The Basic Background Facts

KIC 8462852 at the center of the image (visualized with Aladin)

What the Planet Hunters Found





KIC 8462852 Kepler Light Curve reported by Boyajian, et.al: http://arxiv.org/abs/1509.03622

What you are seeing in this chart is a plot of the Kepler measurement of the brightness of Tabby's star (with the normal brightness equal to 1) over the roughly four years the telescope was staring at the star. The downward spikes at around Day 800 (henceforth called the D800 dip) and just after Day 1500 (D1500 dip) are real data, not mistakes, and they are much larger than the normal dips in magnitude observed when a planet transits a large star - typically close to 1%. Two of these dips (numbered 5 and 8) are around 20%, which is only achievable by an object more star sized than planet sized. Dips 9 and 10 are also considerably larger than any planetary transit. There are smaller dips as well, and the B2015 paper identifies the 10 biggest events in Table 1 of that paper. Here's a zoomed-in view of the same graph, in which the ten events are shown:









Normalized Kepler data for KIC 8462852 with dipping events numbered.





Another important thing to note is the time scale, which you can see for yourself by examining Figure 1 in B2015. The events took place on a time scale of days, and some of them about 10 days. This is a big part of the puzzle.



I would also note something else I don't know how to interpret, but the team found some evidence that a 48.4 day period is involved. Something orbiting at that period would be about 44 million kilometers from the star, which is quite close. Something that close to such a bright star would be hot, with an equilibrium temperature of 1070 degrees Kelvin, if I did my arithmetic right, and would glow fairly brightly in the infrared at a wavelength of about 3 microns. As we'll see, no evidence of an object emitting infrared light around that wavelength has been found. Based on its temperature (which can be derived from its color) and type, Tabby's Star is probably more than 2 million kilometers in diameter - more than 50% larger than our sun. By comparison, Jupiter is 138,350 km in diameter - so even a large planet would barely cause a dip in luminosity - a few percent at most. Whatever passed in front of Tabby's Star was big - the size of a small star itself, and yet no other close star is in evidence.Another important thing to note is the time scale, which you can see for yourself by examining Figure 1 in B2015. The events took place on a time scale of days, and some of them about 10 days. This is a big part of the puzzle.I would also note something else I don't know how to interpret, but the team found some evidence that a 48.4 day period is involved. Something orbiting at that period would be about 44 million kilometers from the star, which is quite close. Something that close to such a bright star would be hot, with an equilibrium temperature of 1070 degrees Kelvin, if I did my arithmetic right, and would glow fairly brightly in the infrared at a wavelength of about 3 microns. As we'll see, no evidence of an object emitting infrared light around that wavelength has been found.





The Planet Hunters Science Team's Follow Up





The science team followed up the Planet Hunter's discovery by asking all the obvious questions. Could the data be in error? Are we looking at two or more stars instead of one? Is there something unusual about this star? Does the electromagnetic energy coming from the star give us a clue as to what could cause the dips? It turns out there wasn't a whole lot of astronomical literature on this particular star. There are after all, billions of stars that telescopes can see, and most have not been the subject of close scrutiny.



The first thing they did was to check with the Kepler team. Had they seen anything like this before? Could it be something wrong with the telescope, or the sophisticated instrument on the telescope's focal plane that measured the brightness of the stars? the Kepler scientists took yet another close look, and shook their heads - the data was real, and wasn't showing up in other star's light curves, as you would expect if it were a problem with the instrument. The Kepler scientists could find nothing wrong with the data - they believe the brightness curve - sharp dips and all - is real. There is a lot of information packed into the 17 pages of B2015 , and I am going to try and unpack it for you, although I am leaving out many details and interesting wrinkles.The science team followed up the Planet Hunter's discovery by asking all the obvious questions. Could the data be in error? Are we looking at two or more stars instead of one? Is there something unusual about this star? Does the electromagnetic energy coming from the star give us a clue as to what could cause the dips? It turns out there wasn't a whole lot of astronomical literature on this particular star. There are after all, billions of stars that telescopes can see, and most have not been the subject of close scrutiny.The first thing they did was to check with the Kepler team. Had they seen anything like this before? Could it be something wrong with the telescope, or the sophisticated instrument on the telescope's focal plane that measured the brightness of the stars? the Kepler scientists took yet another close look, and shook their heads - the data was real, and wasn't showing up in other star's light curves, as you would expect if it were a problem with the instrument. The Kepler scientists could find nothing wrong with the data - they believe the brightness curve - sharp dips and all - is real.





The next thing the team did was take a closer look at the data. They tried more sophisticated analysis techniques to try to understand if anything in this complex pattern repeated. If you have multiple things repeating at different intervals, they can "beat" against each other and cause patterns that at first glance have too fast or too slow a rhythm.





They found that there is a strong component at a 0.88 day period, which the B2015 team argue strongly is caused by the rotation rate of the star itself. Other data in the observational follow-up is consistent with this. Some other interesting bumps were noticed in the frequencies, including a 10-20 day pulsation, but no one knows exactly how to interpret them. They may just be natural variations in the star's brightness.





Following up with ground based telescopes





The B2015 team were mainly interested in detailed analysis of the star's light, using the science of



One thing they learned from the spectroscopy was that the star is in fact an unremarkable star, of Boyajian's team followed up the Kepler observations with ground based telescope observations. An examination of observations with the United Kingdom Infrared Telescope (UKIRT ) revealed a tiny speck that may or not be a small companion star. Images from the giant Keck telescope reveal that there is another, fainter star apparently close to Tabby's Star, but it is not clear if it is a distant companion or just another star passing close to the same line of sight. Observations were able to rule a close companion or a bright companion, but it is possible that a red dwarf star circles Tabby's star at a distance.The B2015 team were mainly interested in detailed analysis of the star's light, using the science of spectroscopy , which is crucial to our entire understanding of the universe. Spectroscopy can not only tell us what sort of elements stars are made of, but since we know what the light spectrum looks like from laboratory work, it can also tell us how fast that material is moving either towards or away from us, since the wavelengths shift up (away) or down (towards) due to the Doppler effect . Over many generations of studying spectra, astronomers have learned to infer a great deal about a star from its spectrum and measurement of the Doppler effect on it.One thing they learned from the spectroscopy was that the star is in fact an unremarkable star, of a fairly bright, yellow-white class called F , which constitutes about 3% of main sequence stars. The star appears to be rotating fairly fast, with a period of 0.88 days (compare to our Sun's 25 days). They were able to estimate the mass and size of the star, and determine that it is likely to be neither a very young F star, nor very old. If it is in the middle of its life cycle, its brightness should be steady.

Radial Velocity Measurements



Although no one thinks it's at all likely anyway, it can show that the star is not moving at ridiculously high velocities towards or way from us, which could have weird effects.

If Tabby's star has a large, dark companion that orbits closely, you would expect to see wobbles in the radial velocity. The radial velocity measurements performed on Tabby's Star were not the most accurate possible (you need a bigger telescope and more time), but they were good enough to show that there is no large, dark companion close to Tabby's star, and that its movement through space is nothing extraordinary. We can't however, rule out a big dark companion object further away from the star, but it could not cause both sets of dips, since it would take it too long to orbit the star. Radial velocity is the movement toward or away from an observer, and can be determined from looking at the spectrum of light coming from a star or galaxy. It is the same technique Edwin Hubble used in the early 20th century to show that the universe is expanding. The radial velocity can do couple of things for us:The radial velocity measurements performed on Tabby's Star were not the most accurate possible (you need a bigger telescope and more time), but they were good enough to show that there is no large, dark companion close to Tabby's star, and that its movement through space is nothing extraordinary. We can't however, rule out a big dark companion object further away from the star, but it could not cause both sets of dips, since it would take it too long to orbit the star.

Infrared Excess



The heat emission spectrum from an object at 300 degrees Kelvin, from Wolfram Alpha If some solid (or liquid) object is absorbing 20% of the energy of the light from Tabby's star in our line of sight, it should heat up, and then basic physics says that energy will be emitted in infrared light, which is just light with a longer wavelength than the human eye can see. So, if we plot the energy we're seeing vs. wavelength of light, then we should see the normal light from Tabby's star, and then a bump out in the infrared, or even in what are called millimeter waves if the object is far from the star and thus not getting as hot. The "bump" is what is called an "infrared excess", and the B2015 team looked for one in the space telescope data available to them, and with a ground based telescope. They did not see any excess.





The B2015 team and others looked in the survey data from the WISE infrared space telescope, and later followed up with data from the Spitzer Space Telescope and with both an infrared and a millimeter wave telescope on Mauna Kea, and found no clear evidence of anything absorbing the light from Tabby's star and re-emitting it as waste heat. There were some little hints that something might be there in the Spitzer data at 4.5 microns wavelength, but we shouldn't get our hopes up.Where we would expect to see the infrared spectrum peak depends on your hypothesis about how close to the star the absorber is.

B2015 examines some possible explanations

So, what could explain the dips, and everything we know about them? The lack of an infrared excess ruled out large amounts of solid matter passing in front of a star, as you might have resulting from a major planetary catastrophe, such as two big planets colliding. Also, if this were the case, you would expect the first dip to be much messier, and it is nice and sharp, although asymmetric. The second set of dips is more complex, though.





The B2015 team examined all the more obvious explanations:

the star itself exhibits variability

big clumps of dust orbiting the star

catastrophic collisions

planets in the process of formation, possibly with very large ring systems

a swarm of large comets

Earlier this year, Brad Schaefer stated that Ben Montet was working on the question of secular fading over the four years of Kepler primary mission data when Tabby's Star was visible, and that he was seeing fading. A preprint came out last night confirming this , and in fact the fading was quite dramatic at times. There are lots of questions, and I suppose there will be controversy, but it's quite important if it holds up. We'll have more soon.Expecting data soon from the the Kickstarter funded observations by the LCOGT. Stand by...When it comes to Tabby's Star (also known as KIC 8462852), we are all perplexed. This post is for those who are disinclined to read technical papers by professional astronomers, but would like to know just what the heck is going on. What is all this stuff about alien megastructures, swarms of giant comets, infrared excess, and old photographic plates? We'll lay all that out here for you in non-technical terms (or we'll explain the terms as we go). Please, if there are any questions, ask in the comments below, and we'll try and figure out an answer, if there is one. The post is richly hyperlinked, so if you want more detail, you can easily find it. I hope I have given credit wherever it is due.Let me start by stating up front, thatknows exactly what is going on with this star. What we'll try to lay out here is why this otherwise ordinary star is strange. If you have questions, or find errors, or know of updates I should include, please leave a comment here.First, a few basic facts we should all know. Stars vary quite a lot in size, brightness and temperature, but most are small and relatively cool and burn for tens of billions of years. A smaller population of stars are like our own sun, which is called a G class star, and some stars are even bigger and brighter than the sun. Tabby's star (an informal name - it's known in some star catalogs as TYC 3162-665-1 or KIC 8462852 ) is one such, and is called an F class star - a bit bigger, quite a bit hotter, and considerably brighter than our sun. It is just under 1500 light years away (so we are seeing it as it was not quite 1500 years ago), and is in the constellation Cygnus.Stars are born in a collapsing cloud of gas and dust, they burn steadily for a while, and then they die. While they are burning steadily, they are called Main Sequence stars, and when they die they change size and color and can vary quite a lot in brightness . Astronomers can study the light from a star and determine the approximate age, just as your vet can tell about how old a cat is by looking at his teeth. Tabby's star is neither very young or very old - it is a main sequence star, and should burn steadily.It turns out that Tabby's Star was one of the many stars that the planet-hunting Kepler Space telescope was staring at for about four years (starting in 2009) in an effort to find planets around other stars, or exoplanets. Kepler is able to find exoplanets because for some subset of those stars it is keeping vigil on, its planets will pass across the face of the star from our vantage point, and we will see a very slight dimming of the star when this happens. Although the resulting variation in brightness is subtle, it should repeat in a rhythm that is a fingerprint for such transits, and this fingerprint can be recognized by careful analysis. More exoplanets have been discovered with Kepler than any other telescope - 1039 confirmed exoplanets at this writing. Big exoplanets are easier to detect than small ones, and planets close to their star with shorter period orbits repeat their transits more often, and so are easier for Kepler to spot.A problem with the Kepler spacecraft in 2013 meant that it can no longer stare at the same patch of sky that it did at first, so it is no longer keeping watch on Tabby's star. However, the data produced by Kepler continues to be analyzed by astronomers, and notably, by a group of citizen scientists called Planet Hunters . The Planet Hunters study the subtle variations in stellar light curves by eye, augmenting the sophisticated computer analysis used to detect many of the exoplanet candidates Kepler has found, and flag unusual events in the light curves for follow up by the science team.