Tabby's Star

NASA’s Kepler space telescope watched about 150 000 stars for four years, looking for the tiny regular dips in light caused by planets passing in front of them. Volunteers in the Planet Hunters project, checking Kepler’s data by eye, flagged one star as very odd. Its light did not dip by a fraction of a percent like a planet would. On 5 March 2011 it dropped by 15%, and on 28 February 2013 by 22%, in irregular dips lasting from days to weeks.

“Where’s the flux?”

In 2015 the astronomer Tabetha Boyajian and colleagues, including many of the volunteers, described the star in a paper subtitled “Where’s the flux?”. The star itself looked normal: an F-type star a little larger and hotter than the Sun, about 1 470 light-years away in the constellation Cygnus. But something large and irregular was passing in front of it. Their favoured explanation was a swarm of comet fragments.

Aliens?

The astronomer Jason Wright pointed out that the pattern was the kind of thing a search for alien megastructures, such as a Dyson swarm, would look for. The idea spread around the world. Wright himself was careful: as he put it, aliens should always be the very last hypothesis you consider. Radio telescopes listened for signals and heard nothing.

Kepler data also showed that the star faded slowly over its four years of observation, by about 0.34% a year. Claims that it had been fading for a century, from old photographic plates, remain disputed.

Dust after all

In 2016 a Kickstarter campaign raised over US$100 000 to watch the star continuously with the Las Cumbres Observatory telescope network. In May 2017 a new series of dips began; the backers named the first one Elsie. These were only a few percent deep, but they were observed in different colours. The star dimmed more in blue light than in red. A solid structure would block all colours equally; fine dust does not. Most astronomers now think clouds of dust are responsible, though exactly where the dust comes from, and why it moves as it does, is still not settled.