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Macclesfield Astronomical Society Vol. 15.5 December 2004

The all-new on-line version of the Macclesfield Astronomical Society's bi-monthly newsletter.

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Cover picture

Greetings from our Patron, Sir Bernard Lovell and Vice President, Professor Rod Davies. A rare opportunity arose to photograph our Patron and Vice President together at Jodrell Bank. The occasion was the Lovell Lecture on December 9th. Sir Bernard is a regular attendee of these lectures. Professor Rod Davies presented an excellent talk, in which he reminisced about his early experiences at Jodrell when he first began working with Sir Bernard. His talk showed how the work of the Jodrell team has progressed from the study of meteors in the 1940s through to present day work on pulsars, quasars and the background radiation from the Big Bang. If you want details of future lectures, contact Megan Argo who will obtain tickets for you.


The Moon

This article arose as the result of comments from several people at the September 2004 Workshop that they don’t observe the Moon very often and don’t know much about it, despite the fact that it is our nearest neighbour in space -light takes just over a second to get there. It has fascinated the curious since the earliest times in human history. So why do we not observe it more often? What exactly do we know about the Moon?

Everyone is familiar with the “onion skin” model of the interior of the Earth: a thin rocky crust, a thick mantle, and a nickel-iron core comprising of a solid outer and liquid inner core. So one question we might ask is whether the Moon has a similar structure, or whether it is a fairly homogeneous sphere. Just as drilling to the centre of the Earth was not necessary to determine its structure, we do not need to do it on the Moon either.

But before diving into geology, let’s see what we can determine from a small telescope.

When looking at the Moon, the most obvious feature you notice is the contrast between the light and dark areas. The dark regions (Mare Imbrium, Mare Serenitatis, etc.) have long been known not to be “seas” as thought by the astronomers who first named them. Looking closely it is possible to detect a few craters within the mare and, with higher magnification, large mountain ranges surrounding some of them. In contrast, the lighter regions tend to be much more heavily cratered and are known as the lunar highlands.

We know the dark areas are not really seas or oceans, but what we now know to be craters were not always thought to be so either. Before the age of space exploration there was quite a debate as to their nature. One group claimed them to be impact features caused by lumps of rock flying around in the early solar system, while the other (including Sir Patrick himself) suggested that they were more likely to be volcanic features, the calderas remaining after eruptions when the Moon’s surface was forming. We now know them to be craters of course, but in the days before space travel and high resolution imaging this was not obvious. The study of our nearest neighbour has come a long way in the past century.

What can we tell about craters from simple observations? By examining them for signs of erosion we can estimate relative ages: the more eroded and the less distinct a crater is, the older it probably is. Younger craters often have a central peak where the meteorite (known as the “impactor”) hit. Over time, this central peak disappears. On the Earth this is largely due to erosion and weathering, but this cannot be the case on the Moon as there is no atmosphere.

We can also tell something about the size and energy of the impactor from measuring the size of the crater and the extent of the ejector blanket (the stuff that got blasted from the surface during the impact and eventually settled back to the ground). It is the re-settling of ejecta that is largely responsible for making older craters less distinct.

Moon1 Moon2
Close up of Mare Imbrium to illustrate the principle of superposition

Cratering counts

Guessing relative ages from the amount of erosion works to some extent, but only for individual craters. In order to date an area we need another method. In the early solar system as the planets were forming there was a lot of debris around. As these small rocks orbited the young Sun they would eventually hit larger bodies and produce a crater. Over time the number of these rocks decreased as they crashed into proto-planets and hence the cratering rate -the number of craters produced in a given time period -reduced. If the surface of the larger body was also changing with time through its own geological processes such as volcanism or plate tectonics, then older parts of the surface would be the most heavily cratered while those which had been formed later (due to lava flows from volcanic eruptions for example) would have fewer craters. From this we can already say, just by simple observation, that the lunar highlands are older than the mare because they are more heavily cratered.

The principle of superposition

Age estimates of individual craters (and other features) can be made more systematic through the use of the principle of superposition. As well as surfaces being renewed by geological processes, newer impacts can partially or wholly obscure older ones. Recent fault lines can split craters down the middle, and ejecta from nearby impacts can make them less distinct. This principle is a very useful one in geology and is just as applicable on the Earth and other planets as it is on the Moon.

