If you’re looking for a guide to this series, click here.

Last time we covered the first two hypotheses: instrumental issues and a Solar System cloud. This time, let’s move to the interstellar medium.

Hypothesis 3) Absorption from the Interstellar Medium

Between the stars there is lots of gas and dust. The densest, coldest parts of this “interstellar medium” (ISM) form neutral gas and dust, which cause reddening, dimming, and line absorption in stars. For instance, all evidence points to Boyajian’s Star having its light pass through enough dust and neutral gas on the way to Earth to make it about 35% dimmer at visible wavelengths than it would be without that stuff in the way. This is expected for any star in its part of the Galaxy.

But, one asks, maybe there are some especially dense pockets along that our line of sight occasionally sweeps past? I had always rejected this line of thought because if that were a thing, all sorts of stars would do that. After all, Kepler looked at over 100,000 other stars and none of them ever showed this behavior.

But, it turns out rare dense patches are a thing! I was at Berkeley recently and chatted with Carl Heiles, and he pointed me to SINS — small ionized and neutral structures in the diffuse interstellar medium. He was actually one of the folks who first brought these things to the attention of the broader community and postulated that they are short-lived, overdense, corrugated sheets and/or filaments in the ISM, and that when our line of sight aligns with a tangent point of such a structure we get a temporary jump in absorption.

So it turns out that this is a whole field—Carl pointed me to this conference on the topic 10 years ago. So, could this be the answer? Well, one problem is that the columns and sizes implied by Boyajian’s Star are much different from the “tiny scale atomic structure” that Carl describes. Those structures are typically around 30 AU across and block maybe 0.1% of the visible light that passes through them. We need about 100 times as much dust, and we need structures maybe 100–1000 times smaller than that.

But, it’s not clear that we would know about such structures if they did exist—you would basically need to launch Kepler to notice them. The ones they know about are pretty rare (most sources don’t show any evidence for them) so regions this dense and small could be so rare that Boyajian’s Star is the only one that sees them.

So, if we hypothesize that there is a spectrum of these SINS down to even smaller scales than have previously been seen, and that these smaller structures have even higher densities, and if these extensions persist across 2-3 orders of magnitude, then we would expect to very occasionally see stars behave like Boyajian’s Star!

This hypothesis is supported by the fact that the existing dimming implied by the ISM reddening and sodium absorption is enough to explain all of the Schaefer and Montet & Simon dimming — they see 15% or so, and the ISM is causing over twice that. This hypothesis would find support if during a future dip (or if the star’s long-term brightness continues to change a lot, in either direction) we see a corresponding change in the reddening, and in the sodium absorption features.

I like this one. Subjective verdict: plausible!

Hypothesis 4) Absorption from an Interstellar Molecular Cloud

A more obvious way to make a star dimmer is with an interstellar molecular cloud. We don’t expect to see any of these up in the direction of Boyajian’s Star, but such high latitude clouds do exist. Now, normally such clouds are obvious because they are the site of star formation, and young, forming stars light the cloud up at wavelengths from the radio to X rays. But some are quiescent, and if a star were not forming in the cloud it might not be obvious.

Now, such quiescent clouds are usually obvious because they are big (or part of a bigger cloud complex). There are small, isolated ones, though, called Bok globules, and they are usually found because they block a lot of starlight, so you see what looks like a big hole in the sky where stars should be.

At least, you do when they’re big and in front of a rich star field, like in the Galactic Plane. If they’re small and farther away, they can be pretty small, and if the background star density is low, you might miss them entirely. If there were a small (0.1 AU) Bok globule in front of Boyajian’s Star about 300 pc away, I don’t think it would have been noticed before.

Bok globules have smoothly varying densities, so a varying line of sight through it would naturally explain the long-term dimming. The dips would imply that this Bok globule has dense, sub-AU knots within it that our line of sight is probing. That’s a bit of a stretch, but, hey, if SINS exist in the diffuse ISM, why not in a dense cloud?

This hypothesis would find support if we could find the cloud, perhaps with a gas map done by the VLA or something. Even a single-dish radio telescope might at least spot some molecular gas in that general direction compared to neighboring direction, which would lend this hypothesis support.

This one isn’t quite as nice as SINS for me, but I still think it’s got a lot going for it. Subjective verdict: plausible!

OK, enough words. Next time: our final interstellar hypothesis, and then we’ll get in close and take a look at the possibilities for material in orbit around Boyajian’s Star!