April 2011: Arriba! Arriba!

In this episode we talk to 4 members of the Jodrell Bank Centre for Astrophysics: Dr Bob Watson tells us about the cosmic microwave background radiation, Dr Cristobal Espinoza tells us about pulsar glitches, Dr Jaime Pineda tells us about star formation and Matias Vidal tells us about cosmology. As usual, Megan has the latest astronomical news and Ian Morison and John Field tell us what's in the night sky.

The News

• Measurements of the expansion of the Universe show that it is accelerating, an effect thought to be due to dark energy, something that counteracts the gravity of ordinary matter but which has not yet been detected and whose actual nature is still unknown. Measuring the rate of this expansion, and the expansion history - how the expansion rate has varied with time, is essential to our understanding of the evolution of the Universe. This expansion rate has been measured in the distant Universe using several different methods, including measurements from the cosmic microwave background (the echo of the big bang) and observations of thermonuclear supernovae. Predictions of the expansion rate in the local Universe have been made using these results, but this requires various assumptions to be made about several unsettled questions in astronomy, such as the nature of dark energy and its effects on other material, the geometry of space itself, and the properties and distribution of neutrinos. Measuring the expansion rate in the nearby Universe can therefore provide some information on these unsolved problems. But local measurements have their own challenges. Accurately determining distances is one major problem, with nearby distance indicators being used to calibrate more distant ones. Errors in nearby calculations are then propagated out to more distant objects, significantly increasing the errors in measurements of the expansion rate. Now, in a paper published in the Astrophysical Journal, a team led by Adam Riess at Johns Hopkins University in Baltimore have measured this expansion rate more accurately than ever before. Their results use the fact that the absolute brightness of an object and its measured brightness as it appears to us here on the Earth can be used to calculate an object's distance. By carefully observing more than 600 examples of a type of star known as Cepheid variables, stars which vary in brightness with a period related to their absolute maximum brightness, located in the host galaxies of thermonuclear supernovae, exploding stars for which the maximum brightness can be determined, the team were able to produce an accurate distance ladder. This was then used to calculate the value of the Hubble constant, the number which describes the expansion rate of the Universe. The number they calculated, 73.8 kilometres per second per megaparsec, fits well with previous measurements but, due to the large number of stars observed, has the smallest uncertainty yet, at just 3.3 per cent, an improvement of 30 per cent over previous results. This result, combined with recent data from other cosmology experiments, has implications for our understanding of the expansion of the Universe. One idea, an alternative to the idea of dark energy, is the suggestion that the Milky Way may be located within a giant void, a relatively empty region of space some eight billion light years across. If we were located towards the centre of this bubble, galaxies accelerating away from us would be an illusion. But these new results are inconsistent with this hypothesis, still leaving the question of what dark energy actually might be.

• Following the big bang, it was some time before the Universe cooled and began forming stars and galaxies. Over time, these young galaxies gradually assembled into clusters, continuing to evolve into the structures we see today, such as the well-known relatively nearby cluster in the constellation of Virgo. Studies of galaxies in such clusters show that the surrounding environment has a strong influence on a galaxy's morphology and star formation activity, but when in the history of the Universe did such clusters actually form? The massive elliptical galaxies located at the centres of clusters are old, and stopped forming stars a long time ago, although they continue to grow as material is pulled in from the surrounding cluster by the strong gravitational field. This infalling material largely destroys much of the evidence of how these galaxies evolved, so determining their history is difficult. One way round this is to observe more distant clusters, since we see them as they actually were far in the past, and use these observations to reconstruct a typical evolutionary sequence. A team of astronomers, led by Raphael Gobat at Saclay in Paris, have used infrared observations to locate a cluster of galaxies at a redshift of 2.1. Observed as region containing many more compact, red galaxies than would be expected by chance, the team also found strong X-ray emission from the same region. Such emission comes from hot gas between galaxies within a cluster, and is a strong indicator of a gravitationally-bound group of galaxies. By investigating the properties of the galaxies within this cluster, the group found that it is far more evolved than would be expected for a cluster at such a redshift, making it likely to be the most distanct mature galaxy cluster found so far, implying that clusters dominated by massive elliptical galaxies and containing significant amounts of X-ray emitting gas were forming significantly earlier in the history of the Universe than was previously known.

