Orwell Astronomical Society (Ipswich)
Long Period Variables
The class of long period variables covers stars with periods between 80 and 1000 days, the average being around 300 days. An alternative name for the objects is Mira stars, after the best known member of the class, Mira, or ο Ceti. Long period variables are numerous and several are bright. These facts, together with the large magnitude range, probably account for the fact that long period variables were the first type of variable to be discovered.
The visual brightness range of a long period variable is, on average, seven or eight magnitudes but can be higher, e.g. 11 magnitudes in the case of χ Cygni. The cycles are not absolutely constant either in period or magnitude. The maximum magnitude is the most likely characteristic to change from cycle to cycle. For example, ο Ceti exhibits maxima between second and fifth magnitude and the time of maximum may vary by up to a week from the predicted value. Several examples of long period light curves may be seen on the OASI website where it will be observed that, in most cases, the rise from minimum to maximum is more rapid than the decline and there are often irregularities in the rising portion.
The majority of long period variables are old red giants, and the colour is usually quite distinct when the star is near maximum. The brightness variations are the result of pulsations in the distended and rarefied atmosphere of the star, causing its physical size to vary. The cause of the pulsations is still unknown: mathematical models of the phenomenon do not fit observations very well. Ironically a long period variable is brightest when smallest because it is then hottest (typically 2400 K) and faintest when largest because it is then coolest (typically 1500 K). Dust clouds have been detected above the surfaces of long period variable stars, suggesting that they eject matter as they expand. However, there is a mystery associated with the observations. The atmospheres of long period variable stars contain metal oxides (mainly titanium, vanadium and zirconium oxides) which exist only in relatively cool temperatures of around 3000 K and are indicated by the presence of absorption bands in the visible spectrum. At maximum magnitude, however, bright lines of hydrogen become visible, associated with much higher temperatures. Both phenomena are found to exist simultaneously, suggesting that the atmosphere of a long period variable must be complex indeed. However, despite the peculiarities of the situation, a spectrum indicating the presence simultaneously of lines of hydrogen and metal oxides is taken as a defining characteristic of long period variables.
The large change in visual magnitude is rather deceiving because the change in total emission of radiation between maximum and minimum is much less. The radiation emission at minimum visual brightness shifts into the infra-red. There is also increased absorption of light due to the formation of metal oxides at lower temperatures. The variation in infra-red magnitude is typically approximately one magnitude.
The large variation in brightness of long period variables makes them the easiest variables for beginners to tackle. ο Ceti and R Leonis are the beginners' favourites and may be followed with binoculars for most of their periods. There are many others which may be followed using a small telescope. Because of its narrow field of view, the Orwell Park refractor is more suited to these stars when they are at minimum.
Mike Nicholls