was combing through old telescope data from 2020 when he found an otherwise boring star acting very strangely. The star, named Gaia20ehk, was about 11,000 light-years from Earth near . It was a stable “main sequence” star, much like our sun, which meant that it should emit steady, predictable light. Yet this star began to flicker wildly.

“The star’s light output was nice and flat, but starting in 2016 it had these three dips in brightness. And then, right around 2021, it went completely bonkers,” said Tzanidakis, a doctoral candidate in astronomy at the ����Ӱ�Ӵ�ý. “I can’t emphasize enough that stars like our sun don’t do that. So when we saw this one, we were like ‘Hello, what’s going on here?’”

The cause of the flickering had nothing to do with the star itself: Huge quantities of rocks and dust — seemingly from out of nowhere — were passing in front of the distant star as the material orbited the system, patchily dimming the light that reached Earth. The likely source of all that debris was even more remarkable: a catastrophic collision between two planets.

“It’s incredible that various telescopes caught this impact in real time,” Tzanidakis said. “There are only a few other planetary collisions of any kind on record, and none that bear so many similarities to the impact that created the Earth and moon. If we can observe more moments like this elsewhere in the galaxy, it will teach us lots about the formation of our world.”

in The Astrophysical Journal Letters.

Planets form when gravity forces together matter — dust, gas, ice or rocky debris, for example — orbiting a new star. Early solar systems are chaotic — planets routinely collide and explode or go flying off into outer space. Through this process, and over perhaps 100 million years, solar systems like ours winnow their planets down and settle into an equilibrium. 

As common as these collisions probably are, observing one in a distant solar system requires patience and luck. The orbits of the planets must take them directly between us and their star, so that the resulting debris obscures some of the star’s light. The telltale flicker then takes years to play out. 

“Andy’s unique work leverages decades of data to find things that are happening slowly — astronomy stories that play out over the course of a decade,” said senior author , a UW assistant research professor of astronomy. “Not many researchers are looking for phenomena in this way, which means that all kinds of discoveries are potentially up for grabs.”

Tzanidakis, the study’s lead author, studies extreme variability in stars over time. His previous work at the UW identified a system with a binary star and a large dust cloud that caused a seven-year eclipse.

The behavior of Gaia20ehk, however, posed a new mystery. The star’s particular fluctuation — short dips in brightness and then chaos — had never before been observed. The team was stumped, until Davenport suggested that they use data from a different telescope to look for infrared light rather than visible light. 

“The infrared light curve was the complete opposite of the visible light,” Tzanidakis said. “As the visible light began to flicker and dim, the infrared light spiked. Which could mean that the material blocking the star is hot — so hot that it’s glowing in the infrared.”

A cataclysmic collision between planets would certainly produce enough heat to explain the infrared energy. What’s more, the right kind of collision could also explain those initial dips in light.

“That could be caused by the two planets spiraling closer and closer to each other,” Tzanidakis said. “At first, they had a series of grazing impacts, which wouldn’t produce a lot of infrared energy. Then, they had their big catastrophic collision, and the infrared really ramped up.” 

There are also clues that the collision resembles the one that created the Earth and moon . The dust cloud is orbiting Gaia20ehk at roughly one astronomical unit, the same distance from the sun to the Earth. At that distance, the material could eventually cool down enough to solidify into something similar to our Earth-moon system. Scientists like Tzanidakis and Davenport can’t know for sure until the dust settles — literally — in the system. That could take a few years, or a few million. 

In the meantime, their discovery is a call to action to find more collisions. The powerful Simonyi Survey Telescope at the NSF–DOE Vera C. Rubin Observatory will be well suited to the task when it begins its later this year; some back-of-the-napkin math by Davenport suggests that Rubin could find 100 new impacts over the next 10 years. That could ultimately help narrow the search for habitable worlds outside our solar system.