• And finally: As the Moon orbits the Earth, it always keeps the same face towards us because it is in what is known as a tidally-locked orbit. It takes the same time to rotate once on its axis as it does to travel once around the Earth. When the first images of the far side of the Moon were returned by the Soviet Luna 3 spacecraft in 1959, the view was very different to what we are used to seeing on the nearside. Rather than the large darker areas we see on the nearside, areas known as lunar mare, or seas, the entire farside more resembles the lighter areas of the lunar highlands seen near the south pole of the nearside. Exactly what the reason is for this dual nature of the lunar surface has been an ongoing puzzle. The mare themselves are thought to be the result of large impacts on the Moon's surface. Incoming rocks hit the surface, fracturing the crust and allowing the molten basalt underneath to flow upwards through the cracks and fill in the impact basins, producing the darker coloured, flat surface of the mare. That such features are seen mainly on one side of the Moon's surface and not the other is thought to be because the Moon's crust is thicker on the far side, making it harder for molten magma to erupt onto the surface. But the question remains: why is the farside crust thicker? Several presentations at the 42nd Lunar and Planetary Science Conference, held in Texas during March, attempted to answer this question. The Lunar Reconnaissance Orbiter has been imaging the Moon from orbit since 2009, and its Wide Angle Camera has been used to image the entire surface at times when the Sun was low in the sky, producing long shadows which can be used to calculate the height of surface features and investigate the topology of the lunar landscape. The final global dataset is yet to be completed, but a preview, released at the conference, shows the Moon's far side in spectacular detail.

Interview with Dr Bob Watson

Dr. Bob Watson works at the Jodrell Bank Centre for Astrophysics investigating the cosmic microwave background (CMB). In this interview, he explains the techniques used to map the CMB and tells us about the QUIJOTE experiment and how data from this will help us to understand more about the formation of the Universe.

Interview with Dr Cristobal Espinoza

Dr. Cristobal Espinoza is part of the pulsar group at the Jodrell Bank Centre for Astrophysics. In this interview, he tells us about his research into how pulsars are slowing down. During this process, some pulsars present features called glitches, Cristobal explains how these glitches are measured and what they can tell us about the inner parts of a neutron star.

Interview with Dr Jaime Pineda

Dr Jaime Pineda is a COFUND ESO fellow for ALMA based at the Jodrell Bank Centre for Astrophysics. In this interview he tells us about star formation and how he observes the very beginning of this process in the Milky Way. Jaime talks about how he measure the dynamics inside the molecular clouds where stars are forming, the telescopes is he using at the moment, and how he plans to use ALMA in the near future.

Interview with Matias Vidal

Matias Vidal is a PhD student at the Jodrell Bank Centre for Astrophysics studying the cosmic microwave background radiation (CMB). In this interview, he tells us about his work trying to observe the CMB B-modes, a type of polarization of the CMB radiation, and how observing these B-modes would help our understanding of the inflation period at the beginning of the Universe.

The Night Sky

Northern Hemisphere

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

The winter constellations of Orion, Taurus and Gemini now set in the west in the evening, while Leo the Lion is in the south. His brightest star, Regulus, is the Lion's front knees, while the asterism of the Sickle is his mane and head. Several bright galaxies lie beneath him, east of Regulus, and can be seen with a telescope. Even more are to be found in 'the Realm of the Galaxies' behind Leo, between Virgo and Coma Berenices. The bright star Arcturus is nearby in Boötes, and Ursa Major, the Great Bear, lies to the north. It contains the asterism of the Plough, whose handle has a middle star which is actually a double named Alcor and Mizar. With binoculars, Mizar is revealed to be a double star itself. The constellations of Lyra and Cygnus rise later in the night, their bright stars Vega and Deneb forming the Summer Triangle with Altair in Aquila.

The Planets

• Jupiter passes behind the Sun (superior conjunction) on the 6th, but rises just before the Sun late in the month, with a magnitude of -2.1 and an angular size of 33".
• Saturn is at its best, rising around sunset and reaching its furthest point from the Sun in the sky (opposition) early on the 4th, when it will be almost due south. Saturn's rings become temporarily slightly more edge-on this month, dropping from 8.8 to 7.8 to edge-on and causing a decrease in the planet's brightness from magnitude +0.4 to +0.5. They now span 44", more than double the angular diameter of the planet. The Cassini Division between the A and B rings can be seen through a telescope, as can Saturn's brightest moon, Titan, and, in good seeing conditions, other moons as well.
• Mercury passes in front of the Sun (superior conjunction) on the 9th, but can be seen just before dawn late in the month, at magnitude +0.9.
• Mars, having passed superior conjunction last month, re-emerges before dawn in the middle of this month, but is very low in the eastern sky. Its magnitude is +1.2 and its angular size is 4".
• Venus is brilliant in the pre-dawn sky at magnitude -3.9, but quite low down. As it moves towards superior conjunction, its angular size drops from 13.2 to 11.7" during the month, but its illuminated fraction increases to keep the brightness constant.

