It is customary, at this time of year, to relax by the fireside, take a sip of a stimulating beverage, consume yet another mince pie and reflect on the past year – and what a year it has been! At this time last year we received the sad news of the death of Sir Patrick Moore. It was not entirely unexpected but it still produced a shock wave that has reverberated throughout the Astronomical fraternity. On May 1st, the BBC held a celebration of his life and work at which over 400 people attended. But soon after an ugly rumour began to circulate that the Sky at Night programme was to be terminated. Many members of the Astronomical Section sent letters of protest and the BBC was quick to respond denying the rumours and stating that the programme will continue, at least for another year. It remains to be seen exactly what form it will take – Patrick was a difficult act to follow; we will have to wait and see.
In last year's Newsletter, I made an appeal for donations towards the recent major restoration work at the Observatory. Many members responded and I would like to express our sincere thanks for their generous contributions. However I could not have foreseen the outcome of a chance meeting at the Observatory which resulted in our receiving a massive donation of £15,000 from a generous anonymous benefactor. This donation, the largest ever received, has enabled us to purchase a dedicated Hydrogen Alpha solar telescope which will be installed at the Observatory later this month. Huge thanks to our benefactor.
Membership numbers are now at 130 and it is good to see that we are attracting younger members, particularly as Assistants at the Observatory. Open nights are attracting many visitors due to the publicity gained from the BBC's Stargazing Live programmes which are now an annual event.
In July, at the annual Observatory picnic, Doug and Julia were totally surprised when they were awarded the Sky at Night Magazine's award for outstanding contributions to the advancement and promotion of astronomy. Doug & Julia wish to thank those responsible for their nomination – you know who you are! Thank you, but it is nice to know that one's small efforts are appreciated.
The major disappointments of the year were the two Comets – panSTARRS and ISON, neither of which lived up to early predictions. We did manage to just about see panSTARRS as a very faint misty patch with a short tail just after sunset in March, but ISON was the major disappointment. Early predictions led us to believe that we actually had a contender for 'Comet of the Century' outshining the full moon. In fact it never exceeded mag. +3.5. At the time of writing, indications are that ISON did not survive its perihelion passage unscathed; we may get a look at it as it gains altitude and heads for a dark sky. But as it moves away from the Sun, its magnitude rapidly decreases; but it just might 'flare-up' and surprise us all.
Impacts from comets could pose a threat to the continuation of life on Earth, but there are other dangers to consider as the following article on Super-Luminous Supernovae demonstrates.
Peter R Wallis
Supernovae are probably the most cataclysmic events known to astronomers. Stars like our own Sun die relatively slowly when the elements that fuel their nuclear reactions are exhausted. However, stars with a mass between 10 and 20 times that of our Sun explode violently with a brightness a billion times that of the Sun today. When the star has exhausted its nuclear fuel its core of ash can no longer withstand the gravitational pressure above and collapses to a neutron star, releasing enough energy to blow away the outer layers in supernova explosion. If this were to occur near to us in our galaxy, it would be the end of us. Fortunately supernovae are not frequent; one occurred nearly 1000 years ago, leaving the Crab Nebula, luckily some 500 light-years away. Such supernovae have also a good side, in that they distribute into interstellar space the chemical elements higher than Helium [1]. These have been created in the thermonuclear reactions in the star and its explosion; life, and in particular our existence could not have evolved without them. Theory suggests that the presence of "metals" in stars allows them to shed material in winds so that they do not grow very massive; this may well help to limit the number of supernovae in the galaxy.
Astronomers have recently discovered a few new super-luminous supernovae which out-shine the other supernovae by a factor of nearly 100. They do not know what the progenitors are but some have speculated that they might be the much larger stars which formed in the early universe, when there was only hydrogen and helium in the interstellar medium. Such stars would burn these in their thermonuclear reactions to form elements such as carbon, nitrogen and oxygen and would continue to grow. It was shown in 1967 [2] that extremely massive stars larger than 140 solar-masses would become so hot in the interior that pairs of electrons and positrons would be spontaneously generated. The energy to make them would reduce the pressure support against gravity and the star would become "pair unstable". The core of carbon and oxygen would collapse, but still full of this fuel. Burning would increase exponentially, consuming all the fuel in seconds, blowing the star apart and expelling a massive cloud of highly radioactive debris. It was calculated that the rise-time of the radiation from such an enormous star would be at least 100 days as it had to penetrate such a large debris cloud. The radiation would fall over the subsequent 200 or more days as the radioactivity (of e.g. iron, nickel, cobalt) decays.
All this remained as theoretical speculation until a remarkably luminous supernova SN2007bi appeared to fit the prediction. There was however a problem: the galaxy in which it was found was not metal-free. Also, the rising phase of the light, indicative of the size of the debris cloud, was not seen in time. It was just a 'possible'.
But now a recent paper by Nicholl et al [3] reports two supernovae which are super-luminous and have slow-to-fade characteristics similar to those predicted for pair-instability explosions. They are PTF 12 dam, first detected by the Palomar Transient Factory on 23rd May 2012 and PS1-11ap of 2nd January 2011. The events were caught early, with a rise-time of 2 months, implying a moderate debris mass of 10 to 20 solar masses. Their colour is also more blue than expected. The authors conclude that they are not pair-instability supernovae but normal core-collapse supernovae of some 10 to 16 solar masses. They suggest this could apply also to SN 2007bi where the rise-time is not known...
However, they are undoubtedly super-luminous. They suggest that the radiation does not arise from radioactivity but from a highly magnetized neutron star (a "magnetar") spinning at nearly 1000 times a second. They say that, "the lack of unambiguous nearby pair-instability events suggests that their local rate of occurrence is less than 6 x 10-6 times that of the core-collapse rate". Good, the threat of ordinary supernovae is quite enough. If we still wish to find pair-instability supernovae, we'd better look for them nearer the big bang.
-------------------------------------------------------------------------------------------------------------
1 Astronomers call these "metals"! Back
2 Rakavy G. et al Astrophys, J 148 803-816 (1967) Back
3 Nicholl S. J. et al "Slowly fading super-luminous supernovae that are not pair-instability explosions" Nature 502 17th Oct. 2013 Back
-------------------------------------------------------------------------------------------------------------
Last updated 27-Jan-2018