Solar Cycles

As you may know, our sun has a solar cycle of about 11 years, and during that time the sun’s magnetic field changes, oscillating between very strong then there is a minimum, then back to the next cycle. During the minimum, there are far fewer sunspots, and the power output is also at a minimum. The last minimum started about 2017, so now we can expect increased activity. It may come as something of a disappointment that some of the peak temperatures here happened during solar minima as we can expect that the next few years will be even hotter and the effects of climate change more dramatic, but that is not what this post is about. The question is, is out sun a typical star, or is it unusual?

That raises the question, if it were unusual, how can we tell?

The power output may vary, but not extremely. The output generally is reasonably constant. We can attribute the variation in the solar output we receive over different years of about 0.1% of a degree Kelvin (or Centigrade) to that. There may appear to be more greater changes as the frequency and strength of aurorae are more significant. So how do we tell whether other stars have similar cycles? As you might guess, the power input from other stars is trivial compared even with that small variation. Any variation in total power output would be extremely difficult to detect, especially over time since instrument calibration could easily vary by more. A non-scientist may have trouble with this statement, but it would be extremely difficult to make a sensitive instrument that would record a dead flat line for a tiny constant power source over an eleven-year period. Over shorter time periods the power input from a star does vary in a clearly detectable way, and has been the basis of the Kepler telescope detecting planets.

However, as outlined in Physics World (April 5) there is a way to detect changes in magnetic fields. Stars are so hot they ionize elements, and some absorption lines in the spectrum due to ionized calcium happen to be sensitive to the stellar magnetic field. One survey showed that about half the stars surveyed appeared to have such starspot cycles, and the periodic time could be measured for half of those with the cycles. It should be noted that the inability to detect the lines does not mean the star does not have such a cycle; it may mean that, working at the limits of detection anyway, the signals were too weak to be certain of their presence.

The average length of the length of such solar cycles was about ten years, which is similar to our sun’s eleven-year cycle, although one star had a cycle lasting four years. One star, HD 166620 had a cycle seventeen years long, although “had” is the operative tense. From somewhere between 1995 and 2004, HD 166620’s starspot cycle simply turned off. (The uncertainty in the timing was because the study was discontinued due to a change of observatories, and it changed to one receiving an upgrade that was not completed until 2004.) We now await it starting up again.

Maybe that could be a long wait. In 1645 the Sun entered what we call the Maunder minimum. During the bottom of a solar cycle we would expect at least a dozen or so sunspots per year, and at the maximum, over 100. Between 1672 and 1699 fewer than 50 sunspots were observed. It appeared that for about 70 years the sun’s magnetic field was mostly turned off. So maybe HD 166620 is sustaining a similar minimum. Maybe there is a planet with citizens complaining about the cold.

What causes that? Interestingly, (Metcalfe et al. Astrophys. J. Lett. 826 L2 2016) showed that by correlating stellar rotation with age for stars older than the sun, while stars start out spinning rapidly, magnetic braking gradually slows them down, and as they slow it is argued that Maunder Minimum events may become more regular, and eventually the star slows sufficiently that the dynamo effect is insufficient and they enter a grand minimum. So eventually the Sun’s magnetic dynamo may shut down completely. Apparently, some stars display somewhat chaotic activity, some have spells of lethargy, thus HD 101501 shut down between 1980 – 1990, before reactivating, a rather short Maunder Minimum.

So when you hear people say the sun is just an average sort of star, they are fairly close to the truth. But when you hear them say the power output will steadily increase, that may not be exactly correct.


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