Stan Walker

I began serious observing about 60 years ago with a home-made 203mm reflector, then from 1967 to 1990 used the Edith Winstone Blackwell 50cm telescope. Measures were initially visual but with Brian Marino and Harry Williams set up a state of the art, photometric UBV system, which was very productive from 1969 until I retired to NZ's Far North region in 1990 where I used several small telescopes with both UBV and BVRI equipment.

My interests have covered a wide range of observing - light curves of minor planets, occultations of sigma Scorpii and by Pluto of an anonymous star which provided details of Pluto's atmosphere, but mainly in a wide range of variable star objects. Observing is only the first step in understanding and with colleagues Brian Marino, Ed Budding, Frank Bateson, Mark Blackford, and many others I have published a wide range of scientific and popular papers and articles and presented these at many meetings. All of this included measures derived by myself and observing colleagues.

Along the way I became a member of the IAU in 1970, the Auckland Astronomical Society about 1965, RASNZ, RAS (Great Britain), the now defunct, IAPPP, in some of which I received research awards. My association with the RASNZ, VSS - Variable Stars South - began about 1966 and continues to this day. My active observing ceased about 2010 and I now concentrate on analysing other observers' measures, mainly from the International Database maintained by the AAVSO but on a few stars of special interest such as QZ Carinae and V777 Sagittarii.

Title: Dual Maxima Mira Variables - What Causes this Behaviour?

Mira variables are long period, medium to high amplitude, low mass stars found in the asymptotic giant branch, AGB, of the H-R diagram. More than a thousand of these are observed, some over observed, by a wide variety of variable star enthusiasts. In a century and a half about a dozen, say one percent, have shown evolutionary period changes. A similar number have two maxima per cycle which suggests they are transiting a short-lived evolutionary stage. What causes this and why are they found almost entirely in a small part of the sky centred near the Southern Cross? Are there two different types? This discussion examines UBV and BV colours of these DM stars, period and other changes and their relationship with Miras showing other unusual features, mainly a noticeable hump on the rise. Why are these features mainly found in the longer period objects - 400 days and over?

Title: Dual Periods in Cepheid Variable Stars

Cepheids with long periods, 15 days or more, are one of the more massive types of pulsating variables. What we see in other types of pulsators, such as the low mass, longer period Mira stars, may provide indications of where observational research should be directed. Thus, the apparent alternation of periods in Mira stars, which we suspect of being the interaction of a fundamental pulsation in the outer radiative layers with some type of occurrence in the convective outer envelope may also occur in the longer period, massive but cooler Cepheids which should have convective outer layers all or part of the time. Light curves and periods of several such Cepheids are presented and discussed. A surprising result of this analysis is strong support for the idea that beginners in variable star observing both visual and CCD, would be better rewarded by observing such Cepheids rather than Mira stars.