# Thread: Up to 22% Dips Detected in Starlight 1,500 LY Away

1. Originally Posted by kzb
Yes I'm beginning to "see the light". I don't fully understand though, because the energy can't just disappear. It must be either reflected, absorbed or pass straight through.
Reflection doesn't mean reflection in our direction. A diffuse reflector reflects light into a large portion of the sky. Some fraction of the reflected energy is sent to every viewpoint that can see any part of the illuminated surface. We see it block the portion of light that the sun emitted in our direction which it intercepted, we only see it reflect the fraction of intercepted light that it reflected back in our direction.

2. Try looking at it this way: suppose that the planet has a cross-section area A and orbits the star at a distance little 'r'. Let the star have a radius big 'R', and luminosity L.

When the planet passes in front of the star, it blocks an area A of the star's photosphere. The amount of light blocked is

Code:
```               luminosity blocked by planet  =   (area blocked by planet) *  [  (luminosity of star) / (area of photosphere) ]

=   A *  [  (L) / (4*pi*R^2) ]```
Fine. Now, when the planet goes behind the star (or _almost_ behind the star) as seen from the Earth, some of the star's light falls on the portion of the planet which faces us. How much light? Well, as the star's light flies out into space, it spreads out. By the time it reaches the planet's distance, the luminosity of the star is spread out over a sphere with radius 'r', the orbital radius of the planet. The luminosity which falls onto the planet's face is now

Code:
```      luminosity striking planet's face  =   (area of planet) *  [  (luminosity of star) / (area of sphere at orbital radius 'r') ]

=   A *  [  (L) / (4*pi*r^2) ]```
The result is that the amount of energy striking the planet -- and available to bounce off back at Earth -- is smaller by a factor of
Code:
`                             [ (radius of star) / (radius of planet's orbit) ]^2`
which is typically a factor of hundreds to millions.

That's why the blocked light is so much bigger than the back-reflected light.
Last edited by StupendousMan; 2015-Nov-18 at 02:15 AM.

3. kzb
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Thanks for the detailed explanations both of you !

I think what was in my mind was that both the star and the cloud (if that is what it is) are both effectively point sources at our distance, in fact they make up one point source, they can't be distinguished individually. Both star and cloud are effectively shining isotropically.

But you're right, the fraction of a sphere taken up by the cloud at its orbital radius is bound to be the controlling factor.

In fact, I thought later, if my theory were true the full moon would be as bright as the sun, corrected for its albedo. Which it plainly is not. So that's another theory blown out the water.

4. No extraterrestrial laser pulses detected from KIC 8462852

Story here: http://www.spacedaily.com/reports/No...62852_999.html

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Yes, no laser pulses detected from Tabby's Star, using a 50 centimeter telescope, in six observation sessions, each limited to an hour or less, randomly spread over a period of one month. A worthwhile experiment, but searches with larger instruments, over longer periods of time are desirable.
We at least have a sense that we're not being constantly, or very frequently signaled at a light flux greater than 67 photons per meter ^-2, in the wavelength range of 300 to 600 nanometers.

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It seems that Tabby's star has a new mystery for us to solve. The observation records for this star, for the years 1890 to 1989 were gone over carefully. It was found that the star has steadily dimmed by about 16 percent in this time. This is unprecedented in a F star, on the main sequence, which Tabby's Star is held to be.
If we hold to the favorite explanation, disrupted comets blocking the star's light, a great deal of dust would be involved. The equivalent of about 648,000 giant comets would be required to pass in front of the star, each about 200 kilometers (120 miles) in diameter. This would have to continue to our current observations, even though no signs of excess dust have been found in this system.
Alternatively, we could be watching the construction of a Dyson sphere, with 16 percent more starlight collected, and so, blocked from our view, in the course of a century. Given the distance of the star, this would have been happening about 1500 years ago.

link, below to scientific paper on this:

http://arxiv.org/abs/1601.03256

7. That long-term dimming seems to have continued in the 25 years since the last Harvard photographs. I had some data in the B filter from last November which I just checked, B magnitude=12.494 from then, which fits nicely on Schaefer's linear approximation.

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Thanks, ngc3314, I was wondering about that very question, and looking for a current bolometric magnitude for KIC 8462852. From what I can gather, this degree of dimming, in the amount of time concerned, is considered extraordinary in this sort of star.
Last edited by Ross 54; 2016-Jan-14 at 10:51 PM.

