June 2011: Spirit

In the show this time we have interviews about dark matter and evolved stars from the National Astronomy Meeting. Megan rounds up the latest news and we hear what we can see in the June night sky from Ian Morison and John Field.

The News

In the news this month:

• Comets are icy bodies known to contain material left over from the formation of the solar system. They spend most of their time far from the Sun where there is very little heat, but when their elongated orbits bring them close to the Sun they begin to warm up. As the surface heats up, so-called volatile materials, chemicals with low boiling points, begin to vaporise, resulting in the sometimes spectacular tails which are commonly associated with cometary bodies. While these tails are large and diffuse, the body of the comet itself is quite small and difficult to study directly. While the composition of a comet can be determined to an extent from studying the diffuse cloud of volatile material, the question of the nature of the nucleus itself is more difficult to answer. One model of comet formation, the gradual accumulation of dust and ice particles, results in a single large, uniform mass. An alternative model involves the merger of a number of mini-comets, resulting in a loose body of material with regions of different composition. But observations of the comet Hartley-2 suggest that, for this comet at least, the answer might be neither. A team of astronomers led by Michael Mumma of NASA's Goddard Space Flight Centre used the Very Large Telescope in Chile and the Keck telescope in Hawaii to take spectra of the coma of comet Hartley-2 over several months. If the nucleus is composed of a uniform material, then the chemical composition of the coma ought to remain constant as the body rotates, but if it is made up of a conglomeration of smaller proto-comets then the coma should vary in composition as different vents rotate into sunlight and become active. Images of the core from NASA's EPOXI mission show that the core is not uniform, with evidence of two, possible three types of ice. The researchers also found that some chemicals, including water, ethane and methanol, varied on a timescale consistent with the known rotation rate of the nucleus. The variation was significant, not just between different rotations, but on much shorter timescales too, suggesting that measurements of the composition of other comets where different chemicals have been searched for at different times may give the wrong idea about a comet's overall composition.

  Despite this evidence suggesting comet Hartley-2 is made of smaller proto-comets, the overall composition of the coma varied very little over several months, suggesting that the nucleus is actually very uniform. What the researchers think is happening is that chunks of ice deep in the comet's nucleus are held together with frozen carbon dioxide. The carbon dioxide evaporates at a lower temperature, carrying chunks of ice with it into the coma where the ice then evaporates, giving the impression that the composition of the comet is more uniform that it actually is. It is not yet known whether all comets behave this way, or if Hartley-2 is unique.

  Hartley-2 is the third comet where periodic variation of volatile compounds has been observed, and the first where imaging has led to an unambiguous association with the rotation of the nucleus. While Hartley-2 is a Kuiper-belt comet, the other comet to be studied in detail is comet Halley which is from the Oort cloud, further out in the solar system. The remarkable similarities in the composition of these two comets may suggest a common origin for at least some of the bodies from these two different parts of the solar system.

• We think of planets as being large gaseous or rocky bodies orbiting around a parent star. Our own planet is roughly five billion years old and orbits a fairly average G-type star at a mean distance of 150 million kilometres. But it seems there is a significant population of planets which exist without a parent star. In a paper published in the journal Nature on May 19th, a team of researchers report the discovery of ten free-floating planet candidates with masses similar to that of Jupiter. Although there is some evidence for lone planets in regions of star formation, these are objects between three and fifteen times the mass of Jupiter, and there are large uncertainties in estimates of the size of the population. Such a population of lone planets has previously been predicted to exist, but this is the first time evidence for them has been found.

  Planets in our solar system are visible to us here on Earth because they reflect the light of the Sun. Having no starlight to reflect, planets existing in free space will be much harder to spot. This new discovery is the result of observations of the galactic centre obtained by a joint Japan-New Zealand team in 2006 and 2007, and confirmed by a Polish team. The collaboration, led by Takahiro Sumi from Osaka University in Japan, used the principle of microlensing to find the so-called orphan planets.

  The detections were made by the MOA collaboration using observations made with the telescope at Mount John University Observatory in New Zealand which is used to regularly scan dense star fields towards the centre of our galaxy for gravitational microlensing events. These occur when something, such as a star or planet, passes in front of another, more distant, star. The passing body's gravity warps the light of the background star, causing it to magnify and brighten. Heftier passing bodies, like massive stars, will warp the light of the background star to a greater extent,resulting in brightening events that can last weeks. Small planet-size bodies will cause less of a distortion, and brighten a star for only a few days or less.

  Another group, the OGLE collaboration, observed many of the same events with a 1.3 metre telescope in Chile, independently confirming the analysis of the MOA group.

  It is possible that these objects may be in very large orbits around host stars, but the teams found no evidence of hosts in the data out to a distance of ten astronomical units, further than the orbit of Saturn. Direct imaging of the fields also found no evidence for a host star for any of the planetary bodies detected.

