From VSSC100


Ian Howarth - director VSS 1977-1980

The 100th edition of the VSSC prompted an exercise in self-indulgence; I went back to my old issues of the Journal to remind myself of just what I'd got up during the few years I'd enjoyed the privilege of directing the Section. It was a salutory experience to be confronted with just how long ago that was, and I don't suppose that the compact format and sober grey covers of the Journal in those days would be recognized by all that many of today's observers. On the other hand,it was pleasing to realise that the team of workers analysing the data back then, might still understandably succumb to the sin of pride, for the productivity they displayed in publishing VS material in the Journal. For a few years, people like Jeremy Bailey, Edward Collinson, John Isles, and George Munford made sure that almost every issue of the Journal included a paper on a VS topic. Looking just at the Section Reports that I prepared, we published analyses of more than 44,000 observations - and this at a time when computations were done on hand-calculators, home computers were largely unheard of, and the leading edge of technology was embodied by data storage on punched cards!

Of course, then, as now, there was a dedicated crew of backroom boys (they were indeed all male) making all this possible, by assiduously checking and logging the records. It's perhaps invidious to single out one such contributor, but, even so, I'd like to take this opportunity to record the particular debt of gratitude that we owe the late, lamented Doug Saw, not only for the considerable burden of administrative and data-processing chores that he shouldered, but also for the personal qualities that he displayed. He was everything the Section (or its Director) could ask for in a Secretary.

Nowadays, I follow the Section's progress merely as an interested outsider. While I find it slightly disappointing that, in recent years, the Section hasn't used the Journal to promote its work as actively as it might (which means that, even within the Association, people outside the Section aren't necessarily aware of just how active it is), it's nonetheless very clear that it is a thriving, and vigorous concern; the work, by Dave McAdam and others, that has gone into making both historical and current observations available in machine-readable form, is particularly laudable. But the heart of any BAA Section is its observers, and with variable stars we usually only ever get one chance to record the vagaries of a light-curve. A good illustration of this came up very recently, with a paper that brought together the results of back garden eye-estimates, and the continuing search for the precise value of the Hubble constant.

Back in April 1974, light from an exploding supernova in the Sc spiral galaxy NGC 4414, in Coma, reached Earth. At discovery, SN 1974G was around magnitude 13, bright enough to be seen in amateur telescopes. Although the supernova wasn't formally added to the Section programme, Walter Pennell - another highly talented, modest observer, whose untimely death deprived amateur astronomy of one of its most skilled proponents, constructed a photovisual comparison star sequence, that was used by a number of UK observers to follow the supernova through maximum, and well into the decline phase. At the time it was quite a thrill just to be able to see a star some 20 Mpc distant; of course, some of today's observers aren't content merely to see supernovae, but successfully set out to discover them! A light-curve and brief discussion were published in the Journal (85, 352, 1975).

There things lay until, two decades later, the Hubble Space Telescope turned to NGC 4414 to obtain observations for the Key Programme on the distance scale of the Universe. Most readers will be familiar with the concept of the ladder of distance determinations in the Universe: loosely speaking, direct trigonometric methods such as parallax can be used to determine the distances, and hence intrinsic luminosities, of only relatively nearby stars. Knowing those luminosities, the distances of similar stars at greater range can be estimated, and used to calibrate the luminosities of rarer, brighter objects at the same distance. This process is gradually extended to larger and larger scales, ultimately to establish the relationship between redshift and distance for the most remote galaxies - and hence to estimate the handful of numbers which succinctly describe the entire evolution, and future fate, of the universe (like the Hubble constant, and the deceleration parameter).

The most luminous standard candles available - and hence those observable to greatest distances - are supernovae, and specifically Type Ia supernovae. (The enigmatic gamma-ray bursters may briefly be more luminous than any supernova, but they don't qualify as standard candles). An important aspect of the Hubble Key Programme is the determination of the distances of (relatively!) nearby galaxies, in which such supernovae have been observed, in order to determine accurately the intrinsic brightness of those supernovae. This is done through the observation of Cepheid variables, which provide reliable distances through the well-calibrated relationship between their pulsation periods and absolute magnitudes. Knowing the distance to the host galaxy (from its Cepheids), the intrinsic luminosity of any supernova in that galaxy, past or present, can be estimated. Knowing the intrinsic luminosity of the supernova, the distances to similar, but more distant, supernovae can be estimated from their light-curves.

The rub comes because there aren't many galaxies well suited to calibrating the supernova luminosities. In the nearby universe - the Local Group of galaxies - there just aren't many well-observed Type Ia supernovae in the records. There are plenty of supernovae observed in very remote galaxies, but they're too distant to reveal their Cepheids. It turns out that there are only a dozen or so galaxies which contain well-observed Type Ia supernovae, and which are also close enough to allow their Cepheids to be measured; NGC 4414 is one of those galaxies, and SN 1974G is one of those supernovae. So those old observations suddenly take on a new significance, and indeed Brad Schaefer has recently undertaken a careful re-analysis of those observations, using a new CCD-based calibration of the comparison-star sequence. He published the results in the Astrophysical Journal at the end of last year (ApJ 509, 80, 1998), finding the peak V magnitude to have been 12.30+/-0.05, and a corresponding Hubble constant of 55+/-8 kms-1Mpc-1. (Our old value for Vmax was 12.2, so Walter Pennell's original sequence was not at all bad!) The paper contains a particularly noteworthy comment: 'It must be realized that without reliable light-curves, the large amount of time used by the Hubble Space Telescope has only poor utility for calibrating the supernovae'.

When the supernova observations were made by Section members, the SpaceTelescope was a pipe-dream, the value of the Hubble constant was uncertain by at least a factor of two, and the use of the observations to investigate the structure of the universe was undreamt of. The moral is clear: theories come and theories go, but good observations, and particularly good observations of time-dependent phenomena, stand the test of time. On the occasion of VSSC 100, I wish all Section members clear skies in their endeavours to secure those observations,and the current Director (and his designated heir) every success in co-ordinating the programme.