Lets start with this: H-K Project
Extra-cyclic activity
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When examining the long-term direct record of sunspot observations, it is immediately apparent that the length and amplitude of the 11-year sunspot cycle is not constant. The figure at right shows the yearly sunspot number over the past 380 years. At the beginning of this record, it appears that the sunspot cycle apparently ceased for almost 70 years. One simple explanation would be that there simply were no observations during that time. Another would be that astronomers of that time period were simply not very good at observing, or did not make accurate records of the sunspot number.
However, neither of these explanations are true. First of all, some of the astronomers of that time period were Galileo, Scheiner, Hevelius, Halley, and Herschel. Their observations of other celestial bodies show their prowess; incompetence simply was not the issue. There is no reason to believe that they ignored the Sun. Several reports of sunspots during this time period actually made note of the fact that their seeing a sunspot was considered out of the ordinary for that time period.
Therefore, we are forced to conclude that the Sun does experience occasional lulls in its activity. Using other proxies of long-term activity, such as the 14C abundance, we can see that the Sun goes into these quiescent periods every few centuries. At present we seem to be experiencing the opposite phenomenon; the most recent four cycles are among the most active ever recorded.
Understanding what causes the 11-year sunspot cycle has eluded scientists for centuries, so it goes without saying that we don't understand why it should vary as much as it does, let alone almost vanish completely from time to time. The 30-year record of activity for sun-like stars compiled by the HK Project is just beginning to investigate this phenomenon.
Extra-Cyclic Activity in Solar-like stars
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A survey of stars close to the Sun in mass and activity shows the distribution at left. The arrows above the distribution show the range of activity seen by the Sun from the expected activity level in the Maunder Minimum to sunspot maximum, with the line left of center indicating the activity at sunspot minimum. From this, it appears that as many as 25% of the stars surveyed could be in a Maunder Minimum-like state. Therefore, the Sun might spend up to 25% of its time at present under similar conditions.
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This figure on the right, shows a similar distribution of activity for two open clusters, Messier 67, which is nearly the same age as the Sun, and NGC 752 which is about 2 billion years younger than the Sun. The arrow above the M67 histogram has the same meaning as in the previous plot. Using open clusters to test the frequency of Maunder minima is preferred because it can be assumed that the age of all the stars in the cluster is nearly equal. Therefore, the distribution observed is more likely to be accurate than using field stars whose ages are not well-known. The distribution for M67 is similar to the field star distribution, and about 1/4 of the stars have low activity similar to the Maunder Minimum. In comparison, the younger open cluster of NGC 752 have very few inactive stars, but the fact that there is a few suggests that Maunder minimum episodes could occur, but a lower frequency than that of stars closer in age to the Sun.
Monitoring Extra-Cyclic Activity
Unfortunately, the Maunder Minimum occurred just after the invention of the telescope. Therefore there are few records immediately preceeding it to see how the Sun behaved as it was beginning. There are records at the end of the Maunder Minimum, and one feature appears to be that the length of sunspot cycle varied more than it does at the present, from 8 to 13 years. In the hopes that we can learn more about extra-cyclic activity, the HK Project has begun observing several inactive stars. Some of these stars may be in Maunder Minimum states, and therefore, long-term monitoring may show changes in their behavior as they come out of this inactive phase.
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At the same time, a few of the stars which have been observed since 1966 are beginning to show long-term behavior. In the left panel of the above figure, two stars with similar mass and rotation period are shown. Because their mass and rotation are similar, they should have nearly the same level of activity. However, the activity of 54 Piscium (HD 3651) has been decreasing steadily since before 1966. The most recent activity maximum is as low as the activity minimum seen in the early 1970's. This star could be entering a Maunder minimum. On the right is HD 9562, a star close to the Sun in mass. It's activity has only a very small increasing trend over 30 years. From its spectrum, the rotation period must be less than 25 days (i.e., close to the Sun's) but its activity it much lower than the Sun's. Therefore, it is very likely that this star is currently in a Maunder Minimum.
Continued observations of all of these stars will hopefully provide clues as to what the Sun's behavior will be over the coming centuries. An important effect of this research is the investigation of how long-term variations of solar activity like the Maunder Minimum affect the Earth. The time period of the Maunder Minimum also corresponds to a period called the ``Little Ice Age'', when summers in Europe were short and winters were severe. Therefore, further analysis of extra-cyclic stellar activity could provide important clues to a better understanding of changes in the Sun and their impact on the Earth's climate.