General Meeting 12 May 1999
Observing Variable Stars
Speaker Mr. Bruce Sumner, acting director Variable Star Section
The tools of trade of the variable star observers are pencil, paper and a watch. With them, Bruce said, we take snapshots in the life of stars. Hidden in variations over time, in the diversity of the stars’ evolution, are the clues on how the pieces of the celestial jigsaw puzzle fit together. Recording the daily information is of course only the beginning in the long process of making sense out of what we see. Understanding why we do what we do, and to what use our efforts contribute, adds enjoyment to the nocturnal vigils.
In sorting this infinity of details the Hertzsprung - Russel Diagram was one of the most revealing steps in man’s understanding of star evolution. Basically it relates the colour of a a star to its brightness. The Danish astronomer Ejnar Hertzsprung in 1911 drew far-reaching conclusions from his life-long observations of variable stars, double stars, and clusters, and from his discovery in 1905 of “whales amongst fishes”. Henry Norris Russel working in Princeton, independently came up with the same idea when he plotted absolute magnitude against spectral type, and published his version of what is now known as the Hertzsprung-Russel Diagram in 1913. Annie Jump Cannon developed the Red-to-Blue colour classification with the catching mnemonic “Oh Be A Fine Girl Kiss Me Now”.
The H-R diagram shows how most stars crowd around a line known as the Main Sequence. These “normal” stars, like the Sun for instance, take billions of years to change in appearance. “Variable” stars in contrast, are stars that are unstable for one reason or another. They branch from the Main Sequence, often altering brightness or colour within minutes, days, weeks, or months in a cyclic pattern, and lead out to the Red Giants or the White Dwarfs. Many phases of activity such as RR-Lyrae or Cepheid variables, (Henrietta Leavitt’s famous distance candles of 1912) are still a mystery.
The first variable star ever recorded was Mira, a Red Giant discovered by the Dutch astronomer David Fabricius in 1590. 12 were known in 1786. Today it is probably 40,000, some 10,000 of which are categorised and named. They include cataclysmic variables, recurring novae, supernovae and binary stars eclipsing one another, opening entirely new areas of physics. One day someone may discover a way to include the rate at which stars change as a third dimension to the diagram, showing a “landscape” of star evolution.
Bruce Sumner, after showing what a huge field Variable Stars has become, pointed out that amateur astronomers by their sheer numbers play a powerful part in its development. And, as stated above, it is not an expensive hobby. It basically needs just a bit of persistent diligence. Practice on the Southern Cross stars judging the magnitude from the brightest to the dimmest. Did you know it goes clockwise? The ASV, like most other Societies, has a section dedicated to such pursuits and provides guidance and preprinted forms for getting started.
All observations are recorded and catalogued in collection centres such as Dr. Frank Bateson of the Royal Astronomical Society of New Zealand. 1996/97 records show individual’s contributions ranging from 4 to a staggering 15,912 observations. For additional information on the ASV Variable Star Section and its activities in 1997/98 refer to Peter Nelson’s report in CRUX Vol 17 No 2.
Enjoy your involvement. Who knows, tomorrow you might contribute to the discovery of a new classification of stars, a comet or another nova.