August 2011: Witless

In the show this time, we talk to Dr Philipp Podsiadlowski about all sorts of different supernovae. As always, Megan rounds up the latest news and we hear what we can see in the August night sky from Ian Morison and John Field.

The News

In the news this month:

• Over the six and a half years since its launch, the Swift satellite has been spotting energetic outbursts all over the sky, but in two papers published in the journal Science on the 8th July, astronomers report the detection of a completely new type of transient event whose properties are unlike those of any previously observed source. With several detectors on board, Swift is designed to spot new objects in the gamma-ray sky and collect information in three different parts of the electromagnetic spectrum. When the satellite detects a new event, it realigns itself so that the object is visible to both the X-ray telescope and the ultra-violet/optical telescope (known as XRT and UVOT respectively), and transmits a message to the ground so astronomers can point other telescopes at the same event to gather more information. On average, Swift detects about two of these gamma ray bursts every week, and most are fairly short-lived (lasting only a few seconds), but one detected on March 28th 2011 turned out to be quite exceptional. Known as GRB 110328A (or Swift 1644+57), this source stayed bright in the X-ray for a considerable amount of time, re-triggering the burst alert telescope three times over the next 48 hours. Even so-called long-duration GRBs fade significantly on timescales of a few minutes and, since long-duration GRBs are the results of catastrophic stellar explosions, they do not re-brighten and cause multiple triggers. The burst was observed at the centre of a small star-forming galaxy located 3.8 billion light years away in the constellation of Draco, but its peak luminosity was roughly 100 times higher than for known bright active galactic nuclei, suggesting that the cause of the burst could be a rare process involving the galaxy's central black hole. The burst was followed up by ground-based optical, infra-red, millimetre and radio telescopes, as well as the Hubble Space Telescope and the Chandra X-ray Observatory. According to the authors of the two studies published in Science, the likely cause of this extremely unusual event is that the massive black hole in the centre of the galaxy has pulled apart a star which got too close, ripping the star apart and swallowing it in pieces, sending out two powerful jets of radiation, one of which was pointing right at the Earth. Since this is the first event of its kind that has been observed in the six and a half years of Swift's operation, they must be rare occurrences.

• Small bits of rock and dust enter the Earth's atmosphere every single day, but most are tiny and go completely unnoticed by people on the ground, larger ones are often seen as meteors by observers under dark skies. At the other end of the scale are the large asteroids and comets (so often used in science fiction movies) which would cause widespread damage if they got close enough to hit our planet. In between these extremes are rocks which are large enough to make it most of the way through the atmosphere, but small enough that they explode before reaching the ground. Large enough atmospheric explosions can cause physical damage at ground level, as happened in Tunguska in 1908 when a large impactor exploded less than 10 kilometres above Siberia, flattening 80 million trees over an area of 2,150 square kilometres (830 sq mi) with an estimated energy of tens of megatons of TNT. Luckily, events on this scale are rare, but there is evidence that explosions in the kiloton range, objects around 10 metres in diameter, happen in the upper atmosphere more than once a year. But with little in the way of reliable records, estimates of the damage caused by objects of different sizes are not well constrained.

  Writing in the journal Geophysical Research Letters, a team led by Elizabeth Silber from the University of Western Ontario in Canada report the detection of an atmospheric impact over Indonesia using infrasound detectors. The explosion occurred on October 8th 2009 at 10:57am local time and was witnessed by people in South Sulawesi who reported thunder-like sounds and the ground shaking. The team used data from a network of infrasound detectors which pick up very low frequency sound waves to look for signatures of the event. The detectors are part of the International Monitoring System, a global network operated by the Comprehensive Nuclear-Test-Ban Treaty Organisation, set up to detect nuclear explosions. The energies of atmospheric explosions caused by bolide explosions such as the Tunguska or Indonesian events are comparable with those in nuclear explosions, so the network is an ideal instrument for studying the size, energies and frequencies of extraterrestrial impactors. Infrasound describes sound waves of frequencies of 20 Hertz or less, waves which can travel very large distances in the atmosphere, making them ideal for studying atmospheric explosions. Using the network, the team found positive detections of the Indonesian event in data from 17 separate stations, one as far away as 17,500km which was recorded almost 15 hours after the explosion. Using the data collected from these stations, together with modeling the likely track through the atmosphere, the team estimate the energy of the explosion to be equivalent to roughly 50 kilotonnes of TNT, corresponding to an [[chondritic]] object between 6 and 10 metres in diameter. This makes it much smaller than the Tunguska event which had an estimated energy of 10 to 15 megatonnes of TNT. Based on records collected to date, objects the size of the Indonesian event are thought to impact the atmosphere on average once every five years. The authors point out that, aside from eyewitness reports and a few handheld videos of the dust trail, no other records of the event exist, so information from this network of sensors provides valuable data on the frequency and energies of atmospheric impacts.

