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Variable stars are those that change their brightness, hence variable. These brightness changes can range from a few hundredths to as much as twenty magnitudes over periods of seconds to years, depending on the type of variable star. Stars change in brightness when they are very young, very old, or dying.
There are now over 30,000 stars known to be variable, and 14,000 more that are suspected to be changing in brightness in our galaxy, the Milky Way. Variables are divided into four main classes: pulsating and eruptive, where the variability is due to physical changes in the star or star system, and eclipsing binary and rotating stars, where the variability is due to an eclipse of one star by another, or the effect of stellar rotation.
Observations of variable stars are plotted on a graph called a light curve as the apparent brightness (magnitude) versus time, usually in Julian Date (JD). The light curve is the single most important graph in variable star astronomy. The light curve allows astronomers to unlock some of the secrets of stars and decode the messages hidden within the starlight. Information about the periodic behavior, the orbital period of eclipsing binaries, or the regularity of stellar eruptions can be directly determined from the light curve. More detailed analysis of the light curve allows astronomers to calculate such information as the masses or sizes of stars. Several years' worth of observational data can reveal the changing period of a star, which is a signal of a change in the structure of the star. In the same way that a histogram is a useful tool to inspect the precision of a set of measurements, we can visualize the nature of a star's variation more easily by plotting a light curve of apparent magnitude versus time.
Light curves show that many variable stars are periodic. Periodic phenomena repeat in a regular way that is predictable. However, the line between periodic phenomena and non-periodic phenomena is not always a sharp one. Sometimes a process may seem to be periodic from a large-scale perspective, while investigation on a fine scale reveals nonperiodic variations. Consider the change in apparent altitude of the Sun. If you measure the highest apparent altitude of the Sun each day for a week, you will not notice much change. The Sun seems to reach the same maximum altitude each day. Over a period of a month, however, changes will become obvious. But if we take measurements each day from December 31st until the summer solstice around June 21st, the apparent daily altitude of the Sun becomes higher and higher. This is still non-periodic behavior. If we take our measurements for an entire year, from December 31st to December 31st, we will see a definite pattern. When we examine the change in the apparent altitude of the Sun over several years, we see that in addition to monthly variations, it follows a regular pattern with a period of one year.
Many variable stars do exhibit simple periodic behavior. Some semiregular variables have time intervals of periodic behavior in between non-periodic intervals. Some have two separate periodic cycles superimposed on each other, affecting the overall result. The only way to determine the periodicity is by plotting and analyzing light curves. |
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