This month in the news: cosmological rulers, a distant ocean and an ancient wander.
Scale, in astronomy, is a concept that can be difficult to appreciate since the distances between stars, galaxies or the horizon of the universe are so incredibly large that they are completely divorced from the day to day life on Earth. The problem is that these distances need to be measured to an accuracy of just a fraction of a percent because we rely on them to establish the properties of stars and the universe as a whole. As distance is gradually measured with ever greater accuracy, it becomes possible for new physics to become unlocked. For example, it was not until Gian Domenico Cassini made the first true measurement of the distance between the Earth and Sun that the distances to the stars could be established using a method known as parallax. Likewise, if we are to understand the physics of the larger universe we need another larger measuring stick to use and the most logical is the distance to our closest galactic neighbour, the Large Magellanic Cloud.
Last month, a team of researchers as part of the Araucaria Project published a new set of measurements they had made for distances to eight eclipsing binary stars within the Large Magellanic Cloud. Eclipsing binaries are star systems where one star will pass in front of the other with respect to the Earth's line of sight. By measuring the time it takes for the two stars in an eclipsing binary to orbit each other, the velocities of the two stars with respect to each other and the shape of the light curve the system produces, it is possible to get a robust measurement of the stars distance. Using this method, the researchers managed to measure the average distance to the Large Magellanic Cloud to be 162,902 light years away to an accuracy of just 2.2%.
However, there is a history with the Large Magellanic Cloud for researchers to quote distances to a level of accuracy that is much less than the actual scatter on the distances made by independent teams. Although in just several years, the GAIA satellite, an ESA mission to measure the distances and proper motions of up to 1 billion stars using the parallax method, will be able to conclusively determine the true distance of the Large Magellanic Cloud.
Between 1995 and 2003 the Galilean moons, Io, Ganymede, Callisto and Europa were extensively studied by NASA's Galileo satellite. One of the key goals for Galileo was to confirm whether or not there was a liquid ocean beneath the icy surface of Europa. By mapping changes in Jupiter's magnetic field they detected small perturbations had developed around Europa. Quantifying this change allowed the Galileo researchers to deduce that Jupiter's magnetic field was inducing another magnetic field in Europa which could only be created by the movement of an ion rich liquid beneath the surface such as a salty liquid water ocean. Unfortunately, beyond this confirmation that liquid water was contained within Europa, Galileo's other instruments were not sufficiently sensitive to be able to probe the chemical constituents of Europa's surface which would have allowed for greater insight into the environment beneath the icy sheets.
Modern day ground based telescopes have come a long way in the thirty years since the launch of Galileo and even though they are up to 600 million miles away, they are now able to measure precise spectra of Europa. This is exactly what a pair of researchers at the Jet Propulsion Laboratory did and they found markers for the salt, magnesium sulphate. The presence of magnesium on the surface indicated that there must be a method of transferring material from the ocean beneath to the surface, since there would be no other reason for a salt of magnesium to be present. However the oceans on the Earth are rich in chlorine based salts such as magnesium or sodium chloride but not sulfur based salts which would be poisonous to life. Interestingly though, they could only find evidence for magnesium sulphate on one side of Europa, the side that, because Europa is tidally locked and does not rotate relative to Jupiter, is always exposed the volcanic eruptions coming from the neighbouring moon Io. The researchers coupled this with previous research showing that Europa has a thin atmosphere containing potassium and sodium. They reasoned that most likely the surface as a whole is dominated by the presence of chlorine based salts, therefore indicating that the oceans beneath Europa's cold exterior may by a dark reflection of our own Earth oceans.
Finally, HD 140283 is a star with a long history. It first caught the attention of astronomers over a century ago as it was observed to be moving at remarkable speeds on a trajectory that would send it far out into the outer halo of the Milky Way where only the oldest of stars reside. It was conjectured that the orbit of HD 140283 was caused when its original dwarf galaxy host was destroyed and consumed during the formation of the Milky Way. However this star holds an accolade far stranger than simply an unusual orbit. Soon after its discovery astronomers found that using all the tools and knowledge we had accrued in recent history for the task of dating the age of stars gave an extremely worrying result when applied to HD 140283. It was found that the age of the star exceeded the age of the universe. Measuring the age of stars in the past relied mostly upon circumstantial evidence such as assuming that all stars within a single cluster will form at the same time, then by measuring the distance to the cluster, the stars' spectra and counting how many stars are in particular stages of evolution, the age of the cluster can be estimated as a whole. However dating an individual star is far more difficult and beyond the theoretical and technical abilities of astronomers in the past, hence giving stars like HD 140283 ages that exceeded the age of the universe by several billion years.
