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SYFY WIRE Bad Astronomy

OK, it's still not aliens, but we're finally catching Tabby’s Star in the act

By Phil Plait

Tabby’s Star is at it again. But this time, we’re ready for it. And it’s still not aliens.

Tabby’s Star — aka KIC 8462852, if that helps any — started making headlines back in late 2015 when it was found to be dimming in strange ways. Lots of stars get dimmer and brighter over time (we call those variable stars, and there are dozens of different types), but this one was different. The drops appeared to be mostly random in time, and while some were just little dips, some were extraordinary events. At one point, it got fainter by 15%, and then another time, it dimmed by a staggering 22%!

That’s a lot, and very difficult to explain. If it were in a binary system, two stars orbiting each other, they can sometimes block each other, changing the overall system brightness, but that happens at regular intervals. Tabby’s Star was seen in the first place because it’s in the field of view of the Kepler space telescope, which looks for periodic dips in star light caused by planets orbiting the star getting in the way (we call those transits; think of them as mini eclipses). But even a big planet only blocks 1% of the star’s light, not 22%. Also, we’re talking an F3 star, more massive and half again wider than the Sun. It would take a planet literally a million kilometers across — more than ten times larger than Jupiter, and such a planet doesn’t exist — to do this.

Another idea is that aliens are building gigantic structures around it to capture its light to power their advanced civilizations. OK, I know how unlikely that is, but that’s a testable hypothesis, and so far nothing’s turned up when we listened for them.

A more likely idea is that something is orbiting the star, maybe swarms of comets or colliding asteroids, which release huge clouds of dust that block the star. That doesn’t exactly fit what we’re seeing, though. However, something like that should be pretty obvious if we observe the star using infrared telescopes, since warm dust emits a lot of infrared light. The problem is that, up until now, we’ve been getting data on the dips after they happen from Kepler’s automated observations, sometimes months later. We need real-time visible light and infrared observations to get a better handle on this.

... and now, the star is finally cooperating! Last week, some Spanish astronomers noticed the star was behaving oddly, and they alerted Tabitha Boyajian, the astronomer who initially worked on the star with the help of some citizen scientists. Observations using small telescopes indicated the star is definitely starting another dip, and has dropped by 1-2% already.

Boyajian issued an alert to astronomers Friday night, May 19, and a lot of ‘scopes are swinging toward the star now. Over the weekend observations have been pouring in, with many reported to Twitter. All that data is being collected, but given the flux of it (haha, flux, I kill me) it’ll be awhile before it’s analyzed and collated, I’d wager.

On Sunday morning, Boyajian and her colleague David Kipping, an astronomer who studies exoplanets, held a live YouTube Q&A to talk about the star. They covered a lot of territory, including showing quite a few of the “light curves”, graphs of the brightness versus time, for the star:

One thing that I’m finding very interesting is the quasi-periodic nature of these events. It’s not at all clear, but there may be a kinda-sorta 750 or so day cycle to these events. This graph Kipping showed on the live stream is compelling:

graph of Tabby's Star brightness

He took the Kepler data from about four years ago, shifted it in time, and overlaid it with data taken this past week. The match isn’t perfect, but it’s very intriguing. If this event really is periodic, then the Kepler data showed one instance, and now we’re seeing it again. There was another Kepler event (called Event 1) but, as astronomer Jason Wright points out, it doesn’t look much like these.

Still, if we’re seeing something happening every ~750 days, that implies an orbital period, something orbiting the star on that timescale. But it would have to be huge, far larger than the star.

But that doesn’t mean it’s a solid object. If, for example, you had a big asteroid or even a planet orbiting the star that got whacked by something big, like another asteroid or a comet, it could have blasted so much dust and debris off the surface that it surrounds the object in a cloud, and even spread out along the orbit. From our viewpoint, we see that material blocking the star every couple of years. It could explain some of what we see, including why each event looks different from the last (the cloud might have dissipated a bit, or a denser clump might get in the way of the star)… but maddeningly not all of it. What about all the random smaller dips? Why don’t we see a big drop every two years? And why isn’t the dust visible in infrared observations even when it’s not directly blocking the star?

The most important things needed are accurate brightness measurements (what we call photometry, literally the measuring of light) and spectra; that’s when you take the light from an object and break it up wavelength by wavelength — think of it like a rainbow, but with thousands of colors instead of seven. Spectra will tell astronomers a lot about the star’s behavior, like whether these dips are a feature intrinsic of the star itself, or caused by an outside object. Also, hopefully, spectra can help support or rule out the giant dust clouds. Some preliminary spectral data don’t show anything unusual, but we’ll see.

This star is a real mystery! There’s some good evidence that it’s been fading more or less steadily for a century or so, and took a dip of nearly 3% over the course of four years. That’s really odd behavior, and difficult to explain without some sort of ad hoc argument. Clouds of material in the way still makes the most sense, but we need more data.

It looks like that data is really coming in. Stay tuned. I’ll have more when I hear it.

Image Credit: Michael Pusatera, used by permission.