The mystery of KIC 8462852 (or Tabby’s Star), the Kepler star with unusual structures apparently in orbit discovered by Yale’s Tabetha Boyajian, continues… I’ve covered it twice before, but practically every day a new paper appears on the astronomical preprint server examining one aspect of Tabby’s Star or another, all with the same intent: figure out what is going on with it. Is it some weird form of variable star we haven’t seen before? IS it a swarm of comets? Is it a giant complex system of planets? ARE aliens building some megastructure after all? (It’s still jumping the gun to say yes to that one). Now there’s a new fascinating wrinkle (and controversy) around the star itself.
To recap, Tabby’s Star is an F3V type star – slightly bigger than the Sun (1.4 solar masses), somewhat hotter than the Sun (6600 Kelvin rather than 5700 Kelvin), somewhat brighter than the Sun (4.3 times brighter), and with an estimated lifetime of 3 billion years, or 3,000,000,000 years (Rather than 10 billion). Put simply, a normal ordinary F3V star should not change much at all over a period of a mere 100 years. Neverthless, Bradley Schaefer of Louisiana State University is reporting that KIC 8462852 Faded at an Average Rate of 0.165+-0.013 Magnitudes Per Century From 1890 To 1989 (conveniently, that’s the actual title of the paper!). This story was picked up all over the place. Stars can be variable on scales of years, but there’s no known kind of variability that would cause a steady dimming totalling 20% of the star’s light over a period of 100 years, and no other examples of this happening to otherwise-normal F type stars. This is WEIRD on a level that might even exceed the dimming effects of the light itself. This is NOT something that comets (the currently preferred explanation for the infamous light dips) could do, which would mean there’s something strange about the very star itself.
On the other hand, there was a quickly issued rebuttal by two other astronomers, Michael Hippke and Daniel Angerhausen, who wrote an also-usefully-titled paper “KIC 8462852 did likely not fade during the last 100 years.” What they found was that Schaefer’s data analysis left out some subtle effects that produced a fake dimming signal.
But how did Schaefer, Hippke, and Angerhausen get a hundred years of data on a star that’s been noteworthy for less than a year? Do they have time machines?
Not really. What they have is the DASCH project, which stands for Digital Acess to a Sky Century at Harvard. Basically, between the late 1880s and the mid-1990s, Harvard’s astronomy department had done an enormous number of all-sky surveys, barring one period in the 1950s when the observatory was run by a Donald Menziel, who stopped the surveys to save money (In the 1880s, photographic plate astronomy was a costly new technique; in the 1990s, it was a costly outdated technique). This was done with a variety of telescopes and different brands of photographic plates, in both hemispheres, extending our complete sky coverage back 140 years into the past. It’s the next-best thing to a time machine, and that’s incredibly valuable in astronomy where practically any kind of change takes aeons and we need as much time coverage as possible. (I’ve previously shown off how the Crab Nebula changed in just 40 years; imagine extending that to 130).
The biggest problem with Harvard’s massive archive is that it’s on 500,000 fragile glass sheets in a climate-controlled archive in a basement. Putting them all online is a massive undertaking. They had to build their own plate scanning machine. You can’t just use a flatbed scanner; they had to make a machine that could fit and scan an entire plate at once, at high enough resolution that they missed absolutely no details, and with all the color depth so that they wouldn’t wash out any fine grayscale gradations on the plates. They’ve had to enlist the help of volunteers to copy down all the observing logs so they know what the particulars (time and date, exposure time, weather) were for each glass plate. There’s also historical interests to be preserved; a large number of these plates have annotations written by Annie Jump Cannon herself, for instance.
As of right now, DASCH has scanned 138,000 plates, and Schaefer was able to go back and use modern techniques to get over a thousand measurements of Tabby’s Star stretching back to 1890. The problem he ran into, though, is that not all of the plates were exactly the same. The kind of chemicals used (and just plain quality of manufacture) changed multiple times over the hundred years, which means the kind of light they were sensitive to was marginally different. The plates were also taken on different telescopes, which might themselves have different responses to light. And then there’s the sad fact that the plates themselves age, and the oldest ones are pushing 140 years old now, which may change things despite the fact that Harvard has taken care to preserve them.
DASCH is trying to correct for all these things so that detailed research can be done, but the second paper by Hippke and Angerhausen looked at several other F3-type stars and found that a number of THEM show small but persistent dimming trends too; with jumps where Harvard changed the type of photographic plate they used. So, it sounds like there’s a bit of recalibration that needs to be sorted out. UPDATE: On the other hand, there’s this news item where Schaefer counter-argues that he knew about the data calibration issues, and that Hippke and Angerhausen were less careful than he was about which plates they used. I guess we’re going to need a THIRD study…
This is definitely not the end of investigations of Tabby’s star.