The Southern Cross




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“The Southern Cross”

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ERMANUS ASTRONOMY CENTRE NEWSLETTER


JANUARY 2014
As the Centre enters its seventh year, we wish you all the very best for the coming year. As usual, we also hope you find this month’s issue of the newsletter informative and enjoyable.


2014 membership fees For those who have not already renewed their membership, the fees for 2014 are as follows:
Member: R130
Member’s spouse/partner/child, student: R70
Six-month membership from July – December 2014:

Member: R70 Member’s spouse etc, student: R40


Important information regarding making payments In order to avoid high bank charges, payment by cheque or directly into the Centre’s account are no longer options. Those which are still available are either cash payment directly to the Treasurer at meetings, or online payment.
For online transfer, the ABSA bank details are as follows:
Account name – Hermanus Astronomy Centre
Account number – 92 3016 3786
Branch code – 632005
When making an online payment, please include the word ‘membership’ and your name, or it is not possible to attribute the payment to you.



2014 monthly meeting dates These have been finalized and are listed in the relevant section in the newsletter. This month’s meeting will be on Thursday 9 January (details below).
Hermanus Binocular Observation Programme This new interest group will begin its activities this month. Details about this development, and contact details, if you are interested in participating can be found in the newsletter.
WHAT’S UP?

Jupiter The largest planet in our solar system can be seen ‘below’ and to the right of Orion this month. Located presently in the constellation of Gemini, it is the third brightest object in the sky after the Sun and Venus. With a mass of only one-thousandth that of the Sun, the gas giant is, however, 2½ times the total mass of all the other planets in the solar system. Some or all of the four brightest of its many moons, Io, Europa, Ganymede and Callisto (together called the Galileans as they were first identified by Galileo in 1610 using one of the earliest telescopes), are visible with binoculars. Only a small telescope (5 inches) is needed to be able to see some of Jupiter’s bands and the Great red spot. Because it rotates so quickly, a Jovian day is a mere 10 hours long, and it requires only a few minutes observation to see it turning. Composed mainly of hydrogen and helium (3:1 ratio), the absence of a solid surface and its rapid rotation mean that it has an oblate spheroid shape, with a slight, but noticeable, equatorial bulge.
LAST MONTH’S ACTIVITIES

Monthly centre meeting Held on 6 December, this took the form of our annual Christmas party. Thirty-one people attended the event which was held at Anya’s Mum Café. Before the festivities began, a one-minute silent tribute to Nelson Mandela’s passing was followed by playing of a recording of the national anthem.

As usual, John and Irene Saunders had worked hard decorating the tables in festive colours and providing a range of party toys. John summarised the highlights of another successful year before crackers were pulled and streamers cast far and wide. The food was delicious, its enjoyment interspersed with some brain strain; an astronomy general knowledge quiz, another regular event at the party.


Interest groups

Cosmology Seventeen people (15 members, 2 visitors) attended the meeting on 2 December. They watched the fifth pair of programmes in the DVD series ‘Black holes explained presented by Prof Alex Filippenko from the University of California (Berkley). The topics were: Part 9: ‘Shortcuts through the Universe and beyond?’ and Part 10: ‘Stephen Hawking and black hole radiation’.
Astro-photography There was no meeting in December.
Other activities

Sidewalk astronomy Over 30 people attended the session on 27 December. Clear weather enabled them to view several summer constellations and a number of smaller objects. These included the Orion nebula, Pleides open cluster and the Eta Carinae nebula. Observation of a sickle-shaped Venus also demonstrated, to great excitement, hat Venus is more than just a bright, featureless ‘blob’.
Cloudy weather prevented viewing on the 28th, although member Krynauw du Toit persuaded Peter Harvey to meet him and his visitors at Gearing Point for an enjoyable question and answer session.
Educational outreach

