Astronomers have found another 21 stars dimming as erratically as Tabby's star

When KIC 8462852 - affectionately known as Tabby's Star - arrived on the scene in 2015, it made one heck of a bang. We'd never seen a star like it before. It kept growing dimmer, erratically, in a manner that defied explanation.

It still does - but it's looking like KIC 8462852 isn't quite unique. A new sky survey has scooped up a bunch of stars that exhibit similar dimming behaviour: 15 stars that seem quite similar to KIC 8462852, and a further six that seem even more extreme.

The unusual behaviour of KIC 8462852 was first published by Louisiana State University astronomer Tabetha Boyajian and colleagues in 2015.

Dimming stars are nothing new to astronomy - in fact, it's one of the main ways we detect exoplanets, when they pass between us and the star, dimming the star's light fractionally - by less than 1 percent, usually.

But, although KIC 8462852 itself seems to be a pretty normal yellow-white dwarf, its dimming behaviour is weird. Real weird.

Those planetary dips are usually on a regular timescale, and will dim the star by the same amount every transit. The dimming of KIC 8462852 is completely random and unpredictable - and the depth of light blocked varies. It's dipped as little 1 percent, and as much as 22 percent, and a range of depths in between. These dimming periods have varying durations, too. Whatever's causing this dimming, it's definitely not a planet.

In addition, some wavelengths of light are blocked more than others, which, astronomers say, rules out a large, solid opaque object (such as, say, an alien megastructure - an early suggestion that has since been ruled out).

But finding more stars that behave the same way could provide additional clues; for instance, if they're all the same type of star, or if they're all in the same sort of environment, that could tell us something. Likewise if they're all different.

So astronomer and physicist Edward Schmidt of the University of Nebraska-Lincoln went looking.

He combed data collected between April 1999 and March 2000 by the Northern Sky Variable Survey to find candidate stars with irregular variability, ruling out all stars with explicable dimming - those consistent with eclipsing binaries, for example. This produced 21 stars.

For these 21 stars, he downloaded light curve data from the All-Sky Automated Survey for Supernovae (ASAS-SN). This was compared against KIC 8462852's light curve. And there were some interesting similarities.

"In my candidates I can see no periodicity and the depths of the dips vary considerably.  So those [behaviours] are like KIC 8462852," Schmidt told ScienceAlert.

This randomness is an important distinction, because KIC 8462852 isn't the only weirdly dimming star out there. EPIC 204376071 was seen dimming to a depth of 80 percent in one single, solid opaque event earlier this year - like an anomalously large, ringed planet had passed in front of it. EPIC 249706694 has erratically timed dips in light, but those dips are all similar in depth.

Schmidt's stars are a lot closer to KIC 8462852.

He divided them into two types. The "slow dippers", of which 15 were identified, were the most like KIC 8462852 in terms of the timing. But there were also six "rapid dippers", which had similar dips, but much more frequently.

What this difference means is yet unknown, but it does indicate that the dimming behaviour exhibits a range of characteristics, that the phenomenon that causes it has a range into which KIC 8462852 could fall.

And it's not just the behaviour that's similar.

"The argument for these stars exhibiting the same phenomenon as KIC 8462852 is their location in the same area of the temperature-luminosity diagram," Schmidt said - that is, they are the same type of star. "This makes it seem likely they are the same."

But, he added, there is still work to be done. For instance, he has not yet looked into whether the dips of the 21 new stars block specific wavelengths. That's one thing.

Another is that, based on archival data, KIC 8462852 was determined to be slowly fading - between 1890 and 1989, it faded by 0.193 magnitude.

Both of these things will need to be investigated further in order to determine whether or not these newly identified 21 stars are Tabby-alikes - but, even if they are, we still have to figure out what is causing the dimming.

"I think it is most likely that the dips are caused by transiting objects but that doesn't necessarily explain the long term dimming. The transiting objects are likely dust based on the color changes during dips," Schmidt told ScienceAlert.

"I think we are still a ways from explaining everything."

The research has been published in The Astrophysical Journal Letters.

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