Highlights

• Details can be made out on the surface of Saturn with a telescope this month, and its rings can be seen ever more clearly, even though they won't appear at their broadest until 2016.
• Lying to the right of Saturn is the double star Gamma Virginis, or Porrima. It consists of two stars of magnitude +3.5 each. Due to their binary orbit, their angular separation decreased from 6" in 1919 to just 0.3" in 2005. Since this point of closest approach to one another (periastron), they have been moving apart again, and are 1.7" apart this month, allowing them to resolved as a pair using a small telescope.
• Low in the west after sunset on the 5th, you can see 'the old Moon in the new Moon's arms', which is when the shaded part of the Moon can be seen glowing faintly within the curve of the waxing crescent Moon. This is caused by sunlight reflecting off the Earth and then the Moon, and is more noticeable when the day side of the Earth is cloudier and more reflective.
• The Moon's Alpine Valley is visible using a telescope on the 11th. It is to the right of the crater Plato, and is 7 miles wide and 79 miles long. A rill runs along its length, but is difficult to spot.
• The Lyrid meteor shower peaks on the 22nd and 23rd and is best observed around 02:00 BST (British Summer Time, one hour ahead of Greenwich Mean Time), when you may see 15 meteors per hour. It has been recorded for 2600 years and consists of dust from Comet Thatcher, which was discovered in 1861.
• Jupiter, Mercury, Mars, Venus and the crescent Moon are all close together just before dawn on the 29th and 30th, but you will need to look with binoculars (making sure you are not still looking through them when the Sun comes up), very low to the east. The Moon will be even closer to the planets on the morning of the 1st of May.

Southern Hemisphere

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

Autumn has arrived in the southern hemisphere. Over the last month our daylight hours have continued to get shorter as we passed the autumnal equinox, and will continue to do so until the winter solstice in June. One advantage of the longer night-time hours is that we can start our star-gazing at a more reasonable hour. Low in the west after twilight, the head of Taurus the Bull is visible, marked by the V-shaped cluster called the Hyades, with the bright foreground star Aldeberan marking one of his eyes. The Pleiades Cluster, marking the back of Bull, will be lost in the twilight sky during April. They will not reappear in the morning sky until June, around the time of the southern hemisphere's winter solstice. One of the zodiacal constellations, Taurus, sits along the path of the Sun, the Moon, and the planets as observed from Earth. The Sun will move in front of the background stars of Taurus during a period from the 13th of May to the 21st of June. The less bright of the two eyes, Epsilon Tauri, was found in 2007 to have a planet 7 times the mass of Jupiter with an orbital period of 1.4 Earth years around it.

There are 20 stars brighter than magnitude 5 in our night sky known to have planets orbiting them. A number of these are visible in our southern sky. The neighbouring zodiacal constellation Gemini's second brightest star, Pollux, shining at magnitude 1.15, also hosts a planet estimated at 2.7 times the mass of Jupiter. Only slightly fainter than Pollux is the bright southern star Fomalhaut - 'the Mouth of the Fish' in Arabic - in the constellation Pisces Austranis, the Southern Fish. The star is known to have at least one planet orbiting it. This planet was the first to be directly imaged, by the Hubble Space Telescope in 2008. There is a disc of debris around the star and it is possible that other planets may be forming within the disc.

Gemini is home to the star cluster M35. This open cluster is about the same size as the full Moon and, at magnitude 5.3, should be visible to the unaided eye from a dark location and easily seen with binoculars or a small telescope. The cluster is estimated to be 2800 light years away and consists of over 300 stars covering an area about 24 light years across. Gemini represents the heavenly twins, Castor and Pollux, who travelled with Jason and the Argonauts on their quest to find the Golden Fleece in Greek mythology.