Youth club John Saunders reports: ‘The Youth Club meeting on 29 November was an overview of the Cape Town trip on the 9 November, with a PowerPoint presentation of photos I took and members’ own drawings of the things that they saw that day.
As the Planetarium show was of constellations in the night sky, each club member was given a blue A3 sheet of card to create their own constellation with silver paper for stars to be cut out and stick onto the card. They were shown symbols of galaxies, nebulae, globular and open cluster etc. to enhance their constellation. One member did a constellation of a ‘bakkie’ others did horses and lions. Two added pulsars and quasars from the symbols I showed them, not knowing what they really were.”
Jenny Morris adds: “Committee members who saw the reports which the learners had written about their day out were struck by how much they had enjoyed the outing, the first visit to Cape Town for almost all of them, and also by how much of what they experienced impacted on them. Apart from the loudness of the noon gun and awe at the size of the huge model cell phone at the Science Centre, mention was also made of the planetarium show, which included information they already knew, enabling them to answer questions, and the museum and McLean telescope at the Observatory. John definitely earned the praise given by committee members for organising such an interesting experience and learning opportunity for the Youth Club.”
Hermanus Youth Robotic Telescope Interest Group On 17 November, some committee members and the science teacher at Curro School participated in a very useful practical introduction to using the Las Cumbres telescopes. Although cloud cover at the Hawaii site limited the extent of imaging, they were able to learn about and use the telescope, and become familiar with the interface. This experience will be utilized when sessions with interested learners begin after the start of the new academic year.
For further information on both the MONET and Las Cumbres projects, please contact Deon Krige at deonk@telkomsa.net
THIS MONTH’S ACTIVITIES

Monthly centre meeting The first meeting of 2014 will take place on 9 January. It will take the form of a DVD presentation of Part 2 of the three-part BBC series ‘Orbit: Earth’s extraordinary journey’, presented by Kate Humble and Dr Helen Czersky. Part 1 of the series was presented at a 2013 monthly meeting and was very well received.


After the presentation, weather permitting, there will be an opportunity for stargazing from the SANSA car park. An entrance fee of R20 will be charged per person for non-members and R10 for children and students.
Interest group meetings

The Cosmology group meets meeting on the first Monday of the month at 7 pm at SANSA. This month’s meeting will take place on 6 January. Presentation of the 12 part DVD series ‘Black holes explained’ presented by Prof Alex Filippenko from the University of California, Berkley will end this month. The final two topics are: Part 11: ‘Black holes and the holographic Universe’ and Part 12: ‘Black holes and the Large Hadron Collider’.


An entrance fee of R20 will be charged per person for non-members and R10 for children and students. For further information on these meetings, or any of the group’s activities, please contact Pierre Hugo at pierre@hermanus.co.za
Astro-photography This group meets on the third Monday of each month. This month’s meeting will take place on 20 January.
To find out more about the group’s activities and the venue for particular meetings, please contact Deon Krige at astronomy.hermanus@gmail.com
Hermanus Binocular Observation Programme The 12 month programme begins this month. Its aim is to assist participating members of the HAC to become proficient binocular observers of the night sky.
Program Organisation:

  • Material will be disseminated by means of the internet. Monthly guidance summaries will be prepared for participants and will be sent by e-mail.

  • Participants will be encouraged to use their own binoculars and make their own observations, either at home or a venue of their choice.

  • Quarterly communal discussions will be arranged, probably at SANSA. The aim of these discussions is to share experiences, table problems and provide solutions.

  • Work assignments will be given monthly in order to keep track of the progress of the group.

  • The program controller will be available to answer questions and to discuss problems either by e-mail or telephonically.


Overview of the Programme

  • The principles of binocular observation.

  • Equipment required.

  • How to set up for binocular viewing.

  • How to use star maps and where to get them

  • Planning an observing session.

  • Recording an observing session.

  • Specific observation assignments

  • The Moon

  • The planets

  • How to find stars (star hopping with binoculars).

  • Constellations, asterisms, binary and multiple stars.

  • Deep sky objects, how to observe them and record DSO observations.

If you are interested in participating in the activities of the Interest Group, please e-mail petermh@hermanus.co.za
FUTURE ACTIVITIES

Trips Details of the first trip planned for 2014 will be circulated shortly.

2014 MONTHLY MEETING DATES

Unless affected by public holidays, these meetings will take place on the first Thursday of each month at SANSA, beginning at 7 pm. The dates are listed below.


9 January DVD presentation. Title: BBC series ‘Orbit: Earth’s extraordinary journey: Part 2. Presenters: Kate Humble & Dr Helen Czersky

6 February AGM

6 March Details to follow

3 April Details to follow

8 May Details to follow

5 June Details to follow

3 July Details to follow

7 August Details to follow

4 September Details to follow

2 October Details to follow

6 November Details to follow

5 December Xmas party


ASTRONOMY EDUCATION CENTRE AND OBSERVATORY (AECO)

It is hoped that the Fernkloof Management Plan will be ready for publication within the next month or so. Following publication, there will be a thirty day period for receipt of public comments and objections and a period of time to respond to each individual objection before the plan is tabled for consideration by the Council of Overstrand Municipality.