“How rare is the event that created the Earth and moon? That question is fundamental to astrobiology,” Davenport said. “It seems like the moon is one of the magical ingredients that makes the Earth a good place for life. It can help shield Earth from some asteroids, it produces ocean tides and weather that allow chemistry and biology to mix globally, and it may even play a role in driving tectonic plate activity. Right now, we don’t know how common these dynamics are. But if we catch more of these collisions, we’ll start to figure it out.”

For more information, contact Tzanidakis at atzanida@uw.edu and Davenport at jrad@uw.edu.

This research was funded by Breakthrough Initiatives.

By their own admission, and are interested in unusual stars. The ����Ӱ�Ӵ�ý astronomers were on the lookout for “stars behaving strangely” when an automated alert from the survey pointed them to Gaia17bpp. Survey data indicated that this star had gradually brightened over a 2 1/2-year period.

As Tzanidakis reported on Jan. 10 at the in Seattle, follow-up analyses indicated that Gaia17bpp itself wasn’t changing. Instead, the star is likely part of a rare type of binary system, and its apparent brightening was the end of a years-long eclipse by an unusual stellar companion.

“We believe that this star is part of an exceptionally rare type of binary system, between a large, puffy older star — Gaia17bpp — and a small companion star that is surrounded by an expansive disk of dusty material,” said Tzanidakis, a UW doctoral student in astronomy. “Based on our analysis, these two stars orbit each other over an exceptionally long period of time — as much as 1,000 years. So, catching this bright star being eclipsed by its dusty companion is a once-in-a-lifetime opportunity.”

Since the Gaia spacecraft’s observations about the star only went back to 2014, Tzanidakis and Davenport, a UW research assistant professor of astronomy and associate director of the , had to do a little detective work to reach this conclusion. First, they stitched together Gaia’s observations of the star with observations by other missions stretching back to 2010 — including , / and the .

Those observations, coupled with the Gaia data, showed that Gaia17bpp dimmed by about 4.5 magnitudes — or roughly 63 times. The star remained dim over the course of nearly seven years, from 2012 to 2019. The brightening that the Gaia survey had uncovered was the end of that seven-year dim.

No other stars near Gaia17bpp showed similar dimming behavior. Through the program, a digital catalog of more than a century’s worth of astro-photographic plates at Harvard, Tzanidakis and Davenport analyzed observations of the star stretching back to the 1950s.

“Over 66 years of observational history, we found no other signs of significant dimming in this star,” said Tzanidakis.

The two believe that Gaia17bpp is part of a rare type of binary star system, with a stellar companion that is — quite simply — dusty.

“Based on the data currently available, this star appears to have a slow-moving companion that is surrounded by a large disk of material,” said Tzanidakis. “If that material were in the solar system, it would extend from the sun to Earth’s orbit, or farther.”

During its eclipse, the unseen companion was blocking about 98% of Gaia17bpp’s light, according to Davenport.

A handful of other similar, “dusty” systems have been identified over the years, most notably Epsilon Aurigae, a star in the constellation Auriga that is eclipsed for two out of every 27 years by a relatively large, dim companion. The system that Tzanidakis and Davenport discovered is unique among these few dusty binaries in the length of the eclipse — at nearly seven years, it is by far the longest. Unlike the Epsilon Aurigae binary, Gaia17bpp and its companion are also so far apart that it would be centuries or more before an astute observer on Earth witnesses another such eclipse.

For Epsilon Aurigae and similar systems, the identity of the dusty companion is a matter of debate. Some preliminary data indicate that Gaia17bpp’s companion could be a small, massive white dwarf star. The source of its debris disk is also a mystery.

“This was a serendipitous discovery,” said Tzanidakis. “If we had been a few years off, we would’ve missed it. It also indicates that these types of binaries might be much more common. If so, we need to come up with theories about how this type of pairing even arose. It’s definitely an oddity, but it might be much more common than anyone has appreciated.”

Additional team members on this study are , a UW research assistant professor of astronomy, and , a UW graduate student in astronomy.

For more information, contact Tzanidakis at atzanida@uw.edu and Davenport at jrad@uw.edu.

NOTE: A previous version of this press release mis-reported the degree of dimming and brightening that Gaia17bpp underwent from 2012 to 2019.

“We’ve known these are big flares, much larger than we see on our own sun,” said co-author , a research assistant professor of astronomy at the ����Ӱ�Ӵ�ý. “Now we see superflares occur at high latitudes, near the ‘poles’ of the star, which means that the bursts of radiation are not directed toward the paths of orbiting exoplanets.”

Flares are magnetic explosions on the surface of stars that expel intense electromagnetic radiation into space. Large flares like superflares emit a cascade of energetic particles that can hit exoplanets orbiting the flaring star, and in the process alter or even evaporate planetary atmospheres.

Using optical observations from the Transiting Exoplanet Survey Satellite — or TESS — the team, led by astronomers at the Leibniz Institute for Astrophysics Potsdam, studied large superflares on red dwarfs, a class of young, small stars that have a lower temperature and mass than our own sun.

Many exoplanets have been found around these types of stars. A lingering question in exoplanet research has been whether these exoplanets are habitable, since red dwarfs are more active than our sun and flare much more frequently and intensely.

The team developed a method to determine the location on the stars’ surface where flares originate. The team achieved this by analyzing so-called “whitelight flares” on fast-rotating red dwarf stars. These types of flares last long enough that their brightness, as observed by TESS, varies as they rotate in and out of view on the stellar surface.

“Since we can’t see the surfaces of these stars, determining the latitudes of things like hot flares and cool spots has traditionally been between difficult and impossible!” said Davenport, who is also the associate director of the Data Intensive Research in Astrophysics and Cosmology — or — Institute at the UW. “This work combines clever data modeling with the uniquely precise data that comes from missions like TESS, and finds something remarkable.”

The team found rotating flares by processing the light curves of more than 3,000 red dwarf stars by TESS. Among these stars, they found four with flares large enough for their new method. The team used the precise shape of each star’s light curve to infer the latitude of the flaring region, and found that all four flares occurred above approximately 55 degrees latitude, which is much closer to the pole than flares and spots on the surface of our sun, which usually occur below 30 degrees latitude. The team also showed that their detection method was not biased toward a particular stellar latitude.

These findings, even with only four flares, are significant: If flares were spread equally across the stellar surface, the chances of finding four flares in a row at such high latitudes would be about 1-in-1,000.

This has implications for models of the magnetic fields of stars and for the habitability of exoplanets that orbit them.

“We discovered that extremely large flares are launched from near the poles of red dwarf stars, rather than from their equator, as is typically the case on the Sun,” said lead author Ekaterina Ilin, a doctoral student at Leibniz. “Exoplanets that orbit in the same plane as the equator of the star, like the planets in our own solar system, could therefore be largely protected from such superflares, as these are directed upwards or downwards out of the exoplanet system. This could improve the prospects for the habitability of exoplanets around small host stars, which would otherwise be much more endangered by the energetic radiation and particles associated with flares compared to planets in the solar system.”

The detection of these flares is further evidence that strong and dynamic concentrations of stellar magnetic fields, which can manifest themselves as dark spots and flares, form close to the rotational poles of fast-rotating stars. The existence of such “polar spots” has long been suspected from indirect reconstruction techniques like Doppler Imaging of stellar surfaces, but has not been detected directly so far.

“Nature is telling us something important about how these little, typically young stars produce magnetic fields that are much stronger than our sun,” said Davenport. “That has huge implications for how we think about the planets that orbit them.”

Co-authors are Katja Poppenhaeger, Sarah Schmidt, Silva Järvinen, Julián Alvarado-Gómez and Ilya Ilyin at the Leibniz Institute for Astrophysics Potsdam; Elisabeth Newton at Dartmouth College; Sebastian Pineda at the University of Colorado Boulder; and Mahmoudreza Oshagh of the Canary Islands Astrophysics Institute and the University of La Laguna in Spain. Ilin and Poppenhaeger also have appointments at the University of Potsdam. The research was funded by NASA, the German National Scholarship Foundation, the Leibniz Association and the UW.

For more information, contact Davenport at jrad@uw.edu.

Adapted from a by the Leibniz Institute for Astrophysics Potsdam.

And sometimes those changes unfold over the course of a few generations.

That is what a team of astronomers from the ����Ӱ�Ӵ�ý, Western Washington University and the University of California, Irvine discovered when they analyzed more than 125 years of astronomical observations of a nearby stellar binary called HS Hydrae. This system is what’s known as an eclipsing binary: From Earth, the two stars appear to pass over one another — or eclipse one another — as they orbit a shared center of gravity. The eclipses cause the amount of light emitted by the binary to dim periodically.

On Jan. 11 at the , the team reported more than a century’s worth of changes to the eclipses by the stars in HS Hydrae. The two stars began to eclipse in small amounts starting around a century ago, increasing to almost full eclipses by the 1960s. The degree of eclipsing then plummeted over the course of just a half century, and will cease around February 2021.

“There is a historical record of observations of HS Hydrae that essentially spans modern astronomy — starting with photographic plates in the late 19th century up through satellite images taken in 2019. By diving into those records, we documented the complete rise and fall of this rare type of eclipsing binary,” said team leader , a research assistant professor of astronomy at the UW and associate director of the UW’s .

The eclipses of the two stars that make up HS Hydrae are changing because another body — most likely a third, unobserved companion star — is turning the orientation of the binary with respect to Earth. Systems like this, which are called evolving eclipsing binaries, are rare, with only about a dozen known to date, according to Davenport. Identifying this type of binary requires multiple observations to look for long-term changes in the degree of dimming, which would indicate that the orientation of the binary is changing over time.

HS Hydrae has such an observational record because, at 342 light- years away, it is a relatively close and bright system and the two stars orbit each other every 1.5 days. Scientists first reported that HS Hydrae was an eclipsing binary in 1965. In a 2012 , astronomers based in Switzerland and the Czech Republic reported that the amount of dimming from HS Hydrae decreased from 1975 through 2008, indicating that the two stars were eclipsing smaller and smaller portions of one another over time. That team also predicted that the eclipses would end around 2022.

Davenport and his team checked in on HS Hydrae using observations of the system in 2019 by the NASA’s , or TESS. They saw only a 0.0075-magnitude drop in light from HS Hydrae, a sign that the two stars were barely covering one another during eclipses. For comparison, eclipses in 1975 saw a more than 0.5-magnitude drop.

“Fifty years ago, these two stars were almost completely eclipsing each other. By the early 21st century, the degree of eclipse was around 10%, and in the most recent observations from 2019, they barely overlapped,” said Davenport.

With these new data, the team now predicts that HS Hydrae eclipses will cease around February 2021.

The observations from the 1960s through 2019 catalogue the decline of HS Hydrae as an evolving eclipsing binary. But Davenport and his team also uncovered evidence for its rise. The , or DASCH, is a digital catalog of photometric data taken from more than a century’s worth of astro-photographic plates at Harvard University. The team mined this record and found observations of HS Hydrae from 1893 through 1955 that they could analyze to search for signs of dimming.

The researchers broke down DASCH observations of HS Hydrae by decade. From the late 19th century through the roaring ’20s, HS Hydrae showed no measurable dimming. But things began to change in the 1930s, where they measured a modest 0.1-magnitude drop in brightness. The degree of dimming rose through the 1940s and peaked in the 1950s with a 0.5-magnitude drop in brightness.

Based off this 126-year history of HS Hydrae observations, the team predicts that the system will start eclipsing again around the year 2195. But, that assumes that the third companion — which other teams have predicted is a small, dim M-dwarf star — continues to behave as it has to date.