A good example of the principle of superposition can be seen in the Mare Imbrium area of the Moon. The first event that occurred was the major impact which formed the Imbrium basin itself (image 1). Further impacts then occurred as more lumps of rock hit the surface creating craters such as Archimedes within the basin (image 2). Then a major lava flow filled the Imbrium basin and flooded or “embayed” other features within the basin (image 3). The last stage in this process is further impacts on the cooled lava basin (image 4).

This is a simple and very useful technique which can be applied to the rest of the Moon and any of the rocky planets or moons in the solar system. It does, however, only give relative ages of features. To date features specifically we need to use other forms of measurement.

Dating the events

This was one of the scientific outcomes of the Apollo missions. Over the six missions, several different areas of the surface were visited and a total of 382 kg of rock was returned to Earth for analysis. As is well known, Apollo 11 landed in Mare Tranquillitatis (or the Sea of Tranquillity), but the other missions went to other parts of the surface. Apollo 12 landed in Oceanus Procellarum, and Apollos 16 and 17 landed in different areas of the lunar highlands.

Two missions landed in areas important to the example of the Imbrium basin above: Apollo 14 landed in the Fra Mauro region and was able to sample ejecta from the Imbrium impact, while Apollo 15 landed in the Apennines, the mountain range stretching around part of the Imbrium basin, and was able to sample the Imbrium infill -the lava that filled the basin after the Archimedes impact occurred. By measuring the relative abundances of specific compounds within the samples it was possible to date the areas where they were taken, giving us a timescale to put on our sequence of events. The Apollo 14 samples showed that the Imbrium basin was formed by an impact which occurred about 3.85 billion years ago, and the samples taken by the crew of Apollo 15 showed that the subsequent lava flow which flooded the basin took place about 3.5 billion years ago. From this we can also say that the lunar highlands formed before this, so they must be older than 3.85 billion years, and that craters within the Imbrium basin which have not been embayed must be younger than 3.5 billion years.

Going further back

We now have a sequence of events and some dates for events on the surface of the Moon, but the observations so far tell us very little about the formation of the Moon in the first place. The standard theory for the formation of the Moon is that a Mars-size impactor collided with the young Earth and the mantle of both Earth and the impactor were blasted into orbit. The Moon formed as the orbiting material began to condense. This explains why the Moon is there at all, but not the surface features we see today.

A simple model that explains the major observations of the surface structure is known to geologists as the magma ocean theory. In this model, the young Moon gathers material through impacts, a process known as accretion. The energy from these impacts melts the top few hundred kilometres of the lunar surface creating a “magma ocean”. As more material is swept up in the formation of the planets, the frequency of the impacts begins to decrease and the magma ocean begins to cool. As it cools, different minerals crystallise and begin to sink or float, depending on their relative density. Heavy minerals such as olivine and pyroxene sink towards the solid lunar interior while lighter minerals such as anorthosite rise to the surface forming giant “rock bergs”.

As the cooling progresses, a crust of light anorthosite forms at the surface, as well as an olivine/pyroxene-rich lower layer. Sandwiched between these two layers is a “residual melt”, a layer rich in potassium, rare earth elements and phosphorous, which generates heat by radioactive decay on timescales of 0.1 to 1 billion years.

As the formation of the solar system progresses, large impacts, such as those that caused Imbrium, occur. These events fracture the outer anorthosite crust. The residual melt, not quite solid due to the heat from the radioactive decay, can seep through the fractures and fill in the basins. This is the process that is thought to have filled in the Imbrium basin: the basaltic magma from the residual melt migrated upwards through the fractures in the surface caused by the impact, filling in the impact basin to form the mare we see today. As the magma is made up of different minerals than the largely anorthosite crust, the flooded mare appear darker than the surrounding lunar highlands.

Problem solved?