Meanwhile, in order to enable payment of outstanding costs, the ‘Friends of the Observatory’ campaign continues. We are very grateful to members who have already contributed to this, and hope that the generosity of the Centre’s membership continues. Both single donations and small, regular monthly donations, of any amount, are welcome.
Contributions can take the form of cash (paid at meetings), or online transfer, The ABSA bank details are as follows:

Account name – Hermanus Astronomy Centre

Account number – 92 3016 3786

Branch code – 632005

If you make an online donation, please include the word ‘pledge’, and your name, unless you wish to remain anonymous.
ASTRONOMY NEWS

Comet ISON dies as it rounds the Sun 2 December: Our star apparently destroyed this surprisingly fragile celestial visitor during their close encounter.



Comet ISON comes in from the bottom right and moves out toward the upper right, getting fainter and fainter, in this time-lapse image from the ESA/NASA Solar and Heliospheric Observatory. The image of the sun at the center is from NASA's Solar Dynamics Observatory.

ESA/NASA/SOHO/SDO/GSFC
Comet ISON survived for more than 4.5 billion years in the frigid depths of the solar system, but it fizzled during its brief moment in the Sun on 28 November. Through a combination of ISON’s delicate makeup, the Sun’s intense heat, and — most importantly — our star’s powerful tidal forces, the comet’s nucleus failed to survive its brush within 730,000 miles (1.16 million kilometers) of the Sun’s surface.
As the comet approached perihelion (its least distance from the Sun), it continued to brighten at roughly the rate astronomers had predicted. Images from coronagraphs aboard both the Solar and Heliospheric Observatory (SOHO) and the Solar Terrestrial Relations Observatory (STEREO) showed the comet as a bright point of light trailed by one distinct dust tail and a narrow dust streamer.
However, ISON started to fade even before its closest approach to the Sun. The Solar Dynamics Observatory (SDO), which is equipped with the best cameras for close-up observations of our star and its surroundings, failed to see the comet at perihelion. And once ISON had moved far enough beyond the Sun that it could reappear in SOHO’s coronagraphs, it was nowhere to be found.
As astronomers began to write their post-mortems, however, the unpredictable comet rose from the dead like the legendary Phoenix. Some 24 hours after perihelion, SOHO once again captured images of ISON showing a thin dusty tail and a diffuse central condensation that some interpreted as a small remnant of the comet’s nucleus. However, the revival soon began to peter out and by late on 29 November, the glow had faded to around 6th magnitude. Most scientists think the nucleus has dissipated, and any remaining dust likely will be too faint to see through anything but large telescopes. Even though ISON’s saga seems over, astronomers will spend months poring over their observations of this unique visitor.

By: Richard Talcott


Do black holes come in size medium? 3 December: Black holes can be petite, with masses only about 10 times that of our Sun, or monstrous, boasting the equivalent in mass up to 10 billion Suns. Do black holes also come in size medium?



The magenta spots in this image show two black holes in the spiral galaxy called NGC 1313 (Topsy Turvy Galaxy). Both black holes belong to a class called ultraluminous X-ray sources (ULXs). The magenta X-ray data come from NASA's Nuclear Spectroscopic Telescopic Array and are overlaid on a visible image from the Digitized Sky Survey.

