November 2011: Mischievous

In the show this time, we talk to Prof. Rob Izzard about the mischievous J-stars. As always, Megan rounds up the latest news and we hear what we can see in the November night sky from Ian Morison and John Field.

The News

In the news this month:

• For much of human history, stars were assumed to be static points of light which did not change. Now we know that they are much less constant, giant nuclear furnaces which evolve over timescales much longer than a single human lifetime. Stars spend most of their lives fusing hydrogen to make helium, and the lifetime of a star depends on its mass; more massive stars have more hydrogen, but they burn hotter and faster, using up their fuel much quicker than their less massive cousins. For a particular stellar cluster made up of stars formed at the same time, we expect to see stars at different stages of evolution depending on their mass. But for older clusters, the most massive stars will already have evolved, becoming red giants or exploding as supernovae, leaving a cluster with no hot, blue stars. Many old stellar clusters in the Milky Way do, in fact, contain a small but significant population of hot, blue stars which appear to be much younger than the rest of the cluster. So how did these so-called blue stragglers come to be?

  There have been many suggestions as to the origin of these apparently young stars, and now a pair of researchers have found evidence which narrows the possibilities. Aaron Geller and Robert Mathieu, both of the University of Wisconsin-Madison, looked at the blue stragglers in an old open cluster in the Milky Way and examined the properties of their companion stars.

  The two most likely formation mechanisms for blue stragglers are the transfer of material from one star to another in a binary or multiple system, or collisions and mergers of stars. Many observations and detailed theoretical models favor formation through stellar collisions, but these new observations provide strong evidence that mass transfer is the dominant mechanism, certainly in this particular case.

  NGC 188 is an open cluster with an age of around seven billion years containing many blue stragglers, including a high percentage with binary companions. The researchers studied these binary systems, looking specifically at the twelve examples with orbital periods of around 1000 days. They found a surprisingly narrow range of companion masses, with most of the blue stragglers having companions with masses of 0.55 times that of the Sun. The two researchers modeled populations of blue stragglers formed through a number of different processes, including the transfer of mass from both main sequence and white dwarf companion stars, and collisions between stars in binary and triple systems. The predictions which best match the observations from NGC 188 are from models where the blue stragglers are the result of mass transfer from a white dwarf in a binary system.

  This new study shows that a collisional origin for blue stragglers is far rarer than expected, casting doubt on whether it occurs at all. Although mergers in triple systems are not completely ruled out by the data, the observations suggest that this is a far less likely scenario. Further observations using the Hubble Space Telescope are planned by the researchers, and these should allow them to distinguish clearly between these two formation mechanisms.

• A large part of the Earth's surface is covered by water, but the composition of the bulk of the rock which makes up our planet suggests that the early Earth would in fact have been a pretty dry place, so where did all the water come from? Now, an international team have found evidence for at least one possible source of terrestrial water. The Earth's composition is similar to that of the class of meteorite known as enstatite chondrites, a type of meteorite thought to have formed in the early solar system. The early Earth would have been so hot that volatile compounds such as water would have evaporated, leaving the Earth's surface a much drier place than it actually is today. The fact that we have oceans implies there must have been an additional source of water sometime after the planet's formation. One possibility is that water and other volatile material could have been delivered by the impact of asteroids and comets, but measurements of the isotopic content of water in comets has so far not matched that seen in the Earth's oceans.

  Deuterium, also known as heavy hydrogen, is an isotope of the element hydrogen containing an extra neutron in its nucleus. On Earth, the ratio of deuterium to hydrogen is roughly 1:6400, so the source of the Earth's water, whatever that was, should have the same ratio. The ratios of deuterium to hydrogen in the six comets measured so far, all with origins in the Oort cloud, are much higher than the D/H ratio measured here on Earth. Now, data taken with the Herschel Space Telescope have shown that at least one comet has a D/H ratio similar to that in the Earth's oceans. In a paper published in the journal Nature on October 5th, an international team led by Paul Hartogh at the Max-Planck Institute for Solar System Research show that comet Hartley-2 has a deuterium to hydrogen ratio pretty close to that of the Earth's oceans. Unlike the other comets where this ratio has been measured, Hartley-2 is thought to have originated in the Kuiper belt, much further out in the solar system. This time last year, Hartley-2 passed closer to Earth than at any time since its discovery, allowing highly detailed observations by both ground and space-based telescopes. Observations made with sensitive instruments on board the Herschel satellite allowed the scientists to determine the amount of heavy water present in the comet. Both water and heavy water, water where one of the two hydrogen atom has replaced by an atom of deuterium, have characteristic emission patterns in the infra red part of the spectrum. Even though the amount of heavy water contained in the comet is very small, the sensitivity of Herschel's instruments meant that it could be detected and measured. The results show that, for comet Hartley-2, the ratio is 1:6200, very close to that found on the Earth. While comet Hartley-2 itself was not the source of the Earth's water, this result implies that similar comets formed in the same region of the early solar system could have been.

