The atmosphere of the Sun (and other stars) contains calcium. It contains most of the elements, actually, just like the Earth does. As light that emerges from the sun passes through this cooler atmospher, two specific colors of very blue light, corresponding to specific transitions of electrons in a calcium ion, have a hard time getting through because they get absorbed by the calcium. These colors are “missing” from the solar spectrum, and Fraunhofer, who established much of our notation for spectral features, labeled them “H.” Later astronomers gave the two wavelengths separate names, and today we call them the H and K lines.1
The sun is “dark” at these wavelengths (this light doesn’t get through the lower atmosphere), so the much hotter upper atmosphere of the sun (the chromosphere) stands in good contrast against it, especially because the chromosphere is bright at these wavelengths (this is not a coincidence—the same transitions that make calcium in the lower atmosphere a good absorber make the upper atmosphere an efficient emitter at the same wavelengths.)
You cannot see the usual “surface” of the sun at these wavelengths; that light has all been absorbed by calcium ions. In this image you are looking at the upper atmosphere of the sun. Here, the brighter regions are hotter, and they tend to cluster around sunspots. This is because sunspots are caused by intense magnetic fields on the sun, and these fields reconnect and deposit energy high in the sun’s atmosphere, heating it and making it shine at this wavelength. The sun has an 11-year activity cycle, and if one makes measurements like in this image, one can clearly see this cycle as the total number of sunspots rises and falls over the course of a decade.
Now, in other stars we cannot see sunspots, but we can measure the amount of H and K line emission. Imagine this image of the sun was taken from so far away, you could not make out the sun’s disk. The five bright “active” regions (near the sunspots) would add up to make the point of light that is the sun look brighter at this color than it would if those regions weren’t there. This means that you could tell how much magnetic activity—sunspots and related things—was going on on the sun by how bright it was at this color. Watch long enough, and you could tell that the sun had activity cycles!
This is the philosophy behind the pioneering Mount Wilson H & K project, undertaken by Art Vaughn, George Preston, Sallie Baliunas and many others from 1966-2002. They measured the brightness of around 100 sun-like stars for decades to watch the rise and fall of their activity levels. The technique is now used at many observatories.
One of the big things people look for is an analog to the solar Maunder Minimum, a period from just after the discovery of sunspots by Galileo lasting about 70 years during which there were almost no sunspots. No one knows why the sun apparently stopped its magnetic cycle for so long, but if we could catch another star doing it, then maybe we could figure it out. The Mount Wilson project identified several sun-like stars with no sunspot cycles—victory!
But in 2005 I published a paper as a graduate student showing that this was actually a mistake. All of these “Maunder-minimum-like” stars had had their distances measured since the Mount Wilson project made their discovery, and most or all of them all turned out to be much farther away than expected—which means they were much brighter than we thought. Why? Because they’re not really much like the sun—they are subgiants, not ordinary main sequence stars, and we don’t expect subgiants to have strong magnetic fields.2 So it turned out the Maunder minimum was still sort of a mystery.
But wait! In star clusters one knows the distance to all of the stars, so one won’t get fooled by subgiants. Mark Giampapa and others have looked at truly sun-like stars in M67, an open cluster of stars a lot like the sun, and found that some of them have calcium H & K emission way below what the sun has even at solar minimum—there they are! Maunder-minimum-like stars!
In an amusing symmetry, my former graduate student, Jason Curtis, has looked into this and discovered that because M67 is so far away, you have to worry about another source of absorption: the interstellar medium. This gas between the stars is very sparse—the Mount Wilson stars are all too close to have their light affected by it. But M67 is very far away, and there is a lot of this gas in the way. This gas is made of the same stuff everything else is—including calcium!
Maybe you can see where this is going. The calcium in the interstellar medium absorbs calcium H & K light, making the stars appear dimmer at those wavelengths, and so our magnetic activity measurements end up giving erroneously low values. Once you correct for that absorption, it turns out that there aren’t really any anomalously inactive stars in M67.
So Jason’s new paper on this topic points out that, once again, stars that we thought were good Maunder minimum stars are, in fact, not—in this case, they’re just behind more interstellar calcium than we’re used to seeing in front of nearby stars.
You can read his (single author!) paper here on the arXiv now that it has been accepted to the Astronomical Journal.
1Jay Pasachoff pointed me to this history of notation for the H & K lines. Fraunhofer did not discover them, and the “K” line terminology came much later. Jason Curtis points me to this amusing mistake, where the letters are misinterpreted as standing for “hydrogen” and “potassium”.
2More on this, including an amusing anecdote about a “Marshall McLuhan moment” at my first colloquium, here.