variable stars actually expand and contract, like a beating heart. Some pulse rhythmically, while others flicker irregularly, suffering from the stellar equivalent of cardiac fibrillation, while still others vary on a semi-regular basis caused by two or more overlapping periods. Some, such as Cepheid variables (named for the genre's prototypical star, Delta [δ] Cephei), take only hours to complete a cycle. Others, called long-period variables, can have periods in excess of one year.Eruptive
variable stars are the most dramatic of all. Eruptive variables usually lie low, only to change brightness suddenly and unpredictably in just a few days, hours, or even minutes. The best known example of an eruptive variable is a nova, which can intensify by 5 or more magnitudes in less than a day. Other eruptive variables, like R Coronae Borealis, shine near their maximum magnitudes, only to drop precipitously as they exhale a cloud of obscuring carbon soot that blocks some of their light.
Regardless of type, variable stars are always cataloged by constellation using capital Roman letters. The first variable to be discovered in a constellation is dubbed "R," the second "S," then "T," and so on. For example, R Coronae Borealis was the first variable found in Corona Borealis, while S Scuti would have been the second discovered in Scutum, and so on. Once the ninth variable was cataloged in a particular constellation, denoted by "Z," the next was identified by "RR," followed by "RS" to "RZ," then "SS" to "SZ," and continuing in this fashion until "ZZ" was discovered. After this, "AA" to "AZ" are assigned, then "BB" to "BZ," etc. The system continues, excluding "JJ" to "JZ," until "QZ" is used.
Using this confusing system, astronomers can catalog 334 variable stars in a single constellation. This might seem to be adequate to cover all variables in a constellation, but some along the main stream of the Milky Way contain even more. In these cases, the 335th variable identified in a particular constellation is listed as "V335" (V for variable), with each subsequent discovery placed in numerical order.Many amateur astronomers enjoy monitoring variable stars.
Andy Beaton is a dedicated variable star observer from Toronto. "In spite of the severe handicaps with my location," he says, "I have a list of 60 variable stars that I monitor regularly. I try to observe 5 to 10 of them every clear night." Astronomy
columnist Glenn Chaple also observes variable stars from his yard in the Boston suburbs every chance he gets. He is closing in on a lifetime total of 70,000 magnitude estimates an achievement exceeded by only a handful of other observers, regardless of location.
Amateur observations of variable stars also constitute a very important source of data toward our understanding the universe. The American Association of Variable Star Observers
(AAVSO) is an international organization devoted to the study of these stars. Through systematic monitoring, members keep a careful watch over variable-star activity and report magnitude estimates to AAVSO headquarters in Cambridge, Massachusetts. The AAVSO acts as a liaison between amateur astronomers contributing data and professional astronomers requesting data.
So, not only can you observe variable stars from a city, you can also contribute valuable scientific observations. Best of all, estimating the magnitude of a variable star is not as difficult as it may sound. The AAVSO has a wide variety of detailed charts available for selected variables. Each chart shows the targeted star and its immediate surroundings.
Below's a sample chart showing Z Ursae Majoris. Z is a great star for beginners because its circumpolar position means it is above the horizon throughout the year for most stargazers in the Northern Hemisphere. It is also bright enough to remain visible in small telescopes throughout its magnitude cycle.