  As is so often the case however, the results also raise further questions. It was thought that the D/H ratio would vary with position in the solar system, with the ratio becoming larger at greater distances from the Sun. Hartley-2, however, comes from further out than the other six comets for which this ratio has been measured, but has a smaller D/H ratio. This implies either that this particular comet originated closer to the Sun than was thought, or that the distribution of deuterium in the young solar system was not what we assumed. Either way, while adding weight to the theory that the Earth's water is cometary in origin, these results still leave some unanswered questions about our own solar system.

• When stars explode as supernovae, they release large amounts of fast-moving material into the interstellar medium. Just as a smoke ring blown in the air soon loses energy and slows down, the spherical shell of material ejected in a supernova explosion eventually loses speed and the rate of expansion slows. Just how quickly the material slows down depends on the density of the surrounding material in the interstellar medium. So, if you know when a particular supernova occurred, and you know how fast material should be moving because of the explosion, you can calculate the size which you expect the remnant to be. A long-standing mystery with one particular supernova is that its actual size is some 2-3 times larger than expected, based on what we know about supernova explosions, the speed the ejecta should be moving and the estimated density of the interstellar gas in the Milky Way. Now, in a paper published in the Astrophysical Journal, a team led by Brian Williams of North Carolina State University have found evidence why this particular supernova remnant is so much larger than expected.

  The remnant, known as RCW 86, is thought to be the remains of a supernova explosion seen from Earth nearly 2000 years ago, the first known recorded supernova in human history. Knowing the date of the explosion from records made at the time, astronomers calculated the size of the remnant today based on a normal density for the interstellar medium and how fast the ejected material appears to be moving. The result is 2-3 times smaller than what is actually measured by telescopes today, assuming the remnant is at the measured distance of 8000 light years. The speed of particles in the remnant today are too slow to have created a remnant this large in just 2000 years. The logical conclusion is that the initial supernova occurred in a cavity, a bubble-shaped region of lower than average density, allowing the ejecta to expand faster than normal until they reached the edge of the bubble and were forced to slow down. Normally, cavities such as this are expected from core collapse supernovae, the type of explosion which results from the death of a star with a mass greater than eight times that of the Sun. Such stars have powerful stellar winds before they explode which push away the surrounding interstellar medium, effectively blowing a low-density bubble.

  The team used the Spitzer and WISE infra red satellites to image the entire remnant of RCW 86, covering an area of sky more than 40 arcminutes in diameter (larger than the full Moon), and compared these images with data from optical and X-ray detectors. Hydrodynamic models of the system suggest that the most likely scenario which explains the observed properties of the nebula is a supernova explosion into a low-density bubble created by the central object some time before the final explosion. Such a model fits the observed properties of the remnant but does not distinguish between a core collapse or type Ia origin. However the large amount of iron seen in the X-ray observations is a strong indicator that the supernova was far more likely to have been a type Ia, the kind of event resulting from material falling onto a white dwarf in a binary system. This finding is particularly interesting because these low density bubbles such as the one seen in RCW 86 were previously only associated with core collapse supernovae.

• And finally: In the second high-profile re-entry in as many months, the ROSAT X-ray observatory made its fiery descent into the Earth's atmosphere on October 23rd. Launched on June 1st 1990, its mission ended in 1999 but, due to having no on-board propulsion system, a controlled re-entry was not possible. The Roentgen Satellite, to give it is full name, was a German spacecraft which made the first all-sky X-ray imaging survey. Originally intended to operate only for 18 months, the mission was highly successful, ultimately operating for a total of eight years until the failure of its primary star tracker in 1998. On Sunday October 23rd at 0150 UTC, the satellite entered the atmosphere somewhere over the Bay of Bengal. Although it is possible that larger parts of the craft may have survived reentry, it is not yet known whether any parts of the satellite actually reached the ground.