By using modern day detailed models for how a star evolves alongside the extraordinary resolution of the Hubble space telescope, a team of astronomers have been able to refine the date of HD 140283. By observing HD 140283 as it races through the constellation Libra, the team has established that the star is 190.1 light-years away. Using this in conjunction with modern understanding for rates of fusion processes in stellar cores, a model which considers a process known as helium diffusion, where helium is pulled deep into the star making it shine brighter, and new measurements showing the star is rich in oxygen come together to give a new estimated age of 14.46 billion years, accurate to within 800 million years. This measurement still dances precariously above the 13.8 billion year age of the universe but unlike previous measurements there is now significant overlap in the uncertainties. Regardless of HD 140283's true age what we do know is that it is among one of the oldest stars in the universe and most likely formed only shortly after the Big Bang.
Interview with Dr Matt Auger
Dr Matt Auger from the Institute of Astronomy at the University of Cambridge tells us about dark matter. He begins by telling us what dark matter is and goes on to tell us how he is investigating the properties of dark matter within galaxies and the methods of observing the effects of dark matter via graviational effects, including gravitational lensing. He also talks to us about the differences between various types of galaxies and why these differences exist.
The Night Sky
Ian Morison tells us what we can see in the northern hemisphere night sky during April 2013.
The constellations of Orion, Taurus, Gemini and Auriga are setting in the west in the evening, with the planet Jupiter visible in Taurus. Cancer lies south of Gemini, with M44, the Beehive Cluster, at its centre. M67, another open star cluster, is to its lower left. Continuing around the sky, Leo the Lion is visible, its head and mane forming the asterism of the Sickle with the bright star Regulus at the bottom. A pair of binoculars or a small telescope reveals several galaxies below the Lion's belly. Virgo is to the south-east, and between the stars Denebola in Leo and Vindemiatrix in Virgo is the Realm of the Galaxies, a rich area of the sky for galaxy-hunters with medium-sized telescopes on a dark night. To the east, the orangey-yellow star Arcturus is in Boötes, beside the arclet of stars called Corona Borealis, the Northern Crown. Hercules is in the north-east, with four bright stars making a trapezium called the Keystone. Two thirds of the way up the right-hand side of the Keystone is the brightest globular cluster in the northern sky, M13, visible using binoculars. Vega, the brightest star in Lyra the Lyre, rises later in the evening. Ursa Major, the Great Bear, is above Leo and contains the recognisable asterism of the Plough, with Polaris due north in the constellation of Ursa Minor. Draco the Dragon winds its way around the Little Bear.
- Jupiter is in the west after sunset, making its way eastwards night by night. Shining at magnitude -2.1 at the beginning of the month, it is 5.5° above Aldebaran, the Eye of the Bull. Its angular diameter drops from 35.8 to 33.6" during the month, but a small telescope still shows surface detail such as the equatorial bands, as well as the four Galilean moons orbiting the planet. As Jupiter rotates, the Great Red Spot can be seen at certain times.
- Saturn is becoming more visible, rising in Libra about half an hour after nightfall as the month begins, and crossing the south at around 25° elevation at 02:00 BST (British Summer Time, one hour ahead of Universal Time). It reaches opposition (opposite the Sun in the sky) on the 28th, crossing the south at 01:00 BST. Saturn's rings are now inclined at 18° to the line of sight, giving the best view of the last six years. They span 43" across and 13" in width and obscure most of the planet's northern hemisphere from view. The largest gap between the rings, Cassini's Division, can be seen with a small telescope, with a fainter inner ring also visible on a dark night. The moon Titan can also be found.
- Mercury is close to the Sun in the sky, but can just be seen on the eastern horizon at dawn at the beginning of April. You will probably need binoculars to spot it, but don't use them once the Sun has risen.
- Mars passes behind the Sun (superior conjunction) on the 18th, and will not be visible until it reappears in the pre-dawn sky in over a month's time.
- Venus reached superior conjunction on the 28th of March, but becomes just visible at the end of April, low in the west-north-west. It may be spotted using binoculars about 20 minutes after sunset, at magnitude -3.9.
- Saturn has the first of two good observing months during April, lying 9 Astronomical Units from the Sun and having a magnitude of +0.3. Its 18" disc is surrounded by its prominent ring system. To find it, follow the Plough's handle down past the bright star Arcturus to the first-magnitude star Spica in Virgo. Saturn appears as a yellowish star just to its left. Its brightest moon, Titan, is visible in binoculars at magnitude +8.2. A telescope of 200x magnification, with an aperture of 6-8 inches, shows the planet and rings in their full glory. Saturn rotates every 10.5 hours, giving it an equatorial bulge. It has bands of muted colour. The 27° tilt of Saturn's rotation with respect to the plane of the Solar System gives a changing angle to the rings as seen from Earth, and the rings will next be at maximum visibility in May 2017.
- Comet PANSTARRS is visible throughout the night in April, but binoculars may be required to see it. It moves northwards through the constellation of Andromeda and passes close to the Andromeda Galaxy from the 1st to the 6th.
- Jupiter is still prominent in the south-west after sunset this month, and its features and large moons are nice to observe with a telescope.
