Tabby's star, otherwise known as KIC 8462852, is dimming drastically in brightness, giving astronomers an opportunity to figure out what has been causing this star's weird behavior.

UPDATE (June 19, 2017): And the dimming has begun again! Starting on June 11th, the star began a slow decline, dipping by about 1%. Keep up with the latest happenings at Wherestheflux.com.

UPDATE (May 22, 2017): Updated brightness measurements show that the dimming event has ended — the star appears to be back at its normal brightness. But the monitoring isn't over yet and the next few days will tell if the star dips again.

UPDATE (May 21, 2017): The Las Cumbres Observatory plot shown below has been updated to reflect the latest data.

Ever since the Kepler space telescope captured a series of random-seeming dips from a certain mysterious star designated KIC 8462852, astronomers and the public alike have been baffled by its behavior. Then, following the end of the main Kepler mission, the star went quiet.

Now, at long last, the star has begun a steep decline in brightness — it’s already 2% dimmer after a single night of observation — and telescopes all over the world are at the ready. For the first time, astronomers are catching Tabby's star in the act, and that means the mystery of what causes the star to dip and dim in brightness may soon be solved.

What We Already Know About Tabby’s Star

What is now famously known as Tabby’s Star is a normal-looking F-class star in the field of the Kepler space telescope. Kepler’s mission was to monitor more than 150,000 stars, watching for the minute dips in brightness that would signal an exoplanet moving across the face of its parent star from Earth’s perspective. But in Tabby’s Star, Kepler — and the watchful eyes of citizen scientists involved in the Planet Hunters project — found something completely different.

Tabby’s Star was observed to dim 10 times, sometimes by 1% (typical of a giant exoplanet transit) and sometimes by 10% to 20% (not at all typical of exoplanet transits, or anything else for that matter), each dimming lasting days to weeks at a time. The dips were irregular both in terms of how long they were and when they occurred.

Things became even more complicated when a longer-term behavior was discovered: Tabby’s Star is fading away over the course of years, and perhaps even centuries.

Explanations for the star’s behavior ranged from the mundane (starspots) to the interesting (a comet breaking up around the star) to the sci-fi-inspired (a Dyson sphere syphoning the star’s energy for an alien civilization).

Eventually, various astronomers involved in the project, including Tabetha Boyajian (Louisiana State University) and Jason Wright (Penn State), seemed to settle on two main explanations: a circumstellar object of some sort, such as a giant comet in an elliptical orbit, or some dusty clump in the stuff between stars.

Notably, the comet scenario predicts a dimming event this very month. From Boyajian and colleagues’ paper, published in January 2016: “A more robust prediction is that future dimming events should occur roughly every 750 days, with one in 2015 April and another in 2017 May.” Read Benjamin Montet (University of Chicago) and Boyajian’s article in the June 2017 issue of Sky & Telescope for a full rundown of all the possible explanations.

There are clear ways to tell these scenarios apart, but those ways require spectra during the dips — and we don’t have that kind of data from Kepler. For example, if it’s dust, then the star will dim more at bluer wavelengths. A Dyson sphere, on the other hand, is presumably a solid object and so the star would dim the same at all wavelengths. Certain spectral fingerprints, such as those left from sodium or calcium, could also enable astronomers to learn more about the obstruction.

Infrared data will also be key, as any material close to the star ought to be hot — and therefore ought to show up as excess infrared radiation.

So Boyajian, Wright, and several others made their predictions and settled in for a wait: All they needed was for the star to stop pretending to be ordinary. And sure enough, Tabby’s Star dropped its act.

Tabby’s Star Dips

On April 24th, the robotic telescope at Fairborn Observatory spotted what might have been a dip in brightness — or a statistical fluctuation. The brightness returned to normal levels within a week. Nevertheless, the event (labeled "Event 1" in the light curve below) caught astronomers' attention and monitoring was increased. Then on May 18th, a more significant drop began.

At 4 a.m. on May 19th, Boyajian called Wright: Fairborn Observatory in Arizona had issued an alert that Tabby’s star had dimmed by 2% — a big dip in the star’s brightness. The team immediately sent out the call for more observations.

ALERT:@tsboyajian's star is dipping This is not a drill. Astro tweeps on telescopes in the next 48 hours: spectra please! — Jason Wright (@Astro_Wright) May 19, 2017

As soon as the Sun set around the world, astronomers trained their telescopes on Tabby’s Star — from the amateur astronomers involved in the American Association of Variable Star Observers (AAVSO) to the spectroscopists at the Keck I and II telescopes in Hawai‘i. Additional spectroscopy will come from the MMT Observatory in Arizona.

The Las Cumbres Observatory Global Telescope Network (LCOGT), which was already regularly monitoring the star around the clock, also stepped up its observing cadence. The LCOGT and Fairborn Observatory light curves are shown below. For now, it seems, the dimming event has ended.

Even space-based telescopes slewed toward Tabby’s Star. While Spitzer can’t point in that direction of the sky, the Swift space telescope monitored the star’s brightness at optical and ultraviolet wavelengths.

And this is far from an exhaustive list — many more telescopes participated in follow-up observations. That in itself is no small feat considering that telescopes are typically scheduled weeks in advance.

“This is the first time we’ve seen a clear dip since the Kepler mission, and also the first we’ve caught in real time,” says Wright. “The changes are as steep as we ever saw it change brightness with Kepler.”

“It’s going to be a busy couple of weeks.”

Watch a live stream from earlier today with Wright and SETI Director Andrew Siemion as they discuss the recent changes and incoming observations:

