Tabby’s star (or Boyajian’s star), named after the lead author on the paper announcing its unusual fluctuating light, is also called WTF (Where’s the flux?) star because of the unexplained large fluctuation of light energy. The observed drop in light intensity, which is about 20 percent of the normal brightness, is too large to be explained as being due to a transiting planet blocking its path to the earth. After several guesses were made, the reason for this fluctuation has now been partly explained in a paper in the journal Physical Review Letters by a group from University of Illinois at Urbana Champaign. In a novel way, this work has brought in ideas from the theory of phase transitions in magnetism to understand the star’s wavering light.
A part of the Cygnus constellation, Tabby’s star shows anomalously large fluctuations of light intensity which has puzzled astronomers. Observations by Kepler spacecraft have shown that Its light intensity drops by more than 20 percent at times, with regular smaller fluctuations which are also in excess of that seen in many other stars.
Mohammed A. Sheikh and collaborators have come up with an explanation for this behaviour. The work draws an interesting parallel between the fluctuations of magnetic field in the star and what is seen in a ferromagnetic transition, for the first time. However, there is more to this phase transition than is seen in magnetism: “The order parameter (magnetization) in ferromagnets describes a conventional second order phase transition [a transition in which the order parameter varies smoothly across the phase boundary]. In our paper, we hypothesize that the order parameter describes a dynamic second order phase transition. In other words, the system is not in equilibrium (as it is for conventional second order phase transitions),” says Sheikh in an email. “We do not yet know for sure if the transition is the result of a change of magnetization. We do not yet have an exact mechanism for the transition,” he adds, hinting that there is more work in store to decipher this puzzle.
While the star shows small dimming events and large ones also, the group has mainly looked at the small dimming effects and the large ones remain to be studied. As Karin A. Dahmen, an author of the study explains in the email, “In the mean field model, small dimming events would correspond to small avalanches. Large events correspond to run-away avalanches, caused by a weakening of the system after failure.
The dynamics of large and small avalanches is different, small avalanches propagate in a very jerky way with many starts and stops, while large avalanches accelerate fast and only decelerate when they are about to stop, so their growth is fairly smooth, not as jerky as the small avalanches are.”
Tabby's star is not alone in having large anomalous dimming events. “EPIC 204278916 [a star in the Scorpius constellation] also shows dimming events which are possibly unexplained. However, the two stars are very different. Tabby's star is a very standard F class main sequence star. EPIC 204278916 is pre-main sequence star with a protoplanetary disk, which may be the cause of the dimming events observed for EPIC 204278916,” says Richard L Weaver, an author of the study.
“The stellar classification is based on spectral characteristics of the star. Roughly speaking, F class stars are stars with surface temperatures of 6,000-7,500K, slightly more massive and slightly larger than the sun (approx 1-1.5 times more mass and 1.15-1.4 times larger radius),” he explains.