NASA/JPL-Caltech/IRAP
NASA's Nuclear Spectroscopic Telescope Array (NuSTAR) is busy scrutinizing a class of black holes that may fall into the proposed medium-sized category. "Exactly how intermediate-sized black holes would form remains an open issue," said Dominic Walton of the California Institute of Technology in Pasadena. "Some theories suggest they could form in rich dense clusters of stars through repeated mergers, but there are a lot of questions left to be answered."
The largest black holes, referred to as supermassive, dominate the hearts of galaxies. The immense gravity of these black holes drags material toward them, forcing the material to heat up and release powerful X-rays. Small black holes dot the rest of the galactic landscape. They form under the crush of collapsing dying stars bigger than our Sun. Evidence for medium-sized black holes lying somewhere between these two extremes might come from objects called ultraluminous X-ray sources (ULXs). These are pairs of objects in which a black hole ravenously feeds off a normal star. The feeding process is somewhat similar to what happens around supermassive black holes, but isn't as big and messy. In addition, ULXs are located throughout galaxies, not at the cores.
The bright glow of X-rays coming from ULXs is too great to be the product of typical small black holes. This and other evidence indicates the objects may be intermediate in mass, with 100 to 10,000 times the mass of our Sun. Alternatively, an explanation may lie in some kind of exotic phenomenon involving extreme accretion, or ‘feeding’, of a black hole.
NuSTAR is joining with other telescopes to take a closer look at ULXs. It's providing the first look at these objects in focused, high-energy X-rays, helping to get better estimates of their masses and other characteristics. Walton and colleagues report serendipitously finding a ULX that had gone largely unnoticed before. They studied the object, which lies in the Circinus spiral galaxy 13 million light-years away, not only with NuSTAR but also with the European Space Agency's XMM-Newton satellite. Archival data from NASA's Chandra, Swift, and Spitzer space telescopes, as well as Japan's Suzaku satellite, were also used for further studies. "We went to town on this object, looking at a range of epochs and wavelengths," said Walton. The results indicate the black hole in question is about 100 times the mass of the Sun, putting it right at the border between small and medium black holes.

By: Jet Propulsion Laboratory, Pasadena, California



Cassini spacecraft reveals clues about Saturn’s moon Titan 16 December: NASA’s Cassini spacecraft is providing scientists with key clues about Saturn’s moon Titan — in particular, its hydrocarbon lakes and seas.


This colorized mosaic from NASA's Cassini mission shows the most complete view yet of Titan's northern land of lakes and seas. Saturn's moon Titan is the only world in our solar system other than Earth that has stable liquid on its surface. The liquid in Titan's lakes and seas is mostly methane and ethane.



NASA/JPL-Caltech/ASI/USGS
Titan is one of the most Earth-like places in the solar system, and the only place other than our planet that has stable liquid on its surface. Cassini’s recent close flybys are bringing into sharper focus a region in Titan’s northern hemisphere that sparkles with almost all of the moon’s seas and lakes. Scientists working with the spacecraft’s radar instrument have put together the most detailed multi-image mosaic of that region to date. The image includes all the seas and most of the major lakes. Some of the flybys tracked over areas that previously were seen at a different angle, so researchers have been able to create a flyover of the area around Titan’s largest and second-largest seas, known as Kraken Mare and Ligeia Mare, respectively, and some of the nearby lakes. “Learning about surface features like lakes and seas helps us to understand how Titan’s liquids, solids, and gases interact to make it so Earth-like,” said Steve Wall from NASA’s Jet Propulsion Laboratory in Pasadena, California. “While these two worlds aren’t exactly the same, it shows us more and more Earth-like processes as we get new views.”
These new images show that Kraken Mare is more extensive and complex than previously thought. They also show that nearly all of the lakes on Titan fall into an area covering about 600 miles by 1,100 miles (900 kilometers by 1,800 kilometers). Only 3 percent of the liquid at Titan falls outside this area. “Scientists have been wondering why Titan’s lakes are where they are. These images show us that the bedrock and geology must be creating a particularly inviting environment for lakes in this box,” said Randolph Kirk from the U.S. Geological Survey in Flagstaff, Arizona. “We think it may be something like the formation of the prehistoric lake called Lake Lahontan near Lake Tahoe in Nevada and California, where deformation of the crust created fissures that could be filled up with liquid.”
A creative application of a method previously used to analyze data at Mars also revealed that Ligeia Mare is about 560 feet (170 meters) deep. This is the first time scientists have been able to plumb the bottom of a lake or sea on Titan. This was possible partly because the liquid turned out to be very pure, allowing the radar signal to pass through it easily. The liquid surface may be as smooth as the paint on our cars, and it is very clear to radar eyes. The new results indicate the liquid is mostly methane, somewhat similar to a liquid form of natural gas on Earth. “Ligeia Mare turned out to be just the right depth for radar to detect a signal back from the sea floor, which is a signal we didn’t think we’d be able to get,” said Marco Mastrogiuseppe, a Cassini radar team associate at Sapienza University of Rome. “The measurement we made shows Ligeia to be deeper in at least one place than the average depth of Lake Michigan.”
One implication is that Cassini scientists now can estimate the total volume of the liquids on Titan. Based on Mastrogiuseppe’s work, calculations made by Alexander Hayes of Cornell University in Ithaca, New York, show there are about 2,000 cubic miles (9,000 cubic km) of liquid hydrocarbon, about 40 times more than in all the proven oil reservoirs on Earth.

