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

Last time we finished up circumstellar scenarios with a discussion of alien megastructures, the reason this star got so much press in the first place. This installment: intrinsic variations.

I think Robin Ciardullo was the first to mention this possibility to me, though it may have been Richard Wade. The idea is that somehow the star itself is getting slowly dimmer, with occasional dips.

Hypothesis 10) Starspots and Magnetic Cycles

The most natural way stars change brightness over years and days is from stellar magnetic activity. The Sun at the maximum part of its cycle is about 0.1% brighter than usual, and sunspots and plage coming into and out of view can also modulate its brightness by a few tenths of a percent. More magnetically active stars might have even greater variations.

But the timescales and amplitudes are all wrong for Boyajian’s Star. Activity cycles take decades, not centuries, and Boyajian’s Star rotates about once every 0.88 day, not many days. If the dips were caused by spots, they should come and go every 0.88 day, but instead they last many days or longer. Finally, in order to cause the big changes in brightness we see, the spots or cycles would have to be 10–100 times larger than the strongest effects known in other stars.

Another problem here is that Boyajian’s Star is decidedly inactive, being a hot star, not a cool star. Unlike the Sun, it has a radiative outer layer (not a convective one), so it does not have the same dynamos that produce the Sun’s magnetic fields. Hot stars like this have very little in the way of surface features (“stellar dermatology”) and no activity cycles (that we know of). There are hot stars with strong magnetic fields and spots, but they are almost always slow rotators (their fields slow them down), and are generally hotter than Boyajian’s Star.

So, subjective verdict: very unlikely.

Nonetheless, something weird an unexpected is going on, so maybe we should not dismiss this entire line of reasoning entirely. This bring us to

Hypothesis 11) Polar Spots

Montet & Simon point out that a polar spot could be doing this. Some very cool stars are known to have large, dark, polar starspots. This explanation nicely explains why the spots don’t come and go with a 0.88 day period: since they are at the pole, we always see them, and so at best their irregular shapes get modulated. This would explain why we see a 0.88 day photometric signature at all, on what we might expect would be a featureless star. Then, the dimmings could be due to the ebb and flow of this spot’s size and darkness on days- and century-long timescales.

This is very clever, but for the reasons I stated above we have good reasons to think this star can’t have spots like this. (Again, if this were a thing, we should see it all the time!) Montet & Simon offer the hypothesis very provisionally for this reason.

So my subjective verdict on this one is: not likely.

This hypothesis would find support if Gaia finds the star to be much farther away than we expect from the reddening and sodium absorption. This would indicate that reddening is not accompanying the dimming we see, so it could be spots. It would also find support if signs of a polar spot could be identified, such as narrow absorption features from the much cooler atmosphere, perhaps with Zeeman splitting if the spot has strong magnetic fields.

OK, what else could the star be doing? Well, many forms of variable star are pulsating, so how about:

Hypothesis 12) Stellar Pulsations

Stars have a few characteristic timescales on which they change. If you poke a star with a regular cadence, there are certain frequencies that will cause it to respond strongly. Some ways this can happen are:

In the Sun, random convective motions cause it to “ring” with many frequencies, the strongest being around 5 minutes. These “asteroseismic” modes are generally very, very small: they can never cause variations of order 20%, and they are usually periodic, not random like Boyajian’s Star. The timescales are wrong for a Main Sequence star, too: these operate on sound-crossing times and their harmonics, not days or centuries. Many classes of pulsating stars rely on internal instabilities involving the opacity of their constituent gasses. If these gasses heat up, they ionize, which makes them more opaque, which makes it harder for starlight to emerge from the core to the surface, which causes the star to expand, which makes the gasses cool, which makes them recombine, which lowers their opacity, which makes it easier for starlight to emerge, which makes the star shrink, which heats the gasses… etc. etc. These timescales can be of order days, but, like asteroseismic modes, they are periodic, not episodic, and don’t cause century-long dimming. Delta Scuti stars are a kind of star of very similar mass and temperature to Boyajian’s Star, so it’s possible that it has something like this going on, but it’s not clear how that could cause any of the effects that we see.

