Astronomie - Dimming of Tabby’s Star likely caused by something less sexy than aliens


"There is a cloud of dust orbiting the star with a roughly 700-day orbital period."


In recent years, a distant star in the constellation Cygnus, known officially as KIC 8462852 and unofficially as Tabby's Star, has intrigued astronomers due to its irregular but significant dimming. As the star has faded by as much as 20 percent and then returned to its normal brightness, scientists were forced to admit that they didn't have a good explanation for this behavior.

The theories they put forth to explain the dimming included some pretty exotic notions, such as some kind of super-large alien megastructure that passed in front of the star. Alas, astronomers using NASA's Swift and Spitzer space-based observatories have come up with a much more prosaic explanation for the phenomenon—dust.

new study in The Astrophysical Journal analyzed light data gathered across a broad swath of wavelengths collected between October, 2015 and December, 2016. The researchers found that the dimming effect was less pronounced in the infrared wavelengths of light than the ultraviolet light. This finding is consistent with the idea that dust particles would scatter more of the shorter wavelengths of ultraviolet light, while more of the longer wavelength infrared light would make it through a dust cloud.

"This pretty much rules out the alien megastructure theory, as that could not explain the wavelength-dependent dimming," said Huan Meng, at the University of Arizona, Tucson, who is lead author of the new study. "We suspect, instead, there is a cloud of dust orbiting the star with a roughly 700-day orbital period."

NASA's Swift instrument, launched in 2004, was designed to study gamma ray bursts across a large portion of the spectrum. NASA's Spitzer Space Telescope, launched in 2003, is highly sensitive to infrared light. To round out their observations, scientists also used data from AstroLAB IRIS, a public observatory with a 68cm reflecting telescope in Belgium.

Based upon the new data, circumstellar dust—dust that orbits a star—is now the best explanation for Tabby's Star. This dust is large enough to remain in orbit around the star but not so large that it uniformly blocks light from all wavelengths. Around our own Sun, this dust causes the zodiacal light that can be seen on very dark nights near the horizon.

Quelle: arsTechnica


Mysterious Dimming of Tabby's Star May Be Caused by Dust 

This illustration depicts a hypothetical uneven ring of dust orbiting KIC 8462852
This illustration depicts a hypothetical uneven ring of dust orbiting KIC 8462852, also known as Boyajian’s Star or Tabby's Star.
Credits: NASA/JPL-Caltech

One of the most mysterious stellar objects may be revealing some of its secrets at last.

Called KIC 8462852, also known as Boyajian’s Star, or Tabby's Star, the object has experienced unusual dips in brightness -- NASA's Kepler space telescope even observed dimming of up to 20 percent over a matter of days. In addition, the star has had much subtler but longer-term enigmatic dimming trends, with one continuing today. None of this behavior is expected for normal stars slightly more massive than the Sun. Speculations have included the idea that the star swallowed a planet that it is unstable, and a more imaginative theory involves a giant contraption or "megastructure" built by an advanced civilization, which could be harvesting energy from the star and causing its brightness to decrease.

A new study using NASA's Spitzer and Swift missions, as well as the Belgian AstroLAB IRIS observatory, suggests that the cause of the dimming over long periods is likely an uneven dust cloud moving around the star. This flies in the face of the "alien megastructure" idea and the other more exotic speculations.


The smoking gun: Researchers found less dimming in the infrared light from the star than in its ultraviolet light. Any object larger than dust particles would dim all wavelengths of light equally when passing in front of Tabby's Star.


"This pretty much rules out the alien megastructure theory, as that could not explain the wavelength-dependent dimming," said Huan Meng, at the University of Arizona, Tucson, who is lead author of the new study published in The Astrophysical Journal. "We suspect, instead, there is a cloud of dust orbiting the star with a roughly 700-day orbital period."


Why Dust is Likely 


We experience the uniform dimming of light often in everyday life: If you go to the beach on a bright, sunny day and sit under an umbrella, the umbrella reduces the amount of sunlight hitting your eyes in all wavelengths. But if you wait for the sunset, the sun looks red because the blue and ultraviolet light is scattered away by tiny particles. The new study suggests the objects causing the long-period dimming of Tabby's Star can be no more than a few micrometers in diameter (about one ten-thousandth of an inch).


From January to December 2016, the researchers observed Tabby's Star in ultraviolet using Swift, and in infrared using Spitzer. Supplementing the space telescopes, researchers also observed the star in visible light during the same period using AstroLAB IRIS, a public observatory with a 27-inch-wide (68 centimeter) reflecting telescope located near the Belgian village of Zillebeke.


Based on the strong ultraviolet dip, the researchers determined the blocking particles must be bigger than interstellar dust, small grains that could be located anywhere between Earth and the star. Such small particles could not remain in orbit around the star because pressure from its starlight would drive them farther into space. Dust that orbits a star, called circumstellar dust, is not so small it would fly away, but also not big enough to uniformly block light in all wavelengths. This is currently considered the best explanation, although others are possible.


Collaboration with Amateur Astronomers


Citizen scientists have had an integral part in exploring Tabby's Star since its discovery. Light from this object was first identified as "bizarre" and "interesting" by participants in the Planet Hunters project, which allows anyone to search for planets in the Kepler data. That led to a 2016 study formally introducing the object, which is nicknamed for Tabetha Boyajian, now at Louisiana State University, Baton Rouge, who was the lead author of the original paper and is a co-author of the new study. The recent work on long-period dimming involves amateur astronomers who provide technical and software support to AstroLAB.


Several AstroLAB team members who volunteer at the observatory have no formal astronomy education. Franky Dubois, who operated the telescope during the Tabby's Star observations, was the foreman at a seat belt factory until his retirement. Ludwig Logie, who helps with technical issues on the telescope, is a security coordinator in the construction industry. Steve Rau, who processes observations of star brightness, is a trainer at a Belgian railway company.


Siegfried Vanaverbeke, an AstroLAB volunteer who holds a Ph.D. in physics, became interested in Tabby's Star after reading the 2016 study, and persuaded Dubois, Logie and Rau to use Astrolab to observe it.


"I said to my colleagues: 'This would be an interesting object to follow,'" Vanaverbeke  recalled. "We decided to join in."


University of Arizona astronomer George Rieke, a co-author on the new study, contacted the AstroLAB group when he saw their data on Tabby's Star posted in a public astronomy archive. The U.S. and Belgium groups teamed up to combine and analyze their results.


Future Exploration


While study authors have a good idea why Tabby's Star dims on a long-term basis, they did not address the shorter-term dimming events that happened in three-day spurts in 2017. They also did not confront the mystery of the major 20-percent dips in brightness that Kepler observed while studying the Cygnus field of its primary mission. Previous research with Spitzer and NASA's Wide-field Infrared Survey Explorer suggested a swarm of comets may be to blame for the short-period dimming. Comets are also one of the most common sources of dust that orbits stars, and so could also be related to the long-period dimming studied by Meng and colleagues.


Now that Kepler is exploring other patches of sky in its current mission, called K2, it can no longer follow up on Tabby's Star, but future telescopes may help unveil more secrets of this mysterious object.


"Tabby's Star could have something like a solar activity cycle. This is something that needs further investigation and will continue to interest scientists for many years to come," Vanaverbeke said.


NASA's Goddard Space Flight Center in Greenbelt, Maryland, manages the Swift mission in collaboration with Pennsylvania State University in University Park, the Los Alamos National Laboratory in New Mexico, and Orbital Sciences Corp. in Dulles, Virginia. Other partners include the University of Leicester and Mullard Space Science Laboratory in the United Kingdom, Brera Observatory and the Italian Space Agency in Italy, with additional collaborators in Germany and Japan.

Quelle: NASA


Update: 4.01.2017


No alien megastructure around mysterious 'Tabby's star', analysis shows

Stand down space cadets: there is (sadly) no alien megastructure around star KIC 8462852, also known as Tabby’s star

As if a divisive Star Wars film wasn’t bad news enough this Christmas, now an analysis by more than 200 astronomers has been published that shows the mysterious dimming of star KIC 8462852 is not being produced by an alien megastructure.

The evidence points most strongly to a giant cloud of dust occasionally obscuring the star. The cloud was most possibly produced by the collision of two comets or the break-up of a single one. Another option is that the star itself is undergoing some sort of internal convulsion that astronomers have never seen before.

KIC 8462852 is approximately 1,500 light years away from the Earth and hit the headlines in October 2015 when data from Nasa’s Kepler space telescope showed that it was dimming by unexplainably large amounts. The star’s light dropped by 20% first and then 15% making it unique. Even a large planet passing in front of the star would have blocked only about 1% of the light. 

For an object to block 15-20%, it would have to be approaching half the diameter of the star itself. With this realisation, a few astronomers began whispering that such a signal would be the kind expected from a gigantic extraterrestrial construction orbiting in front of the star – and the idea of the alien megastructure was born.

Tabetha Boyajian, then at Yale University and now at Louisiana State University, led the investigations into the mysterious signals. It was after her that the star was nicknamed Tabby’s star. She said at the time that a constant monitoring programme was needed to watch the star for more dips.

Today’s new analysis is the result of that programme. It was funded by a Kickstarter campaign that attracted support from more than 1,700 people and raised more than $100,000. In partnership with the Las Cumbres Observatory, a privately-funded organisation that operates 21 telescopes at eight sites around the world, the astronomers monitored the star from March 2016 to December 2017.

In that time they recorded four more dips, though none were as dramatic as the originals, reaching only a percent or two. These were named by the Kickstarter supporters as Elsie and Celeste, Skara Brae and Angkor. Significantly for the analysis, the dimming events were captured at multiple wavelengths.

If an alien megastructure had been causing the drop in light, being a solid object, it would block all wavelengths in the same way, at the same time. That’s not what the team saw. Instead, different wavelengths of light dropped by different amounts. This is exactly what you would expect from starlight passing through a tenuous dust cloud.

It happens because dust grains scatter light depending on its wavelength. Blue light, which has short wavelengths, is scattered more easily than red, which has longer wavelengths. This is why our sky is blue; that colour has been scattered out of the sun’s direct light by the molecules in the air.

In the case of Tabby’s star, the new observations show that it dims more at blue wavelengths than red. Thus, its light is passing through a dust cloud, not being blocked by an alien megastructure in orbit around the star (#sadface).