May 2012: Piggy-Back

Piggy-Back. In this month's show we bring you the first of many interviews recorded at the National Astronomy Meeting. We talk to Dr Jayne Birkby about Hot Jupiter planets, Dr Nick Cross about a billion star image, Phil Bull about dark energy and Dr Dan Brown and John Tanner about light pollution in the Peak District. Plus, Megan rounds up the latest news and we find out what's in the May night sky from Ian Morison and John Field.

The News

In the news this month:

• High-energy particle showers were first detected in the Earth's atmosphere in the 1930s, caused by the arrival of cosmic rays from outside the atmosphere. Such showers are triggered by the impact of particles with energies higher than 10^18 electron volts (that's a million times greater than the energies involved in particle collisions at the Large Hadron Collider) but, although we know what these cosmic rays are, their origin is still somewhat uncertain.

  Up to now, the most likely idea was that the highest energy particles were created in gamma-ray bursts, the extremely bright but short-lived events where massive stars collapse to form black holes, accelerating particles up to high velocities and producing large numbers of weakly-interacting particles known as neutrinos. But now, a team of researchers have found evidence which shows that either gamma ray bursts are not the only sources of such energetic cosmic rays, or that the efficiency of the process which creates them is much lower than models predict.

  The new results come from IceCube, a neutrino telescope built by an international collaboration and buried deep under the Antarctic ice. Consisting of more than five thousand sensitive detectors buried under the surface of the ice in holes drilled more than two kilometres deep, IceCube is designed to detect the tiny flashes of light emitted when high-energy neutrinos interact with atoms in the ice. Such interactions are rare, so IceCube has to be large - the detectors are spread throughout a volume of more than a cubic kilometre, making it the largest neutrino detector on the Earth. When a neutrino interacts with atoms in the ice, it creates a particle known as a muon which emits a tiny amount of blue light. IceCube's detectors are sensitive to this blue light and, by measuring the direction and intensity of this light, the research team can determine where in the sky the neutrino originated.

  By comparing the number and direction of neutrinos detected by IceCube with the positions of hundreds of gamma ray bursts detected by other telescopes, the research team found that the numbers did not match: the detected neutrinos were not coming from the same place as the gamma ray bursts. This has implications for models of the physics underlying gamma ray bursts since models predict a much greater flux of neutrinos than was actually detected.

  The other main likely source of these high-energy cosmic rays is active galactic nuclei, the supermassive black holes located at the centres of massive, distant galaxies. But this may not be the full story either, since other experiments have, so far, also failed to find a match.

• Galaxies are made up of dust, gas and stars, and to get a full understanding of a galaxy involves observing each of these components. The total starlight from a galaxy depends on the different stars which makes up that galaxy. But stars age and eventually die, and the nature of the stellar population changes over time. In a given galaxy, the number of stars of each type that we see today depends on the distribution of stars at the time the population was created, a distribution known as the Initial Mass Function. In the very local universe, stars of different masses can simply be counted, and such surveys find that the shape of the initial mass function does not appear to change between different environments. Studying the distribution in more distant galaxies is more difficult since, very often, we cannot see individual stars in order to count them.

  This distribution is a vital part of our understanding of galaxy evolution but, despite its importance, it is still uncertain if the form of this distribution is truly universal among different types of galaxy, or if it evolved over time as the universe aged.

  Now, in a paper published in Nature on April 25th, a team led by Dr Michele Cappellari of the University of Oxford, have found evidence for a systematic variation in this mass function which could have implications for our understanding of the star formation history of the universe.

  The group studied a sample of 260 nearby so-called early-type galaxies, those which are elliptical or lenticular in shape, and some of the oldest known objects in the nearby universe. By mapping the velocities of stars in each of the galaxies the researchers could calculate the ratio of the amount of light produced by the stars in each galaxy (dominated by the population of massive stars) to the amount of mass within the galaxy (dominated by the far larger number of low-mass stars), a quantity known as the mass-to-light ratio. By comparing the ratios calculated from the observations with predictions from model populations of stars created using different initial mass functions, the team found a systematic difference between galaxies of different types.

  Low-mass stars are far more numerous than their higher mass cousins, but this research shows that that massive elliptical galaxies have a larger fraction of low-mass stars than younger spiral galaxies such as the Milky Way and, crucially, that this difference is not due to the effects of dark matter.

  This result implies that there is no such thing as a universal initial mass function which is applicable to all galaxies, but that there is in fact a variation with galaxy type. This could have wide-ranging implications for a variety of areas of astrophysics, since the results show that often-used assumption that the initial mass function is universal is inconsistent with galaxies in the real universe.

• And finally: The motions of stars in the outer regions of a galaxy can be used to measure the mass of the galaxy, but such measurements show that the outer stars move faster than they should, given the amount of visible matter that we can actually see, material in the form of gas, dust and the stars themselves. This discrepancy is explained by the existence of dark matter, material with an as-yet unknown nature but which is invisible to conventional telescopes. But now, a new study of the motions of stars in the vicinity of the Sun has found no evidence for dark matter in our region of the Milky Way.

  In the study, accepted for publication in the Astrophysical Journal during April, a team of astronomers in Chile used optical telescopes to map the motions of more than 400 stars within 13,000 light years of the Sun. This is the largest survey of its kind to date, covering a volume four times larger than ever before. Using the observations, the team found that the amount of visible matter they detected matched with the amount expected from the motions of the stars, leaving no room for the existence of dark matter in our region of the Galaxy. Studies of the Milky Way show that stars in the outer regions rotate faster than can be accounted for by just the visible matter, so this result suggests that the shape of the dark matter halo around the Milky Way must be very strange indeed.

Interview with Dr Jayne Birkby

Dr. Jayne Birkby from the University of Cambridge talks us about her work regarding hot Jupiter type planets orbiting low-mass dwarf stars. Hot Jupiter's were first discovered in 1995 and since then there have been many questions about how exactly they form and what they can tell us about planet formation as a whole. Dr. Birkby explains how her work in particular with the WFCAM transit survey and the United Kingdom Infra-Red Telescope on Hawaii will help planet formation scientists choose between two competing planet formation theories. She also explains how her research will aid in the understanding of why hot Jupiter's end up so close to their parent star.

Interview with Dr Nick Cross

Dr Nick Cross works on producing, cataloguing and analysing wide-field infra-red images of the cosmos at the University of Edinburgh. These images form detailed surveys recording a huge number and variety of astronomical objects, and in this interview Nick talks about an infra-red image composed of 150 billion pixels, which made headlines for capturing a billion stars. Covering the plane and part of the bulge of our Galaxy, it consists of thousands of smaller pictures stitched together, each observed with either the UKIRT or VISTA instruments. Nick reveals that this is much more than just a pretty picture: when complete, the archive will also show the sky at various times, allowing distances to variable stars to be calculated. Astronomers from all over the world will be able to search a database of objects in the image and download the observations of those that interest them, including many extragalactic objects such as quasars. As Nick explains, the biggest advantage of using infrared wavelengths is that they penetrate cosmic dust much more than visible light does, allowing us to see through the centre of the Milky Way to the other side of its disc.

Interview with Phil Bull

Phil Bull (PhD student, Oxford University), talks to us about what dark energy is and what the "dark energy problem" is. He goes on to tell us about theories of what dark energy could be including quantum mechanical effects and modified gravity theories. He also tells us about the assumption of homogeneity of the universe. Phil also refers to type IA supernovae in his interview.

Interview with Dr Dan Brown and John Tanner

John Tanner from the Peak District National Park and Dr Dan Brown from Nottingham Trent University gave a talk at NAM where they presented the outcomes of raising awareness of light pollution. They discuss how well the message of light pollution has been spread and describe the events that they have done to promote awareness of light pollution as well as some future events including the Peak Star Party. In their interview, they also mention Dark Sky Discovery.

The Night Sky

Northern Hemisphere

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

Leo, looking like one of the lions in Trafalgar Square, lies to the south-west in the evening this month, while the constellation Virgo with the bright star Spica lies in the south. East from Leo is the cluster of galaxies called the Virgo Cluster. Many of these galaxies can be seen using small telescopes. East of that is the star Arcturus in the constellation Boötes and the circlet of stars called Corona Borealis. Further east is the constellation of Hercules. The four brightest stars in Hercules make up what is called the Keystone, and located two thirds of the way up the right-hand side of the Keystone is M13, a globular cluster visible with binoculars or a small telescope. Rising in the east in the early evening are the constellations Cygnus, Lyra and Aquila. Ursa Major is located directly overhead.

The Planets

• Mercury, at magnitude 0, is low above the eastern horizon in the early morning at the beginning of the month. It then disappears from view as it passes through superior conjunction (behind the Sun in the sky) on the 27th of May.
Venus still dominates the south-western sky after sunset. At the start of the month, it appears 34° above the horizon at sunset, which is as high as it ever gets. Venus is also at its brightest: its magnitude is -4.7. At the end of April, it achieved what is called its greatest illuminated extent, when it has the largest apparent illuminated area. As Venus moves towards the Earth this month, its angular size will increase to 57", but its elevation will decrease to about 6-7° above the horizon at sunset. In the last week of May, the thin crescent should be visible with binoculars and may even be visible to the naked eye. On the 6th of June, as Venus passes through inferior conjunction, it will transit across the face of the Sun. This will be the second and last transit of Venus this century, and it will be discussed more in the June Jodcast.
• Mars lies below the constellation Leo this month. As it moves away from us, its magnitude fades during the month from -0.1 to +0.5, and its angular size decreases from 10" to 8". However, given a night of good seeing, it might be possible to see some markings on the planet's surface.
• Jupiter passes behind the Sun on the 13th of May and will generally be very hard to see. While it will be visible just above the horizon before dawn at the end of May, it will be easier to see in the morning sky in a couple of months' time.
• Saturn reached opposition on the 15th of April and is now visible in the evening sky. The planet is currently moving retrograde through Virgo and is located about 5° north of Spica. Its magnitude will drop from +0.3 to +0.5. The part of the ecliptic that Saturn is passing through is relatively far south in the sky, so Saturn will not appear very high in the evenings. However, the rings are opening up very nicely. They are now inclined at 13° to the line of sight, making Saturn appear brighter than it did during its last apparition. Small amateur telescopes can be used to spot Cassini's Division, the largest gap in Saturn's rings, while larger telescopes may reveal Encke's Division or the inner C ring.

Highlights

• Saturn is at its best in the evenings this month. As mentioned above, the rings are now inclined towards the line of sight, and belts and zones can be seen in the northern hemisphere. With a small telescope, there is a good change to pick out several of Saturn's moons.
• On the 5th and 6th, we have a supermoon. This is when the full Moon corresponds to what is called perigee, which is when the Moon is closest to the Earth in its elliptical orbit. Because of optical illusion effects, the Moon will look particularly large near the horizon on these dates.
• The morning of the 6th will see the peak of the Eta Aquariid meteor shower, which appears to come from the direction of the constellation of Aquarius. It is believed that the Eta Aquariids originate from dust particles released by Comet Halley in an eruption about 4000 years ago. While the expected rate is about 25 meteors per hour, some meteors may be difficult to see because of the full Moon. The best time to look will be an hour or so before dawn, when the Moon is on the opposite side of the sky.
• On the 22nd, it is possible to see two very thin crescents: the Moon and Venus. While the crescent Moon can be seen once a month, it is rare to see it with Venus just above.
• On the 26th, the Moon will occult the open star cluster M67 in Cancer. The first part of the occultation is early in the evening when the sky is still bright, and so it may be difficult to see. However, at around 23:00 BST (British Summer Time, one hour ahead of Universal Time), the cluster will re-emerge from behind the Moon. This will be best seen with either binoculars or a small telescope.

Southern Hemisphere

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

May sees Orion the Hunter low in the west, beneath Sirius, the brightest star in the sky. Sirius sparkles like a diamond when near the horizon, as Earth's turbulent atmosphere splits its light into the constituent colours. It forms the head of Canis Major, while, below it, Canis Minor is marked by Procyon. These two dogs accompany Orion, who is upside-down to southern hemisphere observers. His Sword and Belt are also known as the Pot or the Saucepan. The three stars of the Belt were called Tautoru by Māori astronomers. In the middle of the Sword is the haze of the Orion Nebula, a glowing, star-forming region visible to the naked eye and resolvable as a cloud through binoculars. To Orion's right are the stars Castor and Pollux, marking the heads of Gemini, the Twins. In the south, Crux, the Southern Cross, is high overhead with the pointer stars Alpha and Beta Centauri. The constellations of Carina, the Keel, and Vela, the Sails, run along the Milky Way between Crux and Sirius. A number of bright stars and nebulae reside in this region, including the Eta Carina Nebula. Although bright at magnitude +1, it blends into the Milky Way when viewed with the naked eye. Sweeping around with binoculars, however, reveals many star clusters, along with glowing clouds of gas that are laced with dark lanes.

Scorpius, or Te Matau a Māui, rises in the east after sunset. It is accompanied by Sagittarius, and the two constellations mark the widest and brightest part of the Milky Way. To the west of Scorpius's brightest star, Antares, is the globular cluster M4, in which a small telescope will show curved loops and streams of stars. 4° west of Antares is another globular cluster, M80, appearing as a hazy ball to small telescopes and a dense star cluster to larger ones. The clusters M6 and M7 are both visible to the unaided eye near Scorpius's tail, and their stars can be discerned using binoculars.

The Eta Aquariid meteor shower is visible at the beginning of the month. It can produce up to 60 meteors per hour, but is hampered by a nearly-full Moon this year. The radiant, from which the meteors appear to originate, rises in the north-east after midnight, and will be best seen around 03:00 NZST (New Zealand Standard Time, 12 hours ahead of Universal Time).

The Planets

• Mars is in the north in the evening, shining with a red glow.
• Saturn shines yellow in the same part of the sky as Mars, near to the bright star Spica.
• Venus becomes lost in the evening twilight, and next month will transit across the Sun.

Odds and Ends

At the end of April, the space shuttle made its final flight. But this time it wasn't a space mission, instead it was strapped to the back of a Boeing 747 to take part in a "victory" lap around the US Capital, Washington D.C. This resulted in many excellent photo opportunities and was a fitting send off for the flagship of the space shuttle fleet. Discovery will now take up residence in a Smithsonian Museum.

The British ESA astronaut Tim Peake is soon to begin training in an underwater base off the coast of Florida. The NASA Extreme Environment Mission Operations (NEEMO) is a facility for astronaut trainees to test their teamworking and problem-solving in a situation from which there is no quick exit - the sort of skills that will be necessary for space missions. In an exchange programme, NASA's astronauts will later undergo cave training with ESA. You can follow Tim's training on Twitter (@Astro_TimPeake) and, along with his fellow trainees, on the ESA Astronaut Class of 2009 blog.

A company has been set up to mine asteroids for their resources, this is a long term project which includes building space refuelling stations aswell.

Show Credits