PAMELA's orbit takes it through the area known as the South Atlantic Anomaly, the region where the Van Allen belts pass closest to the Earth's surface. Since it began operations in 2006, PAMELA has detected anti-protons at a rate more than 1000 times higher than that expected from Galactic sources. The researchers say that this implies a belt of anti-protons located between two belts of ordinary matter in the Earth's Van Allen belts.

  The signal detected by PAMELA is ten thousand times times stronger inside the South Atlantic Anomaly than it is outside the Earth's radiation belts, and thousands of times stronger than that expected from Galactic cosmic rays. The likely explanation, say the researchers, is that the Earth's Van Allen belts are acting in the same way as they trap protons, trapping the anti-protons in a layer around the Earth (at least until they encounter a particle of normal matter and annihilate).

  Although antimatter is pretty destructive stuff if it comes into contact with ordinary matter, luckily for orbiting spacecraft there isn't that much of it. In 850 days of data acquisition, PAMELA's detectors collected just 28 anti-protons in the previously unknown antimatter region of the inner Van Allen belts.

• Stars are bright because they generate heat and light through nuclear fusion processes in their cores, planets are visible because they reflect some of that light. The percentage of light that is reflected, a quantity known as a planet's albedo, varies depending on the nature of the planet's surface and its atmosphere. Jupiter, with its thick bands of highly reflective cloud, has an albedo of 52%, while the Earth's is somewhat lower, only reflecting around 37% of the sunlight which falls on the surface. But now, a duo of astronomers have discovered a planet with an exceptionally low albedo, reflecting just 1% of its host star's light, making it less reflective than coal.

  The planet, discovered by the Trans-Atlantic Exoplanet Survey, is known as TrES-2b and lies about 750 light years away in the constellation of Draco. David Kipping (of the Harvard-Smithsonian Center for Astrophysics) and David Spiegel (of Princeton University) used data from the Kepler space telescope to investigate the planet's nature. To calculate its albedo, the astronomers measured its brightness at two different points in its orbit around the host star, once when it was located directly between us and the star, and again when it was on the far side, just before it went into eclipse. The difference between the two measurements is therefore the difference in brightness between the day and night sides of the planet, and tells us how much of the star's light is reflected by the surface of TrES-2b.

  The planet orbits its star at a distance of just five million kilometres, far closer than Mercury is to the Sun. Mercury is a dense rocky planet with a large iron core and a rocky, silicate surface which has an albedo of 12%. Surface temperatures on the closest planet to the Sun range between 90 and 700 degrees Kelvin. In contrast, TrES-2b, an exoplanet in the category of "hot-Jupiters" shows brightness variations of just 6.5 parts per million, corresponding to an albedo of less than 1%. But, this assumes that the only cause of the brightness variations is due to the geometry of the planet. The authors calculate that there is significant emission coming from the day side of the planet. Its proximity to its host star means that its surface temperature is likely to be around 1000 degrees and any atmosphere it has will likely be composed of chemicals such as vaporised sodium and potassium, or gaseous titanium oxide. Such a hot temperature also means that the planet actually emits some of its own light, possibly glowing dimly red like an electric bar heater.

  The Kepler satellite is designed to search for planets using the transit technique, observing one densely-packed star field for its entire operational lifetime, searching for the tiny fluctuations in brightness of a star due to a transiting planet.

  The exceptional sensitivity of Kepler's instruments has led to this particular discovery in just four months of data acquisition. In their paper, accepted for publication in the Monthly Notices of the Royal Astronomical Society, the researchers suggest that, over six years of continuous observation, the telescope may be capable of detecting planets with albedos as low as 0.1%.

• And finally: In 2007 the Allen Telescope Array began operations. Designed and constructed to participate in both conventional radio astronomy studies and the Search for Extra-Terrestrial Intelligence, the array is jointly operated by the SETI Institute and the University of California. One of the goals is to observe planetary systems detected by the Kepler mission, searching for possible signals. However, in April 2011, funding shortfalls for operations of the Hat Creek Radio Observatory (HCRO) where the ATA is located resulted in the Allan Telescope Array being put into hibernation. After launching the SETIstars campaign in June, an appeal to supporters to help raise the 200,000 dollars needed to get the telescope back online, thousands of people from around the world made donations and operations are due to restart in September.

Interview with Dr Martin Bureau

Dr Martin Bureau (Oxford University) studies the molecular gas in local early-type galaxies. In this interview, he tells us what an early-type galaxy is, how studying the molecular gas in these galaxies might change our understanding of galaxies and what telescopes he uses to observe them.

