Project Leader: Stan Walker @stanwk

Project Description

This project is designed to establish seasonal epochs of Cepheid variable stars which are somewhat neglected because their brightness makes them unsuited to CCD or PEP measures. We wish to attract people who can make measures using binoculars or the unaided eye—and later extend to include observers using digital cameras of various types for the low amplitude objects. It includes 15 visual target stars, with 19 digital camera targets at a later stage. One or two with sparse visual sequences will appear on both lists.

What is a Cepheid variable star and why is it interesting? All Cepheids lie in an ‘instability strip’ in the H-R Diagram where a mechanism involving an ionisation and recombination cycle creates a series of shock waves which cause the surface of the star to expand and contract, with variations of temperature and brightness. We are interested in ‘Classical’ Cepheids where the periods range from 2 to 40 days and amplitudes are up to 1.3 magnitudes. Brightness ranges from third to sixth magnitude.

This includes only a small fraction of all known Cepheids but most of these have a baseline of more than a century so we’re building on strong foundations. The longer period stars can also exhibit period changes which provide us with some information about internal evolutionary changes. A light curve of S Crucis produced from measures made at the Auckland Observatory is shown to the left. The phase has been offset by about half a cycle as there seem to have been period changes since the GCVS epoch.

Visual measures will not be this accurate. But if we have enough the lesser accuracy is offset by the increased numbers of measures. We also plan a departure from the traditional visual observing methods. This involves better comparison sequences, accurate to 0.01 magnitudes—which avoids the rounding errors inherent in most sequences—and different techniques. Our targets are longer period Cepheids where large period changes are common.

The Bright Cepheid Project will be restricted to NE/binoculars and we would prefer observers who are not also using telescopes at the same time. This will ensure that the Cepheids get the level of attention that is required—in effect, observers will specialise by devoting their whole time, or at least a whole night’s observing, to this project. Data will be analysed and published annually.

Project Targets

Timeline

2009-10-01: Project launched, with four stars as targets.

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