• And finally: July 18th saw the launch of a new space telescope. The RadioAstron satellite was launched from the Baikonur Cosmodrome aboard a Zenit at 2:31 GMT. With a diameter of 10 metres, it is the largest space telescope currently in orbit, but, rather than collecting optical light as Hubble does, RadioAstron is (as its name suggests) a radio telescope. While 10 metres is on the small side for a radio telescope on the Earth, its main use is as part of an array of existing ground-based telescopes using a technique known as interferometry. Since radio waves have much longer wavelengths than ordinary light, radio telescopes have very poor resolution compared to optical telescopes. To overcome this, radio telescopes work together in arrays to synthesize a much larger instrument with far greater resolution. With an operational lifetime of five years, the addition of the RadioAstron antenna to existing ground-based arrays will enable observations at a resolution of a few milliarcseconds (around a millionth of a degree).

Interview with Dr Philipp Podsiadlowski

Dr Philipp Podsiadlowski of Oxford University studies supernova explosions. Here he talks about different types of supernova, using modelling to try to determine the history of an object from its supernova remnant, and why it is often difficult to connect an observed supernova to an expected theoretical type. Supernovae can happen in a range of different scenarios, from the collapse of an aged massive star to the accretion of material onto a white dwarf, and each involves different physical mechanisms. As he discusses in this interview, the famous Supernova 1987A involved a surprisingly young-looking (i.e. blue) star, which may have resulted from the merger of two stars, one of which was near the end of its life, some 20,000 years earlier.

The Night Sky

Northern Hemisphere

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

On a clear night, you can't help but see the constellations Cygnus the swan, Lyra the lyre and Aquila the eagle, with their bright stars Deneb, Vega and Altair making up the summer triangle. Just below Cygnus is Delphinus the dolphin, below Delphinus is the constellation Equuleus and then there are two constellations below: Aquarius and Capricornus. Over to the left of Delphinus is Pegasus (upside down as we see it in the northern hemisphere) with four stars making up the square of Pegasus. The top left-hand star of the square of Pegasus is Alpheratz (or alpha Andromedae) and is also part of the Andromeda constellation, which contains the Andromeda galaxy, M31.

There are two ways to find M31. Starting at Alpheratz, move one bright star to the left, then turn a bit upwards to a second bright star. Go the same distance again and then turn sharp right to one star not far away. Go the same distance again and you should see M31 as a fuzzy object, close to another fainter star. M31 is easily visible with binoculars, or possibly with the unaided eye on a dark, transparent night with no Moon. Continuing upwards from M31 is the constellation Cassiopeia, the 3 rightmost stars form a V-shape that points towards M31. Taking the top left star of that V and going to the next star in Cassiopeia points towards Perseus. Scan along from Cassiopeia down into Perseus with binoculars and you'll see a fuzzy glow which is a pair of open clusters called the Perseus double cluster.

The Planets

• Jupiter is high in the pre-dawn sky. It rises at midnight at beginning of August and by dawn is at an elevation of 50°. Its angular size is increasing to 44 arcseconds.
• Saturn is now deep down in the western sky after sunset. It is at an elevation of 20° in the southwest an hour after sunset but by the end of the month, its elevation is only 5°. Its brightness is increasing because the rings are opening up, now 8° from edge on.
• Mercury passes between Earth and the Sun (inferior conjunction) on August 16, but quite quickly emerges into pre-dawn sky. At 6am on August 27 it will be 7° above the horizon, down to the lower left of a waning Moon.
• Mars is at magnitude 1.4. At the beginning of August it is in the western half of Gemini and will have risen to 30° at sunrise, so will be easily seen in the pre-dawn sky, just north of east. By month's end, it is in mid Gemini, and rises at 2am getting to an elevation of 40° by sunrise. Its angular size is about 4.5 arcseconds. On August 5 it passes to the lower right of the open cluster M35.
• Venus passes behind the Sun on August 15.