  These results suggest that there may be as many as twice as many unbound Jupiter-class planets as there are main sequence stars with masses less than the Sun, and that unbound planets may be one and a half times more common than planets with host stars. These planets may have formed on their own, much like the class of faint small brown dwarf stars, but what may happen is that they form in a planetary system around a star where multiple giant planets could scatter some of the bodies into very wide or completely unbound orbits.

• And finally: Stars are made of gas, but as they form they are surrounded by a much larger gas cloud and often a proto-planetary disk which contains gas, ice and dust. Many chemicals have been detected in these gas clouds and disks, including crystals of the mineral olivine. Found in a variety of forms, olivine is a silicate mineral which is found here on the Earth in some gemstones or the green sand of Hawaii's beaches. It is also a common mineral on the surface of the Moon and has been found elsewhere in the universe. Olivine, in the form of forsterite crystals, has been detected in the proto-planetary disks of young stars, but in new observations made with the Spitzer telescope a team of astronomers has found evidence of these crystals in the colder outer gas cloud around a proto-star where the temperature is a chilly minus 170 degrees Celsius. This is unexpected as temperatures high enough to melt rock are needed to form the crystals. The team, led by Tom Megeath of the University of Toledo in Ohio, suggest that the crystals were formed near the young star's surface and were carried by jets into the cooler cloud where they slowly fall back towards the star. This may help explain why comets, which are formed in the cooler outer reaches of planetary systems, contain minerals such as these which can only be formed at much higher temperatures.

National Astronomy Meeting

The annual National Astronomy Meeting was held in mid-April in Llandudno, north Wales. In this show we have three more interviews recorded at the conference.

Interview with Professor Gianfranco Bertone

Professor Gianfranco Bertone is a member of the Theoretical Physics group of the Institut d'Astrophysique de Paris, currently visiting the Institute for Theoretical Physics of the University of Zurich. In this interview, Professor Bertone talks about dark matter and various ways of detecting it, including direct detection techniques with the Large Hadron Collider (LHC).

Interview with Professor Mike Edmunds

Professor Mike Edmunds of Cardiff University studies dust and the chemical evolution of galaxies, and also leads a project investigating the Antikythera Mechanism. Here he discusses the unsolved mysteries of stars in the later stages of their lives. Although the outline of stellar evolution is understood, the finer details are important for an understanding the role of stars in dictating the chemistry of the Universe. Professor Edmunds talks about the advances in observational instrumentation and computer modelling that are allowing astronomers to uncover the more subtle aspects of stars' lives, which in turn underpin our picture of galactic evolution. His particular interest at the National Astronomy Meeting was the apparent over-abundance of intstellar dust in the early Universe, as determined from its absorption and chemical spectra. Since dust is often thought to be released by supernovae, how can so much have been created so early on? He speculates that giant stars may produce the initial grains during their later life, before they explode as supernovae.

Interview with Dr Rob Izzard

Dr Rob Izzard is an astrophysicist in the Stellar Astrophysics group at the Argelander Institute for Astronomy, part of the University of Bonn. Here he tells us about barium (Ba) stars, giant stars that are in binary systems and have very strong barium lines in their spectra. He and his group have been trying to model the orbits of the two stars and other dynamics of the system.

The Night Sky

Northern Hemisphere

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

As it gets dark - around 10pm in northern England - Leo is setting in the west. Ursa Major lies above and contains the stars Merak and Dubhe, which are aligned with Polaris, the North Star. It also includes the asterism of the Plough, the middle star of whose handle is actually a double, Mizar and Alcor, which can be resolved with the naked eye. Using a telescope, Mizar itself can be seen to be a double, and the entire system is now known to consist of six stars. Looking down and left from Leo, Virgo is in the south-west, and the planet Saturn currently resides there. Above it is Arcturus in the constellation of Boötes. Up and to the left of this is the arclet of stars known as Corona Borealis, the Northern Crown. Cygnus the Swan, Lyra the Lyre and Aquila the Eagle rise in the east around sunset. Their three brightest stars - Deneb, Vega and Altair respectively - mark out the Summer Triangle. Between Vega and Arcturus is the constellation Hercules, whose four brightest stars make up the quadrilateral shape of the Keystone. Binoculars or a telescope reveal the globular cluster M13 about two thirds of the way up its right-hand side. A spherical mass of up to a million stars almost as old as the Galaxy itself (and not much younger than the Universe), it is the most visible globular cluster in the northern hemisphere. The less prominent globular cluster M92 is above and left of M13. In Cygnus, Albireo is the head of the Swan, and a telescope shows that the main orange star is accompanied by a dimmer, bluer partner. The brighter star is orange because it has finished its main hydrogen-burning phase and become a giant. To the lower left of Cygnus is the small, diamond-shaped constellation of Delphinus, the Dolphin.