Interview with Dr Ben Stappers and Lina Levin

The discovery of a 'diamond planet' orbiting a pulsar caused a stir in the media at the end of last month. Here we find out the real story from two of the researchers involved in the discovery. Dr Ben Stappers of the University of Manchester's Jodrell Bank Centre for Astrophysics tells us about the likely history of the planet, accreted and ablated from a white dwarf companion into a dense carbon-oxygen planet, while Lina Levin of the Swinburne University of Technology in Melbourne, Australia, discusses the international pulsar survey which found it.

The Night Sky

Northern Hemisphere

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

The Summer Triangle of the stars Deneb, Vega and Altair, in the constellations of Cygnus, Lyra and Aquila, is visible in the south after dark. Just to the left of Vega, which is in the top-right of the triangle, binoculars show a double star called Epsilon Lyrae, with two stars just about distinguishable to the naked eye. A small telescope reveals that each of these stars is itself a double, and the system is known as the 'Double Double'. The stars in each pair have an orbital period of several hundred years, while the pairs orbit one another every million years or so. Looking from Vega down to Altair, your line of sight crosses the dark Cygnus Rift in the Milky Way, where interstellar dust obscures the centre of the Galaxy. Brocchi's Cluster, with its asterism of the Coathanger, resides here. Comet Garradd passes to the right of it in the sky on the 2nd and 3rd of the month, moving westward towards Hercules. Pegasus, the Winged Horse, is to the left of Cygnus. Looking to the right of the Square of Pegasus, the Sun-like star 51 Pegasi is visible. It is home to the first extrasolar planet found to be orbiting a main-sequence star: a planet of half the mass of Jupiter but orbiting every 4.2 days, making it very hot and surprisingly close to its star. The planet-hunting spacecraft Kepler observes the stars between Cygnus and Lyra, and has found planets only slightly larger than Earth. After a year or two of observations, it will be able to detect planets with similar orbits to Earth as well as similar masses, and will give an indication of how common habitable planets might be. The Milky Way's neighbour, the Andromeda Galaxy, appears not far from the top-left of the Square of Pegasus. It was here that Edwin Hubble observed a Cepheid variable star in 1924, whose known relationship between brightness and period of brightness variation told him that Andromeda was outside our own Galaxy.

The Planets

• Jupiter is visible throughout the night, rising around 21:00 BST (British Summer Time, 1 hour ahead of Greenwich Mean Time) at the beginning of the month and about 20:00 BST by month's end. It shines at magnitude -2.7 in the constellation of Aries. Its angular size of 47" allows surface detail to be seen with a small telescope.
• Saturn is just visible at the beginning of the month, setting in the west at twilight.
• Mercury has passed inferior conjunction (when it was between the Earth and the Sun) and now appears in the pre-dawn sky. Its peak brightness is -0.3, occurring early in the month. It reaches superior conjunction (when the Sun is between it and the Earth) on the 28th.
• Mars is in Gemini with a magnitude of +1.4 at the beginning of the month, 1° north of the star Delta Geminorum. It rises red in the east before dawn and reaches 30° elevation by sunrise at the beginning of the month and 40° by the end. Its small angular diameter of less than 5" prevents much detail being seen on its surface.
• Venus has passed superior conjunction and reappears just after sunset at the end of the month, but its visibility is still hampered by sunlight as it sets low in the south-west with a magnitude of -3.9.

Highlights

• Jupiter's Great Red Spot is visible at certain times, and this month is a good time to view it.
• Mars is just 0.5° west of the photogenic Beehive Cluster, Praesepe, in Cancer on the 30th
• Mercury is 1.3° above the star Regulus, in Leo, on the 8th.
• The asteroid Vesta is at its closest to the Earth, and lies 1° south-west of the 4th-magnitude star Psi Capricorni this month. Its own brightness declines from +6.2 to +6.9 during the month. The Dawn spacecraft is currently in orbit around it.
• Uranus reaches opposition (when the Earth is between it and the Sun) around midnight at the end of the 25th, and therefore lies due south. At magnitude +5.8, it is just visible to the unaided eye in a very dark sky. It is in Pisces, about 15° below the left-hand side of the Square of Pegasus. Detail is hard to see as it is only 3.7" across, but its blue-green colour can be discerned.
• Neptune, earlier in the year having completed one orbit of the Sun since its discovery in 1846, is near to the sky position where it was found: up and right of the star Iota Aquarii, which is 5° east of Gamma Capricornis. It is blue-grey at a magnitude of +7.8, so is visible in binoculars, and is 2.5" across.