Highlights

• The Perseid meteor shower is on August 11-14 and peaks in the early hours of August 13. This year full moon is also on August 13, at low declination but moonlight will wash out fainter meteors. Look towards Perseus, up in the sky to the east, in the early hours for the best chance. On August 11 there won't be as many meteors but by 4am the Moon will have set, giving half an hour before it gets light to look.
• The mane and head of Pegasus point upwards, beyond the highest star of the head is the globular cluster M15. On August 2 Comet Garrard (magnitude 8) will be 0.5° above M15, should see it with binoculars. At the end of month, on August 27, it will lie 7 arcminutes from magnitude 8 globular cluster M71.
• On August 22, Neptune reaches opposition, at magnitude 7.8 you should see it with binoculars under dark sky. On August 22 it also lies 1.6° above iota Aquarii.
• Vesta reaches magnitude 5.6 this month, might just see it with the unaided eye under a dark transparent sky. On August 5 at peak brightest it passes close to pair of fainter stars. On August 31 it is 20 arcseconds from magnitude 4 star psi Capricornus, down to the lower left of Aquila. It is due south around midnight, binoculars should pick it out.

Southern Hemisphere

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

This month we look at the region of sky stretching northwards from the constellation of Sagittarius to the star Deneb, low above the horizon. The Milky Way is at its thickest and brightest in Sagittarius, thinning along the Cygnus Arm towards Deneb. A dark rift of interstellar gas and dust runs through it, blocking the light of the stars beyond. The celestial birds, Aquila the Eagle and Cygnus the Swan, fly through this part of the night sky. The four brightest stars in Cygnus form the Northern Cross; Deneb is the brightest and marks the tail, while Albireo is the head. Deneb is the 19th-brightest night-time star and is around 1500 light-years away. With binoculars or a telescope, Albireo can be seen to be a double star consisting of a blue star and a yellow companion, the latter itself a double. 31 Cygni is another double, with orange and turquoise components, but appears very low in the New Zealand sky. 61 Cygni was the first star whose distance was measured by parallax, a feat accomplished by Friedrich Wilhelm Bessel in 1838. It is nearby at 11.4 light-years' distance, and is another double. Aquila is marked by a line of three stars, the brightest of which, Altair (meaning 'flying eagle' in Arabic), is 17 light-years from us. Aquila contains a number of observable star clusters such as NGC 6709 and NGC 6755, and planetary nebulae such as NGC 6751 and NGC 6781. A planetary nebula has nothing to do with planets, but consists of the ejected atmosphere of a dead star illuminated by the white dwarf at its core. They are typically visible for just a few thousand years before they dissipate, and they seed the instellar medium with heavy elements which end up in the new stars and planets such as those in our Solar System. M27, the Dumbbell Nebula, is one of the most famous planetary nebulae, sitting below Aquila. Its shape can be seen through a small telescope, and a long exposure photograph will reveal its colours. It is one of the closest planetary nebulae to Earth at a distance of around 1300 light-years, and extends to a quarter of the diameter of the full Moon in the sky. The asterism known as the Coathanger is nearby.

The Planets

• Saturn is the only planet high up in the evening sky. It is 100 times more massive than the Earth and has a similar composition to the Sun, and is, on average, less dense than water due to its large gaseous atmosphere. Its ring system is millions of kilometres across but each ring is just tens of metres in depth, visible due to a make-up of reflective ice and rock. The main rings and Saturn's largest moon, Titan, are visible in a small telescope, and more of the planet's 63 moons may also be seen. Saturn was the father of the gods and the keeper of time in Greek mythology.
• Mercury is low on the western horizon after sunset, near Saturn and the star Spica. It reappears in the dawn sky, but is washed out by the rising Sun.
• Jupiter rises in the east around midnight and is high in the sky by sunrise. Binoculars reveal its four largest moons, and a small telescope shows the cloud bands on its surface.
• Mars follows a few hours behind Jupiter, rising red in the pre-dawn sky.

Highlights

• The asteroid Vesta reaches opposition on the 5th, and, at magnitude 5.6, is just visible to the naked eye in a dark sky. It passes close to Venus in the sky on the morning of the 10th, but both are too close to the Sun to be discerned. NASA's Dawn spacecraft is currently orbiting Vesta and returning close-up images.

Odds and Ends

On July 4, the Hubble Space Telescope logged in its one millionth science observation. Looking at exoplanet Kepler 2b, HST made spectroscopic measurements in a search for traces of water.

The space shuttle Atlantis landed back on Earth on July 21, marking the end of the space shuttle era. Progress is being made by the companies contracted by NASA to build the next generation of spacecraft, with one company, SpaceX, hoping to dock an unmanned capsule with the International Space Station by the end of 2011.

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