The Planets

• Jupiter rises in the east before dawn, but does not get high above the horizon before the Sun comes up. Its magnitude brightens from -2.1 to -2.2 during June, while its angular size is about 35" and gradually increasing.
• Saturn is in Virgo and appears low in the south-west after sunset, setting in the early hours. It dims from magnitude +0.8 to +0.9 over the month. We now see the rings at 7.5° from edge-on; the angle has been decreasing in the last few months due to Earth's orbit, but this month Saturn's orbit sees it revert to a longer trend of increase that will open out the rings significantly by the end of the year. The Cassini Division between the A and B rings is still visible, and a small telescope will show Saturn's largest moon, Titan, and perhaps some of its smaller companions.
• Mercury passes behind the Sun (superior conjunction) on the 12th, but becomes visible at twilight in the last week of June. It is then low in the west-north-west at magnitude -1, following the path of the setting Sun. By the end of the month it is 10° above the horizon at sunset, with a magnitude of -0.9. It is 6" across and 76% illuminated.
• Mars is low in the east-north-east before dawn, 4" across and dimming from magnitude +1.3 to +1.4 during the month. Its angular size and brightness will increase for the rest of the year, making it easier to observe in detail. It is currently in Taurus, between the Hyades and Pleiades Clusters.
• Venus is a pre-dawn object, shining brightly at magnitude -3.8. It is in the north-east, but, like Jupiter and Mercury, the shallow angle between the ecliptic and the horizon in the morning means it appears very low down. It is moving away from Earth and its angular size is now 10", but it is also 95% illuminated, keeping its brightness similar to that in previous months. It passes 6° below the Pleiades Cluster on the 8th, and on the 18th it is 5° above the Hyades Cluster and the closer star Aldebaran, which is the eye of Taurus the Bull.

Highlights

• Saturn is coming to the end of its current apparition, and on the 10th it lies just 15' (0.25°) from the star Gamma Virginis, also known as Porrima. Saturn has been moving westward relative to the stars since the end of January - the opposite of its usual direction. This retrograde motion is caused by Earth overtaking Saturn as our planet has a smaller, faster orbit around the Sun. With a small telescope, Saturn's C ring (within the brighter B and A rings) is visible, as is banding on the planet's surface. Porrima is a double whose stars are each of magnitude +3.5. Their eccentric orbit separated them by 6" in 1919, but put them less than 0.3" from one another in 2005. They are now 1.7" apart, and can be distinguished with a small telescope.
• There is a total eclipse of the Moon late on the 15th. The Moon rises in eclipse at around 21:20 BST (British Summer Time, one hour ahead of Greenwich Mean Time and Universal Time) for observers in northern England. It rises in the south-east, directly opposite the Sun as it sets, and remains low in the sky for the duration of the eclipse. The total eclipse (when the whole Moon is fully shadowed) ends around 22:00 BST, but the partial phase (when some of the Moon is fully shadowed) lasts until about 23:00 BST, and the penumbral phase (when the Sun is partially blocked from the Moon) lasts for another hour after that. The Moon's low position in the sky will also give the well-known illusion that it is larger than when high in the sky.
• The grouping of stars called Brocchi's Cluster is visible throughout the night this month in the Summer Triangle, about one third of the way from Altair at the bottom to Vega in the top-right. Its brighter stars form the asterism known as the Coathanger, and it lies near the small constellation of Sagitta, the Arrow.
• The waning crescent Moon and the planets Venus, Mars and Jupiter all appear low in the east to north-east around 04:00 BST (just before dawn) on the 28th. The pleiades Cluster is above and left of the Moon at this time, while the star Aldebaran is below Mars.

Southern Hemisphere

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

The southern hemisphere is in the grip of winter, with long nights allowing over 15 hours of observing per day in some locations. This means that some stars which set in the evening can be seen rising again the morning. The 22nd of June marks the winter solstice, and the dawn rising of the Matariki (Pleiades Cluster) and Puanga (the star Rigel in Orion) were used by Māori in some parts of New Zealand to mark the changing of the year in the calendar, Te Maramataka. Scorpius, the Scorpion, dominates the south-eastern sky, followed by Sagittarius, the Archer. Scorpius is marked by the red giant star Antares - the Rival of Mars - which is around 600 light-years away and is the sixteenth-brightest star in the night sky. It is about 800 times larger in diameter than our Sun, and intrinsically 10,000 times brighter. It has a fainter companion, but a medium-sized telescope is needed to distinguish it from the glare of Antares. Scorpius is seen as a fishing hook by Māori and many Polynesian cultures, and Antares is known to some Māori as Rehua, marking the eye of the hook. The Milky Way passes through Scorpius, so the constellation is host to a number of star clusters and nebulae which are easily observed. Near to Antares is the bright globular cluster M4, which is one of the closest to us at a distance of 7200 light-years. Observations with the Hubble Space Telescope in 1995 showed that some of its stars are among the oldest in our Galaxy, formed around 13 billion years ago. The faint globular cluster NGC 6144 is also near Antares, and long-exposure photographs of the region around the star reveal tendrils of dark material and glowing nebulosity. A number of double stars under the body of Scorpius can be observed with the unaided eye, binoculars or a telescope. NGC 6231. is a naked-eye star cluster near the stinger of the Scorpion, similar in size to the Pleiades but much more distant at 6000 light-years away. Another open cluster, M7, appears below the stinger as a haze to the eye but can be easily made out with binoculars or a wide-field telescope. The Butterfly Cluster, M6, is nearby but fainter, and is 1300 light-years away. Binoculars can also be used to observe M21, M23, NGC 6167 and NGC 6193.