As Taurus sets in the west, our winter constellation, Scorpius, rises in the east. The brightest star in Scorpius is Antares - 'the Rival of Mars' in Greek - shining at magnitude 1. Depending on the list used, it is either the fifteenth- or sixteenth-brightest star in the night sky. Antares is a red supergiant star about 600 light years distant; it has a diameter 800 times that of our Sun and is estimated to be 65,000 times brighter, with a mass up to 18 times greater. Although the core of this star is many times hotter than that of our star, the Sun, the expansion of its atmosphere makes the surface temperature much cooler and gives it a reddish hue. This colour gave rise to its name as the Rival of Mars. Antares is part of a binary star system and the companion, although bright at magnitude 5.5, is difficult to observe due to the brightness of Antares. Normally a telescope greater than 150 mm (6 inches) is needed to observe the companion. But, occasionally, the Moon can occult Antares and this can allow smaller telescopes to observe this star whilst Antares is hidden. Some observers report it as having a green tint, but this due to an optical illusion as the star is actually a blue supergiant. To Māori in Aotearoa (New Zealand), Antares is known as Rehua, and it marks the eye of Te Matau a Māui (the Hook of Māui). The curve of the Scorpion's body and stinger are seen as the curve of the Hook, and the distinctive triangle made by the stinger becomes the tip of the Hook. In Māori mythology this was the hook that Māui, a great hero, used to pull the North Island of New Zealand out of the ocean. The North Island is known as Te Ika a Māui (the Fish of Māui).

The Hook crosses the a wide and bright part of the Milky Way, and it is in this region of the sky that we look toward the centre of our Galaxy, 30,000 light years away. With such a high concentration of stars, this region is a prime area for observing gravitational microlensing events. Microlensing is based on the gravitational lens effect. A massive object (the lens) will bend the light of a bright background object (the source). This can generate multiple distorted, magnified and brightened images of the background source. As the two objects move into alignment, a bell-shaped curve of light intensity with time is formed. If a planet orbits around the lensing star then an additional peak will appear on the curve. A collaborative team of astronomers from New Zealand and Japan are running a microlensing programme at Mount John Observatory above Lake Tekapo in the centre of the South Island of New Zealand. Using a 1.8-metre telescope and a wide-field CCD camera, they study dense regions of the sky on a regular basis to spot the slight change in brightness that may herald a lensing event. Once a potential event is observed, a wide group of amateur and professional astronomers are notified and can follow the light curve. This potentially allows 24 hours' observation of the event and means that the short-period peak of a planet can be observed. To date, planets have been discovered orbiting fourteen stars in Scorpius, but all that stars are fainter than 5th magnitude.

The Planets

• In our evening sky we have only one planet visible - Saturn rising in the east. Sitting in the constellation of Virgo, Saturn appears as a bright yellow 'star'. The most distant of the naked-eye planets, Saturn takes just under 30 years to complete one orbit around the Sun. Named after the father of the Gods in Greek Mythology, it is sometimes associated with Kronos, the father of time. The rings surrounding Saturn are visible in a small telescope with a magnification of 20 or greater. The first person to observe the rings was Galileo in 1610. The rings are known to be made mostly of water ice with small amounts of other materials. Although the rings are wide they are actually very thin - currently estimated to be 20 metres thick. The rings are made up of a large number of individual rings and some areas are denser or sparser than others. The gaps between some rings are due to the interaction of the gravitational pull of the Saturn, its moons, and material within the rings themselves. Saturn has a similar axial tilt to the Earth and so it experiences seasons. An effect of this tilt is that rings sometimes appear as thin bands around the planet and at other times as far more open discs. This tilting of the rings alters the brightness of the planet and its appearance in a telescope. Saturn's largest moon, Titan, is also visible in small telescopes and was discovered in 1655 by Christiaan Huygens. This moon is similar in size to Mercury but has half the mass. Titan is surrounded by a dense atmosphere of nitrogen, methane and hydrogen. This composition is similar to what we think the Earth's atmosphere was like shortly after it formed.
• Venus is still visible in the morning sky as the Morning Star. The other bright planets are still too close to the Sun to be easily seen.

Odds and Ends

April is the Global Astronomy Month run by Astronomers without Borders. Events are running throughout the month but highlights include a global star party on April 9 and lunar week running April 10 - 6.

April 12 marks the 50th anniversary of manned space flight. To celebrate, Yuri's Night is a collection of events across the world.

NASA's penultimate shuttle mission is scheduled to launch on the 19th of April. Endeavour will makes it final flight in order to deliver things to the International Space Station, including the Alpha Magnetic Spectrometer, which will look for evidence of unusual types of matter such as antihelium nuclei and theoretical supersymmetric particles called neutralinos.

The annual UK National Astronomy Meeting is being held in Llandudno in North Wales in mid-April. There are public events on the evenings of April 18 (in Welsh) and April 20 (in English).

The new Discovery Centre at the Jodrell Bank Observatory is nearly complete and scheduled to open to the public in mid-April. The staff have been keeping a development diary during the construction period.

Show Credits