So we have an age scale for surface features, and a workable theory for the formation of the outer layers of the Moon. Is that all the questions of lunar geology solved? Luckily not! There are variations in the chemical make-up of different bits of the lunar surface: there are iron anomalies, concentrations of mass in particular places and numerous other features not touched on in this article. There is plenty more to explore on the Moon, and even more when you start to compare lunar features to those we see on Mars, Venus and the other rocky bodies in the Solar System.

Further reading

If you are interested in reading more about this subject, a very good book on comparative planetology throughout the solar system is “The New Solar System” by J. Kelly Beatty, Carolyn Collins Petersen and Andrew Chaikin, published by Sky Publishing and Cambridge University Press. As well as the basic facts and figures for each body in the Solar System, the book summarises what we have learned from planetary explorations of the last few decades and contains some introductory planetary science.

Megan Argo


DARK MATTER ‘SUPER-STRUCTURE’?
Chandra X-Ray Centre

A Chandra survey of the Fornax cluster of galaxies has revealed a swept-back cloud of hot gas near the centre of the cluster. The geometry indicates that the hot cloud, which is several hundred thousand light-years in length, is moving rapidly through a larger, less dense cloud of gas. The motion of the core gas cloud, together with optical observations of a group of galaxies racing inward on a collision course with it, suggests that an unseen, large structure is collapsing and drawing everything toward a common centre of gravity. The infalling galaxy group is about 3 million light-years from the cluster core, so a collision with the core will not occur for a few billion years. The elliptical galaxy NGC1404 that is plunging into the core of the cluster for the first time provides insight as to how the collision might look. Over the course of hundreds of millions of years, NGC1404’s orbit will take it through the cluster core several times, most of the gas it contains will be stripped away, and the formation of new stars in it will cease. In contrast, galaxies that remain outside the core will retain their gas, and new stars will be able to continue to form in them.

NEWS OF A NEUTRON STAR
University of Arizona

Scientists have obtained a new measurement of the size and contents of a neutron star. The star concerned is a member of a binary-star system named EXO 0748-676, about 30,000 light-years away in the southern constellation Volans. The observers used Rossi X-ray Timing Explorer data, in which they detected a 45-hertz oscillation frequency, interpreted as implying that the neutron star rotates 45 times per second --quite a leisurely pace for neutron stars, the most rapid of which spins more than 600 times per second. They next capitalised on EXO 0748-676 observations with ESA’s XMM-Newton satellite, which detected spectral lines emitted by hot gas. The lines have two significant features. First, they are Doppler-broadened: the equatorial velocity of about 3000 km/s causes light emitted from the approaching limb to be blue-shifted and that from the receding limb to be red-shifted by significant amounts, broadening the spectral lines. Secondly, the lines as a whole exhibit what is called a gravitational red-shift, because the light in effect loses energy as it climbs out of the enormously deep gravitational potential well of the neutron star.

The gravitational-redshift measurement offered the first estimate of a mass--radius ratio, because the depth of the potential well depends on the mass and radius of the neutron star. The 45-hertz frequency and the observed line-widths from Doppler broadening are consistent with a neutron-star radius of about 11 km. The radius and the mass—radius ratio lead to a neutron-star mass between 1.5 and 2.3 solar masses, with the best estimate being around 1.8 solar masses. The result supports the theory that matter in the neutron star in EXO 0748-676 is packed so tightly that almost all protons and electrons are squeezed together to become neutrons, which swirl about as a superfluid, a liquid that flows without friction.

SIZE OF PULSATING RED GIANTS OVERESTIMATED
New Scientist

The sizes of Mira stars, a class of very large and very cool stars that brighten and dim by a hundred times or more in periods of 100 to 1000 days, have been overestimated by a factor of two, according to new observations. Mira stars begin life about the same size as the Sun, but during the phase of rapid evolution at the ends of their lives after 5--10 billion years they become pulsating red giants with diameters hundreds of times greater. Defining their sizes is difficult, as they appear to be different sizes depending on which wavelength of light is used to observe them, looking larger in visible light, for example, and smaller at near-infrared wavelengths. Of course, it is quite difficult to measure the size of a star at all, because none except the Sun is large enough to be seen as anything more than a point in any telescope, and some sort of interferometer has to be used.