The spectacular Lagoon and Trifid Nebulae reside in Sagittarius, along with many globular clusters. The Lagoon Nebula, M8, appears as a compact cluster of stars surrounded by a circle of nebulosity with a dark rift. It contains a number of dark globules that are protostars, and is one of the few star-forming nebulae visible to the unaided eye, at a distance of 4000-6000 light-years. The Trifid Nebula, M20, is a smaller region of nebulosity close by, and a telescope of at least 200mm in aperture allows dark lanes to be discerned. Long-exposure photographs show that it consists of red emission and blue reflected light. It is 7600 light-years away and shines at magnitude +6.3. Sagittarius is a centaur in Babylonian mythology, but its brightest stars form an asterism called the Teapot. Lambda Sagitarii marks its top, and nearby are the globular clusters M22 and M28. M22 is one of the brightest in the sky, at magnitude +6, as well as being one of the closest and largest in our Galaxy. Charles Messier did not observe individual stars in it; William Herschel was the first to do so, and you can do the same using a small telescope. It would be brighter, but for the quantity of intervening interstellar dust between it and the Earth. Sagittarius contains a wealth of other clusters: M23 is an open cluster of over 100 stars forming curving arcs and chains, 2000 light-years away; M24 is a bright region of stars known as the Sagittarius Star Cloud, which includes a number of dark nebulae; M25 is a bright open cluster, 2000 light-years away, containing a number of deep yellow stars; M55 is a globular cluster of magnitude +7.4, 16,000 light years away, which was discovered in the 18th century - along with many others - as telescopes became larger. The north-eastern part of Sagittarius is also home to a much more distant object: Barnard's Galaxy, NGC 6822, an irregular dwarf galaxy that is best viewed through a wide-field telescope. The Milky Way is at its brightest, widest and densest in Sagittarius, and has been seen variously as a river, a sky road and a bridge between the Heavens and the Earth. To ancient Arabs it was Al Nahr - the river - and it is the River of Heaven (Tien Ho) to the Chinese. The Māori called it Te Ika Roa, the Long Fish. Today we know that is the plane of our own Galaxy, and that the region of Sagittarius is in the direction of the Galactic Centre, some 30,000 light-years away. Astronomers have determined that the stars in the very centre - a region known as Sagittarius A* - are orbiting a supermassive black hole around 3 million times the mass of the Sun.

The Planets

• Jupiter, Venus, Mars and Mercury are in the morning sky before sunrise. Mercury disappears into the morning twilight at the start of the month. Venus is also very low, with Mars a little higher and Jupiter steadily climbing during the month.

Highlights

• A total eclipse of the Moon occurs on the morning of the 16th. The Moon starts to enter the Earth's shadow at 05:24 NZST (New Zealand Standard Time, 12 hours ahead of Universal Time), and the Moon begins to become fully shadowed an hour later, by which time the Moon will almost be setting for observers in New Zealand. Australia will have a longer viewing time, but in the east of the country the Moon will set before the end of totality, which is when the whole Moon is fully shadowed. In the west of Australia, however, the eclipse will be visible for its entire duration.

Odds and Ends

On May 25, NASA sent its last transmission to the Mars rover Spirit, ending attempts to make contact with the rover which last sent a communication in March 2010. Spirit has been on Mars since January 2004 but became stuck in May 2009. While it continued to take data from its stationary position, it is unlikely that the rover survived the Martian winter.

The Atacama Large Millimetre/sub-mm Array (ALMA) is growing all the time on its way to becoming a 66 antenna interferometer. In the past two months the first European built 12-m antenna was handed over to the observatory, as was the first of the Japanese built 7-m antennae. ALMA will begin its Early Science observations at the end of this year using a total of 16 12-m antennae located 5000m above sea-level in the Atacama Desert in Chile. For more on ALMA see the Feb Extra show.

An evening astronomy event is being held at the Museum of Science and Industry in Manchester on June 15 to conincide with the lunar eclipse. See the MOSI website for more details.

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