by Emily St.

On May 19, an astronomer named Tabetha Boyajian sent out a tweet that instantly grabbed the attention of hundreds of astronomers and scientists all over the world.

#TabbysStar IS DIPPING! OBSERVE!! @NASAKepler @LCO_Global
@keckobservatory @AAVSO @nexssinfo @NASA @NASAHubble
@Astro_Wright @BerkeleySETI

Telescopes all over the world turned to a tiny star 1,300 light-years away designated as KIC 8462852, and often called Tabby’s Star. Over the next several days, astronomers photographed the star in every imaginable way, taking images for the first time in real time as the star’s light dimmed and resurged. With them, citizen scientists joined in for their first look, too.

Just what is Tabby’s Star?

Tabby’s Star might have escaped all notice in the first place, had not the Kepler space telescope surveyed and catalogued it. From this survey, it gets the crisp, formal KIC 8462852 designation. Tabby’s Star began as just one of over 145,000 stars Kepler monitors for very slight, telltale dips in their light output that may indicate planetary systems live there. Scientists chose these stars from a half million possibilities in Kepler’s field of view, focusing on those which could have Earth-like planets.

Since astronomers were looking for Earth-like planets, Tabby’s Star is not too different from our Sun. Astronomers call it a “yellow-white dwarf,” but it’s actually a little bit bigger than the Sun. It’s 1,300 light-years from us (about 7.642 quadrillion miles). Seen from Earth, it is a little too dim to see with the naked eye, so it requires a small telescope.

Why did we start watching it?

When Kepler spots a planet, it does so by watching the light from a star dim in a specific way and at a regular interval. Kepler looks for a small dip in the light output coming from a star when a planet passes in front of it from Earth’s point of view, called a transit. These dips are usually very tiny, a fraction of a percent.

Tabby’s Star left a record of dimming that defied explanation. It flickered like a candle in a breeze, sometimes fading as much as twenty-two percent, and for no apparent reason. All of this could happen, in fact, in a matter of days before the star would just as rapidly brighten again.

Kepler’s photometer has limitations, though, because it was built to serve its single-minded mission of identifying stars which might have planets, which Earth-bound instruments can then review more closely later. This frustrated attempts to understand Tabby’s Star—both because astronomers were limited in what data they had and because they didn’t know when they would have the chance to observe its dimming again. What’s more, Kepler’s reaction wheels failed in 2013, keeping it from revisiting Tabby’s Star.

What astronomers knew for certain, though, is that they had seen no phenomenon that could explain what Kepler observed. They’ve kept collecting data from Earth, and the new data have confounded them even more.

What could be going on with Tabby’s Star?

They knew already Tabby’s Star wasn’t intrinsically variable (like some stars which naturally dim sometimes) because it wasn’t the right type and age of star for that. Other things, like circling planets, didn’t fit the new data. Indeed, the dimming was so immense that if a planet had caused it, it would have been a planet bigger than the Sun—but a Sun which could race past in just a few days! As time passed, the data ruled out most explanations involving nearby objects blocking the light.

This left room for wilder speculations, and theories began circling about alien megastructures which were under construction—like Dyson swarms used by advanced civilizations to collect energy from a star. Few astronomers entertained a complex hypothesis like this as being supported by the scant data, but some did identify it as an interesting target for further research. More mysterious and exciting was that Tabby’s Star gave astronomers reasons to imagine some completely new phenomenon might be at work—a real mystery that could lead to new ideas about how stars or planets work or how the galaxy came into being.

Who’s “Elsie”?

Into this speculation and dearth of information came the May 2017 dimming event, which has been called “Elsie”. Elsie gave astronomers the first chance ever to watch this dimming as it happened—the first observation since Kepler’s and the first real confirmation that it wasn’t a fluke. Boyajian’s all-uppercase call to action sent the community, both professional and amateur, into a tizzy. Now observatories all over the world could simultaneously measure KIC 8462852 with many different instruments and in many different spectra.

Elsie lasted, in all, about five or six days, drawing to a definitive end on about May 26, after drawing data from dozens of professional observatories.

How are citizen scientists involved?

Because of the real-time nature of the event, citizen scientists—most especially, backyard astronomers—leapt in from the start. Coordination began across the Internet immediately. Boyajian set up a website,, to send regular updates and rally amateurs, who were also sharing and discussing their theories and findings, and helping each other on the AAVSO forum and a dedicated KIC 8462852 sub-Reddit. Finally, Boyajian herself actively answered questions on Twitter, where discussion happens using the hashtag #TabbysStar.

Citizen scientists busily took measurements using equipment designed to track the star, compare it over time, and measure it in different spectra so that they can help determine what may be causing the dimming. Especially useful for measuring variable stars—besides telescopes—are cameras that can gather light carefully, with little noise, like ones with CCD sensors. Amateurs can take several photos using these over time and measure the brightness of a star compared to others in the neighborhood of known brightness.

There are also inexpensive filters which can break up the light coming from a star into its constituent colors so that the spectra from the star can be recorded. Tabby’s Star observers are especially interested in whether this changes during dips—clues as to what may be blocking the light or whether the star itself is changing.

Many funneled their observations over to the American Association of Variable Star Observers (AAVSO) through their submission site. Their observations served to cover as much time and as many different kinds as observations as possible so that there would be no gaps and so that anomalous results could be corroborated.

Even with the Elsie event now over, amateurs’ role hasn’t ended. The unique nature of this event, seen by so many people with so many instruments instead of just a single one, opens up possibilities for citizen scientists again to continue to engage with the project by analyzing and checking the data as it’s gathered and published. Now both professional and amateur scientists will begin poring over it for new answers, and new questions. They will be better able now to rule out or support their guesses as to what’s happening.

Studies of Tabby’s Star are also needed beyond Elsie in order to collect data during the star’s periods of normal brightness, so amateur astronomers will prove to be even more important as professional observatories must move on to other targets. Maybe an amateur could be the next person to notice the next dimming event while others are busy with all the new data!

Why does Tabby’s Star matter?

The reaction to Tabby’s Star and its newest dimming event shouldn’t surprise anyone. Most mysteries of the universe seem abstract, are difficult to understand in the first place, and give no foothold for getting started, let alone advancing our understanding. They remain far off domains of special people with special tools, special training, special words.

Tabby’s Star, though, presents something that literally anyone with a few hundred dollars and some determination can see. The mystery itself is nothing less profound—what’s out there? It underscores just how little we know and how isolated and tiny we are in the universe. But this mystery hides in the very same sky you and I share every night, and it’s one which each of us is empowered to elucidate, even if just a little. It democratizes science again, and it gives everyone a chance to see the unknown for themselves.

Emily St. is a backend engineer. She is interested in how things work.