Basically, all known pulsation modes are regular or semi-regular, not episodic, and none we know of operate over centuries. If it’s some form of pulsations, it’s one we haven’t seen before or thought of before.

Subjective verdict: not likely

The last one is my favorite, from Steinn:

Hypothesis 13) Post-Merger Return to Normal

What if the star isn’t dimmer than it should be: what if it’s brighter, and we’re seeing it return to normal brightness? In that case Gaia will reveal that it is much farther away than we think based on its brightness and reddening. This would imply that the long-term “dimming” we see is the star returning to a normal state after somehow getting way too bright.

But what could that mean? The best idea we have is some sort of merger: perhaps the star recently coalesced with another star, or a brown dwarf or planet. This would deposit a lot of orbital and gravitational potential energy into Boyajian’s Star, which would eventually get dissipated as heat and escape as starlight, causing a temporary brightening.

It’s not clear why this would create the dips, but perhaps the merger was quite recent, and there are still hydrodynamical effects going on inside the star as the companion is “digested”, causing the internal structure of Boyajian’s Star to adjust on days-long timescales.

One issue here is that the dimming is too fast. When confronted with big changes in energy content or flux, stars evolve on the Kelvin-Helmholz timescale, roughly the time it takes for all of the energy in the star at a given moment to finally escape the surface (while being constantly replenished by the fusion in the star’s core). For Boyajian’s Star this timescale is about 1 million years. This means that if the entire star is processing a big change in internal energy or luminosity, it takes around 1 million years to complete the adjustment. Changing by 15% in 100 years is therefore about 10,000 times too fast.

But, the star’s radiative envelope is not very massive, so perhaps the energy never made it deep into the star? In that case the Kelvin-Helmholz timescale is a bit shorter, so maybe we’re off by only 1,000 times. It’s an order of magnitude argument, so maybe we’re being too pessimistic by a factor of 10, so we’re only off by 100 times. It’s possible that a detailed simulation of such a merger will reveal shorter timescale events, perhaps even things that might produce the dips.

So, I’m intrigued, and I like the idea despite the timescale argument not working out. It’s possible that there are other ways to temporarily brighten a star we haven’t thought of. I’d like to hear from people who model these things before I commit to a plausibility level, so I’ll say:

Subjective verdict: unclear.

So that’s it! That’s all I’ve got. I’ve tried to be comprehensive, but cleverer people than me keep coming up with new ones, so I’m sure I’ve missed something. I’m still not convinced that the right answer has appeared in the literature, but I’m hopeful that one of the explanations in our paper is correct. Before wrapping up in the final post, though, I’d like to discuss some nonstarters that keep coming up:

Popular Nonstarters:

Gravity darkening. This one got a lot of attention for some reason. Basically, stars that spin very fast have equators that are darker than their poles. A transit or eclipse of a gravity-darkened star can therefore have an unusual shape, potentially being asymmetric. Since some of the dips are asymmetric, some people declared the system “solved” with this suggestion. Problem is, Boyajian’s Star is not spinning fast enough to have significant gravity-darkening, and this still doesn’t address the issues of what is causing the dips! Black hole or other nearby companions. Boyajian et al. ruled this one out, but just to reiterate their argument: Boyajian’s Star is RV stable, so it does not have any close-orbiting stellar-mass companions. It’s not any kind of binary system. Gravitational waves. I’ve heard this one suggested, presumably because they were just discovered by LIGO and so are in the zeitgeist right now. They have no explanatory power here (i.e. there is no way they could cause what we see). Asteroids in the Solar System. We discuss Solar System solutions here. Anything closer than the outer Solar System would not persist for years, and asteroid occultations have a very different photometric signature than Boyajian’s Star. It’s an ordinary “dipper”. There are “dipper” stars that have superficially similar light curves to Boyajian’s Star. But these as caused by close-in disks and other circumstellar debris that are revealed by their long-wavelength emission, which Boyajian’s Star lacks. That was the original reason Boyajian’s Star stood out as being so weird: it’s not young, it has no disk, so it’s not a dipper. I don’t understand why this one keeps coming up. Starkiller Base. Yes, I saw The Force Awakens, too. I presume these suggestions were all made facetiously.

OK, next time: summary of hypotheses, and what Gaia will tell us.