Now, an international team of astronomers has studied six nearby Mira stars with an array of linked infrared telescopes on Mount Hopkins in Arizona. They say that Mira stars are half the size that they were thought to be, because shells of material high in their outer atmospheres make them look deceptively large. Titanium oxide molecules, an abundant constituent in the atmospheres of cool stars, makes the shells opaque to visible light but transparent to the infrared observed by the team, who think that previous measurements of the stars’ sizes were based on the shells’ diameters, rather than the stellar surfaces. That will please theoreticians, who when trying to model why these stars pulsate always come up with much smaller sizes than observers see.

GAMMA-RAY OBJECT AT GALACTIC CENTRE
PPARC

An object at the centre of our own Milky Way galaxy, radiating high-energy gamma rays, has been detected by a team of UK astronomers using the ‘High-Energy Stereoscopic System’, an array of four telescopes in Namibia. The Galactic Centre harbours a number of potential gamma-ray sources, each of which could be expected to emit the radiation slightly differently. The radiation that has been observed comes from a region very near Sagittarius A*, the supposed black hole at the centre of the Galaxy. The observed energy spectrum best fits theories of the source being a giant supernova explosion. It is already thought that a giant supernova exploded in that region about 10,000 years ago. Such an explosion could accelerate cosmic gamma rays to the high energies observed, but further observations will be needed to determine the exact source.

MERGER OF GALAXY CLUSTERS
New Scientist

Merger

One of the hottest and most energetic mergers of two galaxy clusters has been imaged in X-rays by the XMM-Newton spacecraft. The collision, in the cluster called Abell 754, which is relatively close to Earth at 800 million light-years away, is one of the most powerful cosmic events identified so far. The observation illustrates the manner in which structures in the Universe are thought to grow, and bolsters recent suggestions that the largest structures in the Universe are still evolving. About 30 clusters have been observed so far in which radio and optical data suggest that a merger has occurred. Observing X-rays is crucial because most of a cluster’s mass lies not inside the galaxies but between them, in hot gas that radiates at X-ray wavelengths. Abell 754 is 3 million light-years across and, if it were visible to the eye, it would appear half the size of the Moon. It appears to be made up of two component clusters, one crammed with 1000 galaxies and the other with 300. Shock waves caused by the merger, which may have begun 300 million years ago, has heated gas in the clusters to extreme temperatures --about 100 million °C. Astronomers took X-ray spectra of the merged cluster and measured its temperature, pressure and density. There are indications that one of the clusters had already made one pass through the second. Now, gravity will pull the remnants of the first cluster back towards the core of the second. Over the next few billion years, the remnants of the clusters will settle and the merger will be complete.

JUPITER MAY HAVE DRIFTED TOWARDS THE SUN IN ITS YOUTH
New Scientist

Jupiter drifted tens of millions of kilometres towards the Sun in its youth, a new study suggests. The idea that planets migrate towards their stars has received considerable attention over the past decade, thanks to the discovery of planetary systems beyond our own. Most contain ‘hot Jupiters’ --gas giants that in most cases orbit closer to their stars than Mercury does to the Sun. Astronomers have thought that the giants formed in the cold outskirts of their systems but moved inwards as they lost angular momentum owing to drag within the dusty disc that surrounds a young star. Clearly, no dramatic migration happened in our system, where all the giant planets like Jupiter are relatively far away from the Sun. But now scientists say that they see evidence that Jupiter did migrate inwards, albeit to a lesser degree than the hot Jupiters.

The evidence comes from a curious group of 700 or so ‘Hilda’ asteroids, which orbit the Sun three times for every two Jovian years. The vast majority of them have slightly elongated elliptical orbits, whereas many other asteroids have near-circular orbits. According to people at the Centre for Astrophysics in Cambridge, Massachusetts, a migrating Jupiter could explain why so few Hilda asteroids have circular orbits. Their simulations show that if the young Jupiter had orbited about 10% farther from the Sun than it does now, and then spiralled in by about 70 million kilometres over a period of 100,000 years or more, its gravity would have ejected any proto-Hilda asteroids with circular orbits from the Solar System, and it would have further elongated the orbits of those that remained. Luckily for us, Jupiter did not spiral in very far, as analogous planets around other stars apparently did, possibly because the dusty disc around the young Sun was relatively thin.