By: Jet Propulsion Laboratory, Pasadena, California, NASA Headquarters, Washington, D.C.


Liftoff for the European Space Agency's billion-star surveyor 19 December: The European Space Agency’s (ESA) Gaia mission blasted off this morning on a Soyuz rocket from Europe’s Spaceport in Kourou, French Guiana, on its exciting mission to study a billion suns. Gaia is destined to create the most accurate map yet of the Milky Way. By making accurate measurements of the positions and motions of 1 percent of the total population of roughly 100 billion stars, it will answer questions about the origin and evolution of our home galaxy.

Gaia is en route toward an orbit around a gravitationally stable virtual point in space called L2, some 930,000 miles (1.5 million km) beyond Earth as seen from the Sun. Tomorrow, engineers will command Gaia to perform the first of two critical thruster firings to ensure it is on the right trajectory toward its L2 home orbit. About 20 days after launch, the second critical burn will take place, inserting it into its operational orbit around L2.

A four-month commissioning phase will start on the way to L2, during which all of the systems and instruments will be turned on, checked, and calibrated. Then Gaia will be ready to begin its five-year science mission. Gaia’s sunshield will block heat and light from the Sun and Earth, providing the stable environment needed by its sophisticated instruments to make an extraordinarily sensitive and precise census of the Milky Way’s stars.

Repeatedly scanning the sky, Gaia will observe each of the billion stars an average of 70 times each over the five years. It will measure the position and key physical properties of each star, including its brightness, temperature, and chemical composition. By taking advantage of the slight change in perspective that occurs as Gaia orbits the Sun during a year, it will measure the stars’ distances and, by watching them patiently over the whole mission, their motions across the sky.

The position, motion, and properties of each star provide clues about its history, and Gaia’s huge census will allow scientists to piece together a ‘family tree’ for our home galaxy. The motions of the stars can be put into ‘rewind’ to learn more about where they came from and how the Milky Way was assembled over billions of years from the merging of smaller galaxies and into ‘fast forward’ to learn more about its ultimate fate.

By comparing its repeated scans of the sky, Gaia also will discover tens of thousands of supernovae, the death cries of stars as they reach the end of their lives and explode. Also, slight periodic wobbles in the positions of some stars should reveal the presence of planets in orbit around them as they tug the stars from side to side. Gaia also will uncover new asteroids in our solar system and refine the orbits of those already known, and it will make precise tests of Einstein’s famous theory of general relativity. After five years, the data archive will exceed 1 Petabyte or 1 million Gigabytes, equivalent to about 200,000 DVD’s worth of data.


By: ESA, Noordwijk, Netherlands

Source of all items: www.astronomy.com/news


DID YOU KNOW?

Some unusual celestial objects Part 4: Cepheid variables

Typical light curve Hubble view of a Cepheid Period-luminosity

of a Cepheid variable variable star relationship


Many stars do not consistently shine with the same magnitude or brightness. Such stars are called variables, their brightness normally changing in cycles which can last from hours to years. Some exhibit irregular non-cyclical variation. Stellar variability can have many causes. These include changes caused by flares arising from the outer layers, wave-like motions on the surface resulting from fluctuations in the flow of energy from their interiors, having a non-spherical shape, having a surface which has some brighter areas than others, and stars in binary systems which are eclipsed by their companion.
Cepheid variables are an important group of very luminous yellow giant or supergiant pulsating variable stars with masses three times that of the Sun. They are named after the prototype Delta Cephei in the far-northern constellation of Cepheus. Their variability is a result of expansion and contraction caused by fluctuations in the energy emitted from their active cores. These stars exhibit radial pulsations, a form of pulsation in which they expand and contract symmetrically over their whole surface. The changes in radius are accompanied by variations in brightness, surface temperature, and spectrum (colour). Their size at maximum is typically 7-15% larger than at their minimum, differences which are in the millions of kilometres in distance. Cepheid variables pulsate with a simple fundamental mode, their pulsation reflecting the simple pattern of period expansion and contraction of the stars outer layers. This contrasts with the more complex vibrations found in other variable stars where the basic frequency is overlaid with others, like the harmonics in musical tones.

