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TRANSIT the Newsletter Of TRANSIT The Newsletter of 05 March 2008 Lunar Eclipse, 21 February 2008. An image of the eclipse captured by John Gianforte at the University of New Hampshire observatory. Sadly the UK was covered in cloud that night. Editorial Last meeting : 08 February 2008. Rod Hine on “Orbits, Spin and Chaos”. A great trip back to Astronomy 101. All we should know about the physics basics of our hobby. Rod was very courageous. Presenters are advised never to share the stage with children, animals or weapons of mass destruction. Next meeting : 14 March 2008 : “We Are Not Alone” by Neil Haggath Another Moonwalker visit : Ken Willoughby & Carleton Community High School present an evening with Colonel Edwin “Buzz” Aldrin (Apollo 11 Lunar module Pilot and 2nd man on the Moon). Sunday 30th March, 2008. Carleton Community High School, Green Lane, Pontefract WF8 3NW Admission £20.00 SAE to Ken Willoughby, 11 Hardistry Drive, Pontefract, West Yorkshire, WF8 4BU, Tel no. 01977 795535 e-mail : [email protected] Letters to the Editor Dear Editor, Comments on February’s “Transit” We had a colourful picture of Olympus Mons on the cover. If it was situated near one of the Martian poles it would then resemble this picture, which is Mt. Kilimanjaro photographed from the International Space Station? (from Astronomy and Space July 2007). Another article also tells us that every second the Sun loses mass which approximates that of sixteen Empire State buildings. Also in two hundred years Mercury will be “moving slower and it will lag behind it’s current orbital position by 5.5 Km”. 2 Or does this mean it will have moved slightly away from the Sun and that its orbit will be 5.5Km longer? So in two hundred years the Sun will have lost 6 billion kilograms x 60 x 24 x 365 x 200. An astronomical number indeed with many zeros. Lastly has anyone any ideas as to why those who receive a printed “Transit” responded to the Editor’s questionnaire made up just under half of the total, whilst of those who receive the e-mail version only under a sixth responded? John Crowther Editor : It is a well researched fact that computer screen readers often operate in an entirely superficial world – they scan screen content and probably discard (or forget) most of what they read to prevent information overload. On the other hand magazine and book readers, having made the effort to obtain a paper copy, pay the material a much greater compliment by actually reading it and committing it to their little grey cells. (also, you can’t take a computer to bed with you for that “best of the day” read before sleep kicks in). Dear Editor, Could I remind the Society that the Planetarium is to host a Public Solar Observing session on Sunday March 09 2008 as part of the National Science and Engineering Week, NSEW, (formerly National Science Week). Website is www.the-ba/nsew This is an opportunity for people of all ages, areas and organisations to take part in science, engineering and technology activities. The session starts at 1.00pm. CaDAS volunteers will be supervising the use of the Planetarium telescopes fitted with stringently safe Sun filters, including visual, hydrogen alpha and calcium wavelength filters. Could the CaDAS volunteers please appear at the Planetarium at least one hour before the start Ed Restall 3 The Largest Stars From Rob Peeling Much in astronomy is concerned with extremes. Consider our Sun where superlatives are generally used to describe its brightness, size, mass, temperature and lifetime. The Sun’s vital statistics almost paralyse our imagination. Yet, astronomy tells us that the Sun compared to other stars, is very far from exceptional in almost all respects. The single exception is, that to the best of current knowledge, the Sun is the only star in existence that has a life- bearing planet orbiting it. The Sun is 1.4 million kilometres in diameter. If that is a “small” star then just how big can stars get and which ones are they? To answer that we need to ask two more questions; what sorts of stars are likely to be very big, and how do astronomers measure them? The Sun is powered by nuclear fusion reactions in its core, consuming hydrogen and converting it to helium. Detailed mathematical models can be devised to describe the behaviour of the Sun. These can be used to predict what will happen when the hydrogen in the core starts to run out. The model shows that the core will contract under its own gravity, releasing energy to heat the surrounding gas that still contains hydrogen to a high enough temperature for nuclear reactions to start. As the nuclear energy is now being released closer to the surface, the outer layers of the star start to puff up, becoming less dense but as they expand they also cool down because the same available energy is now being spread over a larger surface area. The star’s diameter will increase by at least a factor of ten, so in the case of the Sun it is now 14 million kilometres and stretches 10% of the way to Mercury. The star is now a red giant. The model provides a clue to help search for the largest stars. We are looking for stars that have consumed the hydrogen in their cores and are therefore beginning to die. An observer on Earth can identify them because they are redder than most other stars. The next clue is more like common sense than clever physics. If you take a big, hot glowing globe i.e. a star and place it a long way away (i.e. a star) and compare it to a similar but much smaller globe at the same distance, then it is pretty clear that the bigger one is going to appear brighter than the little one. We now have all the clues we need. To find the biggest stars we should investigate the ones that are red and bright. Two candidates immediately come to mind; Betelgeuse and Antares. Both of these are very bright stars and most 4 people looking at them with the naked eye agree that they are at least a little orange coloured. There are a number of other fainter naked eye stars, which seem white to the naked eye, are noticeably coloured when using binoculars. These are a Herculis (Rasalgethi), µ Cephei (Herschel’s Garnet Star) and VV Cephei. Being bright and coloured doesn’t mean a star is necessarily large. It could simply be close to Earth. Astronomers need to find out the distance to the star. The distance to a star is most accurately measured by determining its geometric parallax. This is the small angle a relatively nearby star appears to move against much more distant background stars when observed at an interval of 6 months during which the Earth moves half way around its orbit. As we know the diameter of the Earth’s orbit around the Sun, this makes it possible to directly calculate the distance to a star. The first astronomer to be able to measure very small angles sufficiently accurately ot be able to put this method into practice was Friedrich Bessel in 1838 when he determined the distance to 16 Cygni. The brightness of the stars listed implies that they are may be close enough for geometric parallax to be used to measure their distance. This proves to be the case for Antares, Betelgeuse and Rasalgethi. The distances to the two stars in Cepheus are estimated using other techniques. Most stars are both small enough and far enough away that they always seem to be points of light rather than a disk even with the best telescopes. As the stars in the list can be detected as disks this is a clear indication that they are truly large. Measuring the diameter of the disk and knowing the distance to the star it is a simple calculation to find out how big the star actually is. The answers are astonishing. Star Apparent Visual Distance Radius Magnitude [light-years] [AU] Betelgeuse, 0.50 430 3.6 a Orionis Antares, 0.96 600 4.2 a Scorpii Rasalgethi, 3.48 400 2.0 a Herculis Herschel’s Garnet 4.08 2000 5.7 Star, m Cephei VV Cephei 4.91 2000 8.8 Note: 1 AU is the distance from the Earth to the Sun ~150 million kilometres 5 The diagram shows that if any of these five stars were substituted for the Sun at the centre of our Solar System then all the planets out to and including Mars would be inside the star. The biggest, VV Cephei, would swallow everything out to nearly the orbit of Saturn. These stars are immensely larger than the red giant described earlier. The diameter of the largest star VV Cephei has been confirmed in a surprising way. VV Cephei is not alone in space, it has another star orbiting it and it as it happens the orbit is angled such that from Earth we can see the smaller star eclipsed by VV Cephei itself. This happens every 20 years but the amazing bit is that when it happens the eclipse is detectable as a drop in brightness lasting 1.2 years, a testament to the gigantic size of this star. It is clear that these enormous stars are reaching the ends of their lives and that they are much bigger than standard red giants. These are the red supergiants and their position in the sky gives away their true nature.
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