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Other stars

For many years observations of magnetic activity on stars similar to the Sun has told us that it is not unique in its magnetic cycles. Unfortunately, the direct observation of starspots (the equivalent of sunspots on stars) is not possible over extended periods, but two effects have been consistently measured for over two decades: the apparent variation in brightness of the star, and the emission in the spectral line of ionized calcium (which is known to represent the level of magnetic activity quite well; see the image on the first page).

A comparative study of these stars reveals a number of important facts. Many stars have magnetic cycles with periods ranging from 6 to 14 years, some being regular (like the Sun), some irregular, and some not cyclic at all. In addition, these stars exhibit variations in brightness which vary from 0.1% up to 4% (recall that the measured solar variation over a decade is 0.1%), and equally variable magnetic activity. While some of the differences between stars in the sample can be accounted for by their differing properties (such as rate of rotation), the Sun could easily have variations twice as large as are currently observed, based on a simple comparison with similar stars.

  Figure 8: The brightness (increasing along the vertical axis) and magnetic variations (measured by calcium emission, increasing along the horizontal axis) for a sample of stars that are classified as non-variable (diamonds) and variable (triangles). The Sun is denoted as an asterisk. Note the variations of stars both of similar magnetic activity and brightness variation (0.01 corresponds to 1%, and so on, courtesy R. Radick/Phillips Lab).

Perhaps one of the most interesting facts is that stars about the same age as the Sun, or older, have the same correspondence between increases in magnetic activity and increases in brightness. However, the exact opposite is true for younger stars! It is believed that starspots dominate on these younger stars, and faculae are absent or very weak. These are important clues as we attempt to understand what solar variability may be possible.

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Approved by Peter Fox
Last revised: Wed May 17 11:28:21 MDT 2000