Ordinary Meeting, 2004 December 18
Kepler's Supernova of 1604
Dr Mason remarked that in contrast with the extragalactic supernova discoveries mentioned by the previous speaker, supernova explosions actually within our own Galaxy were very rare. Indeed, none had been observed in living memory: the last had happened over 300 years previously. The present talk would be concerned with one particular such supernova which had burst into prominence in Orphiuchus on 1604 October 9, now known as Kepler's Supernova for reasons to be discussed in due course. It had been lost to twilight on November 6, but had still been easily observable when it re-emerged in 1605 January.
The speaker remarked that the astronomers of the time were particularly lucky: this was the second Galactic supernova to have burst into prominence in only 32 years. It was perhaps a little regrettable that the telescope was not to be used for astronomy until 1609, just too late for these two events, and that there had only been one Galactic supernova in the 400 years since then.
The speaker emphasised that despite its name, Kepler had not been the first to observe the supernova: the first recorded observations were by Italian observers, Altobelli in Verona, Clavius in Rome and Capra and Marius in Padua. However, he could be fairly confident that it had been observed on the first night of its prominence, as by a fortuitous coincidence there was a conjunction of the planets Mars, Jupiter and Saturn in the sky at the time. The supernova itself had appeared right among the planets, peaking at mag –3, in excess of the brightness of Jupiter, and so would have been particularly conspicuous in an area of the sky where many were looking.
Kepler himself had been in Prague at the time, serving as a Court Astronomer, but only came to hear of the event through the reports of a local weatherman, Brunowski. Even after hearing such reports, cloudy skies prevented him from making any immediate observations of his own, and it was seven whole days before he saw the new 'star' for himself. His name would come to be attached to it as a result of his writing a book, De Stella Nova, published in 1606, in which he wrote up all of his extensive observations.
Dr Mason proceeded to list all of the Galactic supernovae of the past 1000 years: six in total. The first, in 1006 in Lupus had shone at a brilliant mag –9, and must have been a truly spectacular sight, though not visible from the UK. In 1054, Chinese astronomers had recorded a supernova in Lyra at around mag –5, the remnant of which was now visible as the famous Crab Nebula, whilst a century later in 1181, a somewhat fainter mag –1 event had been observed in Casseopeia. The next two events had happened only 32 years apart: a mag –4 event in Casseopeia in 1572, followed by Kepler's supernova of 1604, mag –3 in Orphiochus. The most recent event had been very much fainter at sixth magnitude, and whilst historical records seemed ambiguous as to whether it had been in 1667 or in 1680, the remnant had now been detected in the form of the radio source Casseopeia A. Indeed, in all six cases, some form of remnant of the explosion, often in the form of an expanding shell of gas, had now been identified.
Looking through this list, the speaker remarked that three of the six events had been in Casseopeia. He was not sure whether this was mere luck, or whether there was reason to think future events might also be more likely to happen in this part of the sky. The speaker went on to remind members that there were two distinct types of supernovae: core collapse events, and so-called 'Type-Ia' supernovae. In the former case, the inner core of a massive star collapsed in upon itself to form a neutron star in a massive explosion. The latter case resulted when a compact white dwarf star was in a binary system with another more normal star, and accreted gas from the surface of its companion. As this happened, the mass of the white dwarf would gradually increase, but when it exceeded a mass limit of 1.4 times the mass of the Sun, called the Chandrasekhar limit, it could no longer support itself against its own gravity. The result was a monumental explosion, ripping through the white dwarf and completely destroying it, and often ripping apart its companion as well. Dr Mason remarked that one way in which to distinguish the two classes of events was that core-collapse events always left a neutron star or pulsar remnant, whereas Type-Ia events completely destroyed the progenitor star. In both cases, however, a remnant taking the form of a shell of shocked gas could often be seen expanding away from the explosion.
The speaker explained that it was fairly certain that the events of 1006 and 1572 had been Type-Ia events: amongst other evidence, no remnant pulsar had been seen in either case. In the latter case, Dr Mason remarked upon a star by the name of Tycho-G, seen within the expanding shell remnant of the supernova by the Hubble Space Telescope (HST) and Chandra X-ray Observatory, apparently moving at a speed of around three-times that of other nearby stars. It was thought that this was the original binary companion star to the progenitor white dwarf, which had been given a 'kick' by the blast wave from the explosion.
In contrast, a pulsar was clearly visible in the Crab Nebula remnant of the 1054 event, and similarly for the 1181 supernova remnant: both optical and X-ray observations revealed a rapidly pulsating source at the centre. In the case of the 1667/1680 event, no pulsar had been detected within the remnant Casseopeia A, though Chandra images had recently revealed what appeared to be a radio-quiet neutron star. The speaker said it was not fully understood why a neutron star should sometimes be radio quiet, however some had constructed theoretical models of how the star's magnetic field might bring about such behaviour, and it was now widely accepted that this neutron star was the remnant of a core-collapse supernova.
Returning to Kepler's supernova, Dr Mason closed by remarking that this was the only one of the Galactic events listed earlier for which the question of whether it was Type-Ia or core-collapse remained open. After the event, Kepler had made detailed brightness observations until it had eventually faded out of sight on 1605 October 8, and he had published these in his book. In 1924, Walter Baade had used these observations to reconstruct the light-curve of the event as accurately as possible, and had found that the shape of the resultant curve resembled a typical Type-Ia event. However, over the years, many had discredited Baade's methodology, and support had tended to switch towards a core-collapse explanation. But now, recent images from a range of observatories at a range of wavelengths – the Very Large Array (VLA) in the radio, the Spitzer Space Telescope in the infrared, the HST in the optical, and Chandra in X-rays – all failed to show any sign of a pulsar at the centre. In addition, spectral observations of the remnant matched more closely the chemical composition that might be expected after a Type-Ia event – poorer in heavier elements than a core-collapse remnant.
The speaker remarked that many of the historical Galactic supernovae must have been amazingly eye-catching events, and he hoped the wait for another would not be a long one. Following the applause for Dr Mason's talk, Mr Boles introduced the afternoon's final talk, to be presented by Messrs Martin Mobberley and Ken Goward.