- Jupiter and a thin crescent Moon will both appear close the Hyades and Pleiades Clusters on the 13th
- The 25th sees a partial lunar eclipse, causing the top half of the Moon to look slightly darker than the bottom half. The middle of the eclipse is at 21:00 BST, and the Moon also passes 4.5° below Saturn that night.
- The asteroid 4 Vesta passes close to the open cluster M35 in Gemini on the 30th. It can be found using binoculars, low in the west-north-west in the evening and with a magnitude of +8.4.
John Field from the Carter Observatory in New Zealand speaks about the southern hemisphere night sky during April 2013.
Jupiter is low in the west after sunset, with a crescent Moon close by on the 14th and 15th. The second-magnitude star Alpha Librae is occulted by the Moon on the 26th, as seen from Australia and New Zealand. This begins at 22:35 NZST (New Zealand Standard Time, 12 hours ahead of Universal Time, UT) in Auckland, 22:41 in Wellington and 22:43 in Christchurch, ending before midnight. In Austrlia, the start time is 19:21 ACST (Australian Central Standard Time, 9.5 hours ahead of UT) for Alice Springs and 20:29 AEST (Australian Eastern Standard Time, 10 hours ahead of UT) for Hobart. The nearly-full Moon will make observation of the star difficult, and so a high-magnification telescope may be required.
Saturn rises in the east in the evening and is visible throughout the night, reaching opposition (furthest from the Sun in the sky) on the 28th. A small telescope reveals the planet's rings and its largest moon, Titan. The tilt of the rings varies as seen from Earth, and the brightness of the planet is currently invreasing as our view of the system opens out. The constellations of Orion and Gemini appear in the north-west after sunset, while Orion's two hunting Dogs, Canis Major and Canis Minor, follow him across the sky. The dwarf galaxies known as the Large and Small Magellanic Clouds appear as small patches of light near the bright star Achernar, and can be seen with the unaided eye under a dark sky. Binoculars or a small telescope show star clusters with the Large Magellanic Cloud, as well as the star-forming region of the Tarantula Nebula, which is illuminated by hot, young stars at its centre. This nebula is bright enough to spot with the naked eye on a moonless night. Near to the Small Magellanic Cloud is the globular cluster 47 Tucanae, also visible by eye and containing over a million stars, mostly very old. A telescope shows the density of stars decreasing from its centre. Omega Centauri is another globular cluster apparent to the naked eye, near to the Pointer Stars. Alpha Centauri, the brighter and lower of the two Pointer Stars, is the nearest star to Earth that can be seen without a visual aid. It is actually a binary star, 4.3 light-years away, with two stars similar in size and brightness to the Sun. A third star, Proxima Centauri, orbits the pair, while the dimmer of the main double stars hosts an Earth-sized planet. Besides Crux, the Southern Cross, is the Coalsack Nebula, a cloud of gas and dust which obscures the stars beyond. Many similar clouds can be seen along the Milky Way, and will eventually collapse under gravity to form new stars. A cluster of young, luminous stars called the Jewel Box is near to the second-brightest star in Crux. It appears as a fuzzy star to the unaided eye, while binoculars reveal individual stars within it. Two other cross-shaped asterisms can be found in the sky. The Diamond Cross sits near to Crux and contains the Theta Carinae Cluster at one of its points. With a central star surrounded by fainter stars, this cluster is sometimes called the Five of Diamonds of the Southern Pleiades. The False Cross is almost overhead, consisting of four similarly bright stars to one side of the Milky Way. A number of clusters and nebulae inhabit the area between the Diamond and False Crosses.
The planet Mercury is in the morning sky, shining at magnitude 0. On the 20th and 21st it is near Uranus, which can be seen in binoculars. Venus and Mars are too close to the Sun to be seen this month, with Venus reappearing as an evening object next month.
Odds and Ends
Astronomers have recently announced the discovery of a new type of supernova: Type IAx, which has also been dubbed "mini-supernova". Less energetic than their "normal-sized" cousins, Type Ia supernovae, these stellar explosions are interesting as they present a slightly different mechanism and are as yet poorly understood.
The Discovery Centre at Jodrell Bank Observatory has just finished installing its latest exhibit. A 5m diameter orrery! This mechanical representation of the orbits and positions of solar system bodies is moved by over 50 gears. The new exhibit is believed to be one of, if not the largest orrery in the world.
|News:||Stuart Harper and Christina Smith|
|Interview:||Dr Matt Auger, Chris Wallis and Christina Smith|
|Night sky:||Ian Morison and John Field|
|Presenters:||Adam Avison, Libby Jones and Indy Leclercq|
|Editors:||Adam Avison, Claire Bretherton, Mark Purver, Christina Smith and Chris Wallis|
|Segment Voice:||Mike Peel|
|Website:||Christina Smith and Stuart Lowe|
|Cover art:||A view of the trailing hemisphere of Europa in approximate true-colour. CREDIT: NASA|
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