Cepheids pulsate in regular periods of a few days, a pattern which astronomers plot on a light curve which shows a sharp rise in brightness towards maximum followed by a slower dimming towards minimum. The first variable star was identified in 1781 by the amateur English astronomer John Goodricke, but it was in 1784 that he identified Delta Cephei, the variable which was later realised to be the first known example of what became the Cepheid variables, variables with a unique set of characteristics. Delta Cephi is not only important for its initial discovery, but because its own distance is amongst the most precisely established, enabling it to function as a particularly important calibrator for distance measurements.


The importance of these stars for astronomy was identified in 1908 by the American astronomer Henrietta Swan Leavitt when she discovered that their period was directly related to their absolute magnitude (brightness) in an almost linear way. The longer the period, the greater the luminosity. The resulting consistent period-luminosity relationship, which she published in 1912, has given Cepheids the status of being important ‘standard candles’ for establishing galactic and extra-galactic distance scales.
Leavitt’s proof enabled others to use Cepheid variables in a number of important ways. In 1915, Harlow Shapely used those he found in globular clusters* to identify the size and shape of the Milky Way, and the position of the Sun within it. In 1924, Edwin Hubble established the distance of Cepheid variables in the Andromeda galaxy, demonstrating that these variables were not members of the Milky Way. This put to rest the Island Universe debate, which was concerned with whether the Milky Way and the Universe were synonymous. Later that decade, Hubble also used Cepheids in the work which eventually identified Hubble’s law and evidence of the expansion of the Universe.
Two distinct types have been identified. More numerous are classical Cepheids aka Type 1 Cepheids, Delta Cephei Cepheids. These are Population I stars (stars which, like the Sun, lie in the disk of the galaxy and follow roughly circular orbits) with absolute magnitudes 0.7 – 2 magnitudes brighter than Type II Cepheids. The pulsation periods for classical Cepheids range from days to months. They are 4-20 times more massive than the Sun, and up to 100,000 times more luminous. Stars of this type are used to determine distances to galaxies within the Local Group (of which the Milky Way is a member) and are also the means by which the Hubble constant (the speed of an objects recession in the expanding universe) can be established. They have also been used to clarify many characteristics of the Milky Way including the Sun’s height above the galactic plane, and the galaxy’s local spiral structure.
Type II Cepheids are older Population II stars (stars found in the halo and central bulge of the galaxy, those in the halo following very elliptical orbits). They are dimmer than classical Cepheids, have smaller masses (half the mass of the Sun) and brightness, and also contain lower concentrations of heavy elements. Their pulsation periods are much shorter than those of the classical Cepheids, typically between 1 and 50 days. This type is divided into three subgroups by period length. The BL Herculis sub-class has pulsation periods of 1-4 days, W Virginis Cepheids 10-12 days, and the RV Tauri sub-class periods of greater than 20 days. Type II Cepheids are used to establish distances to the galactic centre, globular clusters and other galaxies.
Sources: Ridpath, I (Ed) 2007 Oxford dictionary of astronomy 2nd ed, Astronomy (Dorling Kindersley – Eyewitness companions,www.en.wikipedia.org

*Objects written in bold are covered in more detail in other parts of this series.


For more information on the Hermanus Astronomy Centre and its activities, visit our website at www.hermanusastronomy.co.za
COMMITTEE MEMBERS

Pierre de Villiers (Chairperson and AECO) 028 313 0109

Laura Norris (Treasurer) 028 316 4453

Peter Harvey (Secretary, including membership) 028 316 3486

Jenny Morris (Vice-chairperson and newsletter editor) 071 350 5560

Derek Duckitt (Website editor) 082 414 4024

Lynette Geldenhuys (Education co-ordinator) 028 316 2428

Deon Krige (MONET project and astrophotography) 028 314 1045

John Saunders (Youth club & events co-ordinator) 028 314 0543

Non-committee members with roles:



Pierre Hugo (Cosmology interest group co-ordinator) 028 312 1639

Johan Retief (Monthly sky maps) 028 315 1132


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