Interview with Prof. Rob Izzard

Prof. Rob Izzard is an astrophysicist in the Stellar Astrophysics group at the Argelander Institute for Astronomy, part of the University of Bonn. In this interview, Rob talks about J-type stars, carbon stars which are rich in the carbon 13 isotope. Here he proposes a new theory for how these stars are formed, discusses their odd chemical abundances and explains why these stars are mischievous.

The Night Sky

Northern Hemisphere

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

The Square of Pegasus is in the south-east after sunset. An arc of stars forms the mane and head of Pegasus, the Winged Horse. The globular cluster M15 is up and right of Enif, the nose of the Horse. The top-left of the Square is the star Alpheratz, from which your unaided eye can locate 2.5-million-year-old photons coming from the Andromeda Galaxy. Under a dark sky, the Triangulum Galaxy, our smaller neighbour, can also be found. The w-shaped constellation of Cassiopeia is high in the sky, with Perseus slightly lower to the north-east. Between them, where the Milky Way runs, the Perseus Double Cluster can be made out with binoculars or a telescope. Taurus, the Bull, is lower in the east, containing the Pleiades Cluster. The V-shaped Hyades Cluster marks the head of the Bull. The Bull's eye is the bright red giant star Aldebaran, which is not part of the Hyades. Orion rises in the east later in the evening, and moves across the southern sky during the night.

The Planets

• Jupiter is just past opposition (when it is on the opposite side of the Earth to the Sun) and so is close to us. It is in Aries, the Ram. As well as getting high in the sky, it is close to perihelion, giving it an angular size of almost 50", which is near the maximum possible. You can make out its Great Red Spot at certain times using a telescope.
• Saturn has now passed superior conjunction (when it was behind the Sun in the sky) and reappears in the pre-dawn sky this month. It has a magnitude of +0.8 and lies in Virgo, a few degrees from the first-magnitude star Spica. By month's end, it is at 20° elevation at dawn. The rings are 13.5° from edge-on and continue to open out, with the Cassini Division easily visible with a telescope.
• Mercury is about 2° below Venus in the first half of the month and shines at magnitude -0.3. Its low elevation at sunset means that binoculars are needed to see it (be sure not to look or point a telescope towards the Sun). As the month progresses, Venus climbs higher each night while Mercury drops down out of view.
• Mars resides in Leo at magnitude +1, and passes close to the Lion's brightest star, Regulus, on the 10th and 11th. By the end of the month, it is visible by midnight each night and reaches 40° above the horizon by dawn. With an angular diameter of 7", surface features are now becoming visible, and it will increase to 10" after Christmas.
• Venus has emerged from superior conjunction and has an 11" disc which is 90% illuminated. It can be seen just above the south-western horizon at sunset at the start of the month, with a magnitude of -3.8. By the end of the month it is up for a little longer after sunset.

Highlights

• Mars is 1.3° from Leo's brightest star, Regulus, an hour before dawn on the 11th, remaining close by for five days.
• The Leonid meteor shower peaks on the night of the 16th-17th. The best time to view it is around midnight, when the Moon is not nearby in the sky. The meteors are the dust of Comet Tempel-Tuttle, which orbits the Sun every 33 years, and appear to originate from the from the Lion's head (also known as the Sickle) in the constellation of Leo.
• Saturn is close to the star Spica and a thin, waning crescent Moon before dawn on the 22nd.
• Venus appears near the thin crescent Moon at sunset on the 27th, also giving a chance to spot 'the Old Moon in the New Moon's Arms'.
• Neptune can be found using binoculars or a telescope from the 20th to the 27th, when the Moon is not washing it out. It is near the star Iota Aquarii in Aquarius. On the 22nd, it is in the same position in the sky as it was when it was discovered.
• Comet Garradd is visible this month at magnitude +6 using binoculars in the constellation of Hercules, to the left of the The Hercules Globular Cluster is not far away, to the right of the Keystone.
• The asteroid Eunomia can be viewed with binoculars towards the end of the month at magnitude +8, as it passes high to the east through Perseus. It is almost overhead on the 28th, lying in front of the California Nebula, which is a reddish region of excited hydrogen gas.
• Uranus can just be made out with the naked eye in a dark sky at magnitude +5.8 this month. It is in Pisces, about 15° below the eastern side of the Square of Pegasus.

Southern Hemisphere

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

The winter constellations of Scorpius and Sagittarius are descending towards the western horizon, while the rise of Taurus, Orion and Canis Major in the east heralds the arrival of summer. Early in the month, Mercury and Venus are close together in the west after sunset, and by the middle they approach to the red star Antares, the heart of the Scorpion. Jupiter rises high in the northern sky by midnight, appearing at its brightest as it is close to the Earth, having just passed opposition (when it was on the opposite side to the Sun in the sky). Binoculars or a small telescope reveal its four largest moons, which change position from night to night. Banding on its surface is also visible, as is its Great Red Spot at certain times. Jupiter's rapid rotation and fluid composition make it appear wider than a sphere at its equator.

The three brightest stars in the night sky - Sirius, Canopus and Alpha Centauri - line the south-eastern horizon near the Milky Way. The brightest is Sirius in upside-down Canis Major (the Great Dog), which shines at magnitude -1.46 and is intrinsically around 26 times more luminous than our Sun. The second-brightest star in Canis Major is Adhara, at magnitude +1.5, which is intrinsically some 20,000 times more luminous than the Sun and one of the brightest ultraviolet sources in the sky. The Great Dog's third-brightest star is at the Wezen, at the tip of its tail; it is a yellow supergiant 50,000 times more luminous than the Sun. The open star cluster M41 is in the belly of the Dog, and can be seen with the naked eye or resolved into individual stars using binoculars or a telescope.

Crux, the Southern Cross, is in the south-west after sunset and descends during the night but never sets. Taurus the Bull and Orion the Hunter stand to the north-east of Sirius, with the Hyades Cluster forming the Bull's head. The brightest star in this region is Aldebaran (the Follower), which lies between Earth and the Hyades, and is named for following the Pleiades Cluster across the sky. The Pleiades are part of the Bull's back, and are also known as the Seven Sisters in Greek mythology, as Matariki to the Māori and as Subaru to the Japanese. Near to where the Bull's horns point to the northern horizon is the Crab Nebula, an expanding cloud of gas and dust resulting from a supernova observed in 1054. Orion has his shield and sword raised, and a Belt of three aligned stars with a fainter Sword slightly higher in the sky. A faint haze in the Sword marks the Orion Nebula, a star-forming region 1200 light-years from us. To most southern hemisphere observers the Belt and Sword are called the Pot, the Iron Pot or the Saucepan, while to Mā they are Tautoru. The Maya visualised Orion as a turtle with three stone glyphs on its back, while they saw the stars Alnitak, Rigel and Saiph as a triangular hearth from which the Orion Nebula was rising smoke.

Odds and Ends

The Lovell Telescope at Jodrell Bank has been chosen to represent the letter 'J' in a new set of Royal Mail Stamps featuring iconic UK landmarks. This is not the first time Jodrell Bank has been featured on a stamp around the world but marks the first time since 1966 the Jodrell Bank radio telescope has been on a stamp in the UK.

The VLA (Very Large Array) in New Mexico has been undergoing a substantial upgrade of its systems over the past ten years and, as a celebration of their completion, a competition has been opened to rename the VLA. Members of the public are being encouraged to submit suggestions for the new name for the upgraded VLA (which many have been calling the e-VLA) via the website: www.namethearray.org. The closing date for entries is 1st December 2011 at 23:59 EST and the new name will be announced at the American Astronomical Society meeting in Austin in January.

Ancient volcanic tunnels on the moon, discovered in images from Japan's Kaguya mission, could provide the main structure for a lunar colony. Russian space scientists think they could seal the tunnels with an inflatable shell, avoiding the need to dig into the lunar surface or build walls and ceilings. Such a base could become a reality by 2030.

The new Virgin Galactic spaceport in New Mexico has been dedicated by Sir Richard Branson by absailing down the side of the building drinking champagne. Onlookers were also given a flyby from the commercial crafts being used for the flights (WhiteKnightTwo and SpaceShipTwo). The spaceport is the world's first "built from scratch" commercial spaceport and will be the departure and alighting points for passengers taking the $200,000 trip into space and back. The flights aboard WhiteKnightTwo and SpaceShipTwo will last 2.5 hours in total, including five minutes of weightlessness.

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