9. Originally Posted by Ross 54
It seems that Tabby's star has a new mystery for us to solve. The observation records for this star, for the years 1890 to 1989 were gone over carefully. It was found that the star has steadily dimmed by about 16 percent in this time. This is unprecedented in a F star, on the main sequence, which Tabby's Star is held to be.
If we hold to the favorite explanation, disrupted comets blocking the star's light, a great deal of dust would be involved. The equivalent of about 648,000 giant comets would be required to pass in front of the star, each about 200 kilometers (120 miles) in diameter. This would have to continue to our current observations, even though no signs of excess dust have been found in this system.
Alternatively, we could be watching the construction of a Dyson sphere, with 16 percent more starlight collected, and so, blocked from our view, in the course of a century. Given the distance of the star, this would have been happening about 1500 years ago.
Quite a fascinating thing we are seeing. I am really interested in seeing what is happening. Comets indeed seem unlikely, but an artificial construction seems unlikely as well. For one thing, if you look at the graph at the end of the paper, it appears that it dimmed significantly in 1902 and 1907 but then went back up several years later. That would seem to imply that somebody constructed something really big but then took it apart or something like that. I also wonder, because there is the known variability, some of the plates may have been taken during bright phases and some during dark phases, and I'm not sure how that fits into the curve. The author of that paper doesn't speculate at all on what might be happening.

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16 of 18 data points seem to appear along a gradual, reasonably consistent slope of declining brightness, and appear to amount to a meaningful pattern. Perhaps the observations in 1902 and 1907 happened to be into parts of a Dyson swarm that was more fully completed then the rest, and so, dimmed the light of the star more completely.
These may have the same source as the two largest dips in brightness reported by Boyajian, et al. I will check the dates of the 1902 and 1907 observations to see if they fit the separation in numbers of days of the two large dips detected by the Kepler Space Telescope.

11. We're back to the alien structure thing?
I'll come back in a while.

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I don't find a match between the ~720 days between the two major dips in the Kepler Space Telescope data, for this star and the spacing of the two particularly low data points in the Harvard observations. Given the wide error bars for the dating of the later observations, it's difficult to judge the significance of this result.
Last edited by Ross 54; 2016-Jan-15 at 03:22 AM.

13. Ross 54 wrote this
It was found that the star has steadily dimmed by about 16 percent in this time. This is unprecedented in a F star, on the main sequence, which Tabby's Star is held to be.
And I must ask the question : Are there other studies of long-term behavior of this type of star ? I know the theory don't explain such a variation but what are the evidences ?

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Dr. Schaefer, the author of the new paper, indicates that there are no other known instances of a main sequence F star with a century-long pattern of substantial dimming.
Someone may propose an astrophysical explanation for this, perhaps by questioning the age, status, or classification of the star.

Otherwise, there seems to be a consensus that KIC 8462852 is, in all respects other than its mysterious dimming behaviors, a very ordinary star of its type and age.

15. I passed the question about variability of main-sequence stars on to Brad Schaefer, who says (quoted with permission):

Actually, your question (do other F stars or other main sequence stars fade or brighten on decadal or century time scales?) already has a deinitive answer for many millions of stars. Arlo, and I, and many people have been watching many stars with accurate photometry even going back many decades. And historical records show no substantial intrinsic variability for main sequence stars. But the real definitive work is from Josh Grindlay and the whole DASCH program. They have now scanned and well-measured the >100-year light curves for millions of main sequence stars. And the answer is that main sequence stars never have century-long or decadal fadings or brightenings. This is a pretty sweeping set of conclusions.
This is actually much more than I thought was already known, although such a study was implicit in the digitized Harvard plate collection being done by the DASCH project.

16. That's pretty impressive. It does seem that there is something really strange about that star. It will be interesting to get more data.

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Remarkable, how all these main sequence stars are able to maintain themselves so stably. There were some internet headlines when the Boyajian, et al. paper about KIC 8462852 came out-- 'The strangest star in the galaxy'. It seems they may not have been exaggerating after all.

18. This is fascinating! I love a mystery.....

19. Originally Posted by Ross 54
If we hold to the favorite explanation, disrupted comets blocking the star's light, a great deal of dust would be involved. The equivalent of about 648,000 giant comets would be required to pass in front of the star, each about 200 kilometers (120 miles) in diameter. This would have to continue to our current observations, even though no signs of excess dust have been found in this system.
Alternatively, we could be watching the construction of a Dyson sphere, with 16 percent more starlight collected, and so, blocked from our view, in the course of a century. Given the distance of the star, this would have been happening about 1500 years ago.
I sort of wanted to say that I kind of expect that a mundane physical explanation will be found, but if it turns out to to be something artificial I'll be as excited as anyone else.

