<p>Reading <a href="http://adsabs.harvard.edu/abs/1960Sci...131.1667D" rel="nofollow noreferrer">Dyson's original argument</a> gives some useful information. He says that such a sphere would have a surface temperature of 200-300 Kelvin, as it would reradiate some of the energy it absorbs at infrared wavelengths. That's a temperature comparable to cool <a href="https://en.wikipedia.org/wiki/Brown_dwarf#Spectral_class_Y" rel="nofollow noreferrer">Y-type brown dwarfs</a>. If we can assume a <a href="https://en.wikipedia.org/wiki/Black_body" rel="nofollow noreferrer">black body approximation</a>, and a radius of about 1 AU, we get a <a href="https://en.wikipedia.org/wiki/Stefan%E2%80%93Boltzmann_law#Temperature_of_stars" rel="nofollow noreferrer">luminosity</a> for a 200 K sphere of
$$L_{\text{DS}}\simeq4\pi\sigma R^2T^4\sim10^{25}\text{ Watts}\simeq0.1L_{\odot}$$
That's substantially more luminous than a cool brown dwarf of the same temperature. Additionally, <a href="https://en.wikipedia.org/wiki/Planck%27s_law" rel="nofollow noreferrer">Planck's law</a> gives a peak black body wavelength of about 14,500 nm, placing the emissions in the mid-infrared range, as expected. Relatively hot brown dwarfs should be expected to have high infrared emissions. In other words, a Dyson sphere should look something like, from a distance, a large, luminous brown dwarf.</p>

<p>Dyson spheres, in the classic picture of a solid shell enveloping a star, are actually unstable, so it's unlikely that they would ever be built. We should expect other variants - Dyson swarms, rings, bubbles, etc. - that are composed of large arrays of smaller objects are much more likely to be created.</p>

<p>Fermilab used data from the <a href="https://en.wikipedia.org/wiki/IRAS" rel="nofollow noreferrer">Infrared Astronomical Satellite (IRAS)</a> to look for Dyson spheres as <a href="http://home.fnal.gov/~carrigan/infrared_astronomy/Fermilab_search.htm" rel="nofollow noreferrer">a secondary mission</a>, searching a band in infrared wavelengths centered around 300 K (see <a href="http://iopscience.iop.org/article/10.1088/0004-637X/698/2/2075/meta;jsessionid=8CC048BBAB18C31EE60139CC33C9AC98.c4.iopscience.cld.iop.org" rel="nofollow noreferrer">Carrigan (2009)</a>), covering 100 K to 600 K. The study started with 250,000 sources over 96% of the sky, then whittled them down to about 6,000 based on temperature range and flux. 17 "ambiguous" candidates were finally left. This is, to my knowledge, the most complete survey yet undertaken.</p>

<p>There have, of course, been <a href="http://home.fnal.gov/~carrigan/infrared_astronomy/Other_searches.htm" rel="nofollow noreferrer">others</a>, most using IRAS data. <a href="https://en.wikipedia.org/wiki/Wide-field_Infrared_Survey_Explorer" rel="nofollow noreferrer">WISE</a> and <a href="https://en.wikipedia.org/wiki/2MASS" rel="nofollow noreferrer">2MASS</a> also contributed data. Very few candidates have been found, and it has been estimated that any within about 1 kpc in reasonable temperature ranges (less than 400 K) should have been among these.</p>

<p>There are indeed natural sources that could prove to be false positives. Carrigan lists several:</p>

<ul>
<li>Dust shells around stars possibly undergoing mass loss</li>
<li>Stars embedded in dusty areas, including dense nebulae</li>
<li><a href="https://en.wikipedia.org/wiki/Mira_variable" rel="nofollow noreferrer">Mira variables</a>, which undergo mass loss towards the end of their lives</li>
<li>Planetary nebulae</li>
<li><a href="https://en.wikipedia.org/wiki/Asymptotic_giant_branch" rel="nofollow noreferrer">AGB stars</a>, which may create circumstellar envelopes, as well as post-AGB stars</li>
</ul>

CC BY-SA 3.0
2014-10-04T13:46:21.910Z
I think a Dyson sphere is impossible, because nothing can stabilise it against gravitational collapse into the star. A ring rotating at the right rate is better, but a completely different type of thing and can only capture a tiny fraction of the stars energy.
Walter
CC BY-SA 3.0
2014-10-04T13:57:54.673Z
@Walter I agree with you about that. I also think that, gravity neglected, it would be incredibly difficult for a civilization to build any of the above - if I were an astronomer, I wouldn't search for them. But for the purposes of this answer, I decided to just ignore that.
HDE 226868
CC BY-SA 3.0
2014-10-05T15:29:02.340Z
No serious astronomer does. Those who do, do only because they can market that well with the public and hence get some funding.
Walter
CC BY-SA 3.0
2014-10-05T15:30:46.953Z
@Walter No kidding. I would expect something like this maybe out of SETI, but not of anyone working at somewhere like Mauna Kea. Especially given how hard it can be to get timeslots.
HDE 226868
CC BY-SA 3.0
2014-08-04T22:17:39.590Z
2016-11-04T15:06:04.760Z
2016-11-04T15:06:04.760Z
HDE 226868
<p>A <a href="https://en.wikipedia.org/wiki/Dyson_sphere" rel="noreferrer">Dyson sphere/swarm/ring</a> is a hypothetical structure an extra-terrestrial entity would construct to collect a large fraction of its host star's light, and would likely generate a fairly strong infrared signature that could potentially be detected by modern infrared surveys (such as <a href="http://www.nasa.gov/mission_pages/WISE/main/index.html" rel="noreferrer">WISE</a>). Do we have any meaningful constraints on the existence of these structures within the Milky Way (or beyond) from these surveys? What sorts of natural phenomena could be confused with such structures, if they existed?</p>

CC BY-SA 3.0
2014-03-13T11:09:24.130Z
Why would it be 'fairly strong'? Relative to other IR sources, like the star it would be around, wouldn't it more likely be insignificant in comparison? Like the hard-to-pin-down vague boundary of the corona of a red hyper giant?
Jeremy
CC BY-SA 3.0
2014-08-05T01:03:14.387Z
Since the surface collects visible light and then radiates the energy in the infrared, you would probably see 2 superimposed black body curves, one for the host star and one for the object which peaks in the infrared. (Too bad I can't attached an image.)
LDC3
CC BY-SA 3.0
2014-08-06T18:21:40.507Z
@LDC3 If it was a full-enclosed Dyson sphere, why would there be a black body curve for the host star?
Jeff-Inventor ChromeOS
CC BY-SA 3.0
2014-08-08T00:31:25.417Z
@Jeff-InventorChromeOS A Dyson sphere would only have one black body curve with the peak in the infra-red. I was thinking about a Dyson ring when I posted my statement.
LDC3
<observational-astronomy><extra-terrestrial>
What are the current observational limits on the existence of Dyson spheres/swarms/rings?
Guillochon
# What are the