“We won’t know for sure unless we keep looking,” said Davenport. “The best we can say right now is that HS Hydrae has been changing constantly over the course of modern astronomy.”

Missions like TESS will likely identify more evolving eclipsing binaries in the coming years. This should open new opportunities for astronomers to understand how star systems are built, as well as how they change over time — whether they are busy, dynamic systems like HS Hydrae, or more quiet systems, like ours.

Co-authors on the paper are UW graduate students and ; UW researcher Karen Warmbein; Erin Howard at Western Washington University; and Courtney Klein at UC Irvine. The research was funded by NASA, the National Science Foundation, the Heising-Simons Foundation, the Research Corporation for Science Advancement, the DIRAC Institute, the UW Department of Astronomy, the Charles and Lisa Simonyi Fund for Arts and Sciences and the Washington Research Foundation.

For more information, contact Davenport at jrad@uw.edu.

That’s the view of a new paper by Louisiana State University astronomer and scores of co-authors — including astronomers and from the ����Ӱ�Ӵ�ý. The paper has been accepted for publication in .

KIC 8462852 is nicknamed “Tabby’s Star” after Boyajian, who discovered it with the help of citizen scientists. The star has intrigued astronomers with its irregular, unexplained dips in light of up to about 22 percent. It’s an otherwise average star, about 1,250 light-years away, in the constellation Cygnus, and is about 40 percent more massive than the sun and about four times brighter.

The unusual dimming spawned many to explain the star’s behavior, with some hoping against all odds that the star might be orbited by a massive superstructure to harness energy  — the work of an advanced civilization. But observations from March 2016 to December 2017 from the , a network of robotic telescopes, indicate the explanation is likely more prosaic than that: Space dust.

“Dust is most likely the reason why the star’s light appears to dim and brighten,” said Boyajian, an assistant professor of astronomy and physics at LSU. “The new data shows that different colors of light are being blocked at different intensities. Therefore, whatever is passing between us and the star is not opaque, as would be expected from a planet or alien megastructure.”

In May of 2017, Boyajian saw that the star was again beginning to dim, and to fellow researchers to observe the star immediately. That’s where Morris and Davenport came in.

“Our involvement in this project was to be immediate responders,” said Morris, who is a UW doctoral student in astronomy. “I frequently observe on the ARC 3.5 m Telescope at Apache Point Observatory, and was able to secure a bit of time to observe at the beginning of several dips.”

Davenport, who is a post-doctoral researcher at both the UW and Western Washington University, alerted Morris to Boyajian’s call to action. Morris said he “pleaded” with those scheduled to use the telescope that late that night to get a few minutes of observing time. “At first maybe they thought I was an over-enthusiastic graduate student or a conspiracy theorist,” he said, but when he explained to them the importance of the moment, they agreed.

“Our observations were within the first three high-resolution spectra to be taken of the star after the dimming began,” Morris said. He spent the morning , where, he said, “many open science conversations ensued.” Boyajian proudly showed Twitter followers their first response in a proclaiming “Day-tah!”

Davenport, who did on Tabby’s Star in late 2017 and was among those who Boyajian notified, said his contribution was mostly alerting Morris, whose expertise in observational astronomy made him “the perfect person for doing the follow-up.” Davenport also assisted in analyzing data.

“Tabby’s Star” was first seen to perform its dips by the Kepler Space Telescope in 2015. Davenport noted that more than a year passed before the anomalies in the data were noticed. “We may go back and find that this kind of event is occurring in lots of our data,” he said.

Boyajian praised the citizen scientists and Planet Hunters, who were the ones to detect the star’s unusual behavior in first place.

“If it wasn’t for people with an unbiased look on our universe, this unusual star would have been overlooked,” she said, adding. “Without the public support for this dedicated observing run, we would not have this large amount of data.”

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This was based in part on a by Louisiana State University. For more information on their work, contact Morris at bmmorris@uw.edu or Davenport at James.Davenport@wwu.edu.