Southern Hemisphere

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

The Spring Equinox occurs on the 23rd of the month, after which the days become longer than the nights. The constellations of Scorpius and Sagittarius are in the west in the evening sky, followed by Capricornus, Aquarius and Pisces, while Orion rises in the small hours. Capricornus, depicted with head of a goat and the tail of a fish, appears as an elongated triangle with its tip facing away from Sagittarius. 2500 years ago, the goat's head was below the Sun at the time of the summer solstice, when the Sun was directly overhead for observers on the Tropic of Capricorn - the latitude line at 23.5° south on the Earth's surface. Precession of the Earth's rotational axis means that Sagittarius is now in the place of Capricornus for observers on this line at the summer solstice. In Capricorn, Alpha Capricorni is a double with stars at magnitudes +3.6 and +4.2, and the two can be distinguished with the naked eye. It is called Algedi in Arabic, meaning 'billy goat'. Beta Capricorni is another double, with a yellow star of magnitude +3.1 and a blue-white companion of magnitude +6, and is resolvable in binoculars. The brightest star in the constellation is Delta Capricorni, in the tail, at magnitude +2.9, which is about 40 light-years away and actually consists of four stars. Aquarius, symbolically pouring water onto the Earth, contains several deep-sky objects. M2 is a globular cluster that can be found with binoculars, near the 3rd-magnitude star Beta Aquarii. The cluster is just visible to the naked eye at magnitude +6.3, but only in a dark sky. A telescope can pick out individual stars, which, at around 13 billion years old, are among the most ancient in our Galaxy. South-west of Delta Aquarii lies NGC 7293 - the Helix Nebula - the largest planetary nebula in our sky and the closest to us at 700 light-years' distance. It covers half the angular size of the full Moon, corresponding to a diameter of 2.5 light-years. NGC 7009, the Saturn Nebula, is another planetary nebula, located near Eta Aquarii. The small constellation of Delphinus, the Dolphin, is below Aquarius. Its nose is Gamma Delphini, a double of golden-yellow stars at magnitudes +4.3 and +5.2. Crux, the Southern Cross, is on its side in the south-west after sunset. Though the smallest of the 88 constellations, it is one of the most recognisable in the southern hemisphere. An imaginary line can be drawn from Gamma Crucis at the top, through Alpha Crucis at the bottom, and on towards the bright star Achernar, halfway along which the south celestial pole is located. Continuing the line to the ground shows the direction of south. To Māori, the Cross is Te Punga - the Anchor - while in Tonga it is Toloa - the Duck. An image of the Cross at Machu Picchu labelled it as Chakana - the Stair - in the Inca language. Alpha and Beta Centauri are near to the Cross. Alpha Centauri is the third-brightest star in our sky and the nearest to Earth at a distance of 4.3 light-years. It is a double that can be split using a small telescope, while a larger telescope reveals a third star apparently in the system: the red dwarf Proxima Centauri, currently the closest actual star to the Sun. It may be orbiting the Alpha and Beta Centauri system, or they could both be part of a larger cluster. The activity of our own Sun is now increasing, with more features such as sunspots resulting from the varying magnetic field. This activity follows an 11-year cycle, but emergence from its minimum over the last two years has been slower than expected due to longer-term variation.

The Planets

• Saturn sets in the west just after sunset.
• Jupiter rises in the east just after midnight.
• Mars rises low in the north-east just before dawn, so it is not easily visible.
• Mercury rises just after Mars and so is even harder to spot before sunrise, while Venus is close to the Sun in the sky and so not visible.

Odds and Ends

On August 24, a Russian Progress spacecraft crashed soon after launch.

NASA is aiming to launch its next mission to the Moon on September 8. The Gravity Recovery and Interior Laboratory (GRAIL) will place two spacecraft orbiting around the Moon.

The Astronomy Picture of the Day on August 26 featured the new supernova spotted in M101.

The astronomers at the Royal Observatory Greenwich will be live-tweeting the results of the Astronomy Photographer of the Year 2011 competition on September 8 using the hashtag #astrophoto11.

The programme for the 2011 Manchester Science Festival is now online, and includes Rocket Week at the Jodrell Bank Discovery Centre.

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