OBSERVATIONS OF THE REMNANTS OF KEPLER’S SUPERNOVA OF 1604
NASA

Four hundred years ago, sky watchers were startled by the sudden appearance of a ‘new star’ in the western sky, rivalling the brilliance of the nearby planets. When the new star appeared on 1604 Oct. 9th, observers such as Johannes Kepler could use only their eyes to study it, as the telescope had not then been invented, but modern astronomers can analyse the remains in visible light, infrared radiation, and X-rays. The combined image indicates a bubble-shaped shroud of gas and dust, 14 light-years across and expanding at 2000 km/s.

Kepler

Chandra (X-ray) Hubble (optical) Spitzer (IR) Visible-light images from Hubble reveal where the supernova shock wave is slamming into the densest regions of surrounding gas. Astronomers used the Hubble observations and those taken with ground-based telescopes to obtain the distance to the supernova remnant, about 13,000 light-years. They used Spitzer to observe in infrared light, which shows heated microscopic dust particles that have been swept up by the supernova shock wave. Instruments on Spitzer also provide information about the chemical composition and physical environment of the expanding clouds of gas and dust. The dust is similar to that which was in the cloud of dust and gas that formed the Sun and planets in our Solar System. Chandra X-ray data show regions of very hot gas; the hottest gas, giving the highest-energy X-rays, is located primarily in the regions directly behind the shock front. Astronomers still do not know, however, what type of star exploded. There have been six known supernovae in our Milky Way over the past thousand years; Kepler’s is the only one the nature of whose progenitor remains uncertain.

FREQUENT STARBURSTS STERILISE GALACTIC CENTRE
Harvard-Smithsonian Centre for Astrophysics

According to astronomers from the Centre for Astrophysics, life near the centre of our galaxy never had a chance. Every 20 million years on average, gas pours into the Galactic Centre, creating millions of new stars. The more massive stars soon explode violently as supernovae, blasting the surrounding space with enough energy to sterilise it completely. The gas for each starburst comes from a ring of material located about 500 light-years from the centre of the Galaxy. Gas collects there under the influence of the Galactic bar --an oval of stars 6,000 light-years long rotating in the middle of the Milky Way. Tidal forces and interactions with the bar cause the ring of gas to build up to higher and higher densities until it reaches a critical density, whereupon the gas collapses down into the Galactic Centre and fuels a burst of star formation.

Astronomers see starbursts in many galaxies, most often colliding galaxies where lots of gas crash together. But starbursts can happen in isolated galaxies too, including our own galaxy, the Milky Way. The CfA team thinks that the next starburst in the Milky Way will happen within the next 10 million years. Some 30 million solar masses of matter will then flood inward, overwhelming the 3-million-solar-mass black hole that the team believes to be at the Galactic Centre. The black hole will be unable to consume most of the gas, and with so many stars packed so close together as a result of the starburst; the entire Galactic-Centre region will be impacted dramatically enough to kill any life on an Earth-like planet. Fortunately we are at a safe distance!

NEW TYPE OF STELLAR OBJECT FOUND
NOAO

Astronomers using the Gemini North and Keck II telescopes have found that one of the interacting stars in the binary system EF Eridani has lost so much mass to its partner that it has regressed to a strange, inert body resembling no known star type. Unable to sustain nuclear fusion at its core and doomed to orbit its much more energetic white-dwarf partner for millions of years, the dead star is a new, indeterminate type of stellar object. The system is located 300 light-years from Earth and consists of a faint white-dwarf star with about 60 per cent of the mass of the Sun and a donor object of unknown type, which has an estimated mass of only 1/20th of a solar mass. EF Eri is one of the binary systems known as ‘magnetic cataclysmic variables’, which may produce many more ‘dead’ objects than scientists have realised. The white dwarf in EF Eri is a compressed, burnt-out remnant of a solar-type star that is now about the same diameter as the Earth, though it still emits copious amounts of visible light. Its magnetic field is about 14 million times as powerful as the Sun’s.

NEW GLOBULAR CLUSTER DISCOVERED
NASA

New infrared images from the Spitzer Space Telescope and the University of Wyoming Infrared Observatory have revealed a globular star cluster that is hidden as far as visible light is concerned by the obscuring dust in the plane of the Milky Way. The new-found object, in the constellation Aquila, is one of about 150 globular clusters known to orbit the centre of the Milky Way. The tightly packed knots of stars are among the oldest objects in our Galaxy, having formed about 10 to 13 billion years ago. Each contains typically several hundred thousand stars, most of which are older and less massive than our Sun. The cluster was found in the course of a survey with Spitzer to find objects hidden within the dusty mid-plane of our Galaxy. Astronomers then searched archival data for a match and found only one undocumented image of the cluster from a previous infrared survey of the sky, called the Two-Micron All-Sky Survey. Follow-up observations at Wyoming helped to determine the distance of the new cluster at about 9,000 light-years -closer than most clusters --and set the mass at the equivalent of 300,000 Suns.

NEW PLANET-BUILDING THEORY
NASA

Planets are built over a long period by collisions between rocky bodies as big as mountain ranges, astronomers have proposed. New observations from the Spitzer infrared telescope reveal surprisingly large dust clouds around several stars. The clouds most likely flared up when rocky, embryonic planets smashed together. The Earth’s own Moon may have formed from such a catastrophe. Previously, astronomers thought planets were formed under less chaotic circumstances.

When embryonic planets, the rocky cores of planets like Earth and Mars, crash together, they are believed either to merge into a bigger planet or splinter into pieces. The dust generated by collisions is warmed by the host star and glows in the infrared, where Spitzer can see it. According to the most popular theory, rocky planets start out around young stars as tiny balls in a disc-shaped field of thick dust. Then, through sticky interactions with other dust grains, they gradually accumulate more mass. Eventually, mountain-sized bodies take shape, which further collide to make planets. Previously, astronomers envisaged this process as proceeding smoothly toward a mature planetary system over a few million to a few tens of millions of years. Dusty planet-forming discs, they predicted, should steadily fade away with age, with occasional flare-ups from collisions between leftover rocky bodies.

Astronomers looked for dusty discs around 266 nearby stars of similar size, about two to three times the mass of the Sun, and various ages. Seventy-one of those stars were found to possess discs, presumably containing planets at different stages of development. But instead of seeing the discs disappear in older stars, the astronomers observed the opposite in some cases. Such variability implies that planet-forming discs can become choked with dust throughout the discs’ lifetime, up to hundreds of millions of years after the host star was formed. The only obvious way to produce as much dust as is being seen around some of the older stars is by collisions.

SURVEY FINDS NEW MILKY WAY COMPANION
Sloan Digital Sky Survey

The Sloan Digital Sky Survey (SDSS) has found in the halo of the Milky Way a very diffuse clump of stars unlike any seen before. The object, called Willman 1, is so faint that it was found only as a slight increase in the number of faint stars in a small region of the sky. It was discovered in a search for extremely dim companion galaxies to the Milky Way, and is 200 times less luminous than any galaxy previously seen. Another possibility is that Willman 1 is an unusual type of globular cluster. It is, however, dimmer than all but three known globular clusters, all of which are much more compact than Willman 1. If it is a globular cluster, it is probably being torn to shreds by the gravitational tide of the Milky Way.

The real distinction between the globular cluster and dwarf-galaxy interpretations is that galaxies are usually accompanied by substantial quantities of dark matter, and astronomers say the next step is to make additional observations to try to determine whether there is any dark matter associated with the object. If it does turn out to be a dwarf galaxy, the discovery could shed light on the ‘cold dark matter’ hypothesis that suggests that our own Milky Way galaxy should be surrounded by hundreds of dark-matter clumps, each a few hundred light-years in size and possibly inhabited by a dwarf galaxy. However, only 11 dwarf galaxies have been discovered orbiting the Milky Way. Perhaps some of the clumps have very few embedded stars, making the galaxies particularly difficult to find. If the new object is in fact a dwarf galaxy, it might be the tip of an iceberg of a yet unseen population of ultra-faint dwarf galaxies.

PROMETHEUS STEALS FROM SATURN’S RINGS
New Scientist

In Greek mythology, Prometheus stole fire from the gods. Now, Saturn’s tiny moon Prometheus is found to show similar tendencies, stealing material from planet’s rings, according to a new image taken by the Cassini probe. The image was taken on 2004 October 29 from a distance of 791,000 kilometres. It shows a sliver of light about 300 km inside Saturn’s F ring, which lies beyond its main ring system and contains at least three bright strands of ice and dust. That sliver is the partially illuminated, potato-shaped moon Prometheus, which is about 150 km in length.

Prometheus and another moon --Pandora, which orbits just outside the ring --have been called ‘shepherd’ moons because they appear to keep the ring in place. But the image confirms that the moon sometimes also strips material from its neighbouring ring, as a stream of material appears to be drawn from the innermost bright strand toward the moon. Such a feature --called a streamer --was first seen around Saturn by the Cassini spacecraft earlier in 2004. It is thought to occur when Prometheus, which travels in an elliptical orbit around the planet every 14 hours or so, reaches its closest point to the F ring. It is currently unclear whether the wobbles in the bright central strand near Prometheus are associated with the moon.

A dark horizontal band in the image is thought to be a hole left behind from a previous pass in which Prometheus siphoned off ring material. Dark lanes called striations were also first seen by Cassini earlier in 2004, but streamers and dark lanes have not previously been seen together in the same image.

Bulletin compiled by Clive Down
© 2004 the Society for Popular Astronomy


LIGHT POLLUTION SOLUTIONS ?

At our November Workshop at Jodrell Bank I showed the recently published Phillips’ Dark Sky Map of the British Isles. There was much interest shown and a lot of the members gathered around the map after the presentation. Many, no doubt realising their worst suspicions of how badly light polluted their usual observing area was. Some were deciding how they must travel to find a reasonable dark sky and there was also a lengthy discussion on how to deal with the problems of light pollution.

It occurred to me that there are three possible solutions for us astronomers, to “light blight”. 1 We could travel to a known dark sky site to do our observing. If you are a hunter of deep sky objects, this is about the only immediate option open to you. The problem here is that a clear night is only going to coincide rarely with an opportunity to travel, perhaps a considerable distance. And can you guarantee the clear sky will last long enough? 2 To campaign at a local level for better, more sensible street lighting. I have done this to a limited extent after the introduction of new “more efficient” street lights where I live. The new lights caused more glare to my usual observing site in the back garden than the replaced lights. I contacted my local council about the problem, they then forwarded my complaint to the contractors who installed and now maintain the lights. The contractors subsequently installed a louvered baffle to the offending lamp which reduced the glare, although not enough to my satisfaction. I intend to get the offending light modified still further. 3 We could realise our limitations and observe what is possible at present. When we make those rare trips to dark sky sites then we can observe the “dim fuzzies”. At home with just limited time available don’t forget our own Solar System. After all the Sun, Moon and planets are all observable from the worst light polluted site in Britain -London. There is a whole lifetime of observing with these few objects. There must be many amateur astronomers confined to the metropolis who cannot travel but still observe. Some of the most notable planetary observers are confined to the conurbations.

I suggest the best solution is a mixture of all three. One thing is for sure, outdoor lighting is not going to go away, even it improves and it will not improve without our intervention and pressure. Whatever happens in the future with outdoor lighting, we must never give up. KEEP THE PRESSURE ON -AND KEEP OBSERVING.

Chris Hall


Editor’s note

Having heard some fascinating discussions at the workshops, I am sure that many of our members have interesting tales to tell. Why not tell them in the Society Journal? Perhaps you bought an interesting piece of equipment or tried an unusual observation. Maybe you visited an interesting observatory or Astronomy Centre. There are many possibilities for articles. Anything interesting that you have seen, or done recently, relating to astronomy will be welcome. If you think you might have any contributions, please let me or Alan Banks have them, preferably by e-mail, although hand written contributions are perfectly acceptable.

Dave Ogden


Sky map

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