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Exciting for sure!

21. Originally Posted by Ross 54
Remarkable, how all these main sequence stars are able to maintain themselves so stably. There were some internet headlines when the Boyajian, et al. paper about KIC 8462852 came out-- 'The strangest star in the galaxy'. It seems they may not have been exaggerating after all.
I can only concur !

And Great answers to my question ROSS & NGC !

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Interesting article from New Scientist, linked below. it discusses, among other things, Dr. Schaefer's reservations about the megastructures explanation for the dimming of KIC 8462852. He saw no short term dimming of the star in the old images, and doubts that even a very advanced civilization could have then proceeded to the point where they could collect a fifth of the star's light in only a century.

It's already been discussed that the instances of short term, conspicuous dimming are very sporadic. Perhaps these were missed before. If this is a Dyson swarm under construction, this dimming could have been less obvious a century ago.
I hesitate to apply a time frame to the construction of a partial Dyson swarm. If this could proceed with automated, self-reproducing construction machines, it might go faster than we might tend to expect.
Dr. Schaefer also thought that a Dyson swarm should reveal itself to us by the infrared radiation of waste heat. I wonder if higher energy efficiencies than we can easily imagine might be possible for a very advanced civilization. Perhaps the amount of waste heat is substantially lower than expected; too low to readily detect.

https://www.newscientist.com/article...tar-after-all/
Last edited by Ross 54; 2016-Jan-16 at 06:30 PM.

23. For a modern re-discussion of how we might look for Dyson swarms and similar megastructures from energy loss and leakage, I can't recommend much better than the papers from the G-HAT project, especially the first one:

The Ĝ Infrared Search for Extraterrestrial Civilizations with Large Energy Supplies. I. Background and Justification

The others are

The Ĝ Infrared Search for Extraterrestrial Civilizations with Large Energy Supplies. II. Framework, Strategy, and First Result

The Ĝ Infrared Search for Extraterrestrial Civilizations with Large Energy Supplies. III. The Reddest Extended Sources in WISE

The Ĝ Search for Extraterrestrial Civilizations with Large Energy Supplies. IV. The Signatures and Information Content of Transiting Megastructures

I had missed that last one initially. There are pretty basic thermodynamic reasons for the expected temperature ranges of complete Dyson spheres (the maximum energy-conversion efficiency doesn't rise very much going from a local temperature 300 K down to 100 or so) while the surface area and required mass to intercept starlight rises vastly), although sufficiently advanced technology might also be able to do something like dump excess energy in neutrino beams.

24. Originally Posted by ngc3314
I had missed that last one initially. There are pretty basic thermodynamic reasons for the expected temperature ranges of complete Dyson spheres (the maximum energy-conversion efficiency doesn't rise very much going from a local temperature 300 K down to 100 or so) while the surface area and required mass to intercept starlight rises vastly), although sufficiently advanced technology might also be able to do something like dump excess energy in neutrino beams.
It seems like a difficult solution to a non-problem. Why not just let the heat radiate outward?

25. It seems that, whatever is going on there, there're things happening continuosly.....

26. Originally Posted by Jens
It seems like a difficult solution to a non-problem. Why not just let the heat radiate outward?
The only reason I can think of is if you somehow came up with an effective way to harness the waste heat too. But it's hard to imagine what they'd turn it into, or how they could channel it in a way that wouldn't have any other waste signal.

The only thing I can imagine capturing that sort of energy without waste would be matter. But assuming 25 ish percent all around the star and it was like the sun, that'd be like a million tons of matter per second converted.

27. Whatever it is, it is definitely extremely interesting. Shame it's so long ago, and far far away...

28. Originally Posted by jokergirl
Whatever it is, it is definitely extremely interesting. Shame it's so long ago, and far far away...
I understand what you mean, but I think it will only be a shame if it turns out being ET. If it's some phenomenon it dies't really matter when or where it was.

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Originally Posted by Jens
I understand what you mean, but I think it will only be a shame if it turns out being ET. If it's some phenomenon it dies't really matter when or where it was.
Yes except if it is ET then I think there´s a reasonable chance they still exist. If they were able to reach the stage of such massive engineering project then I think its more likely they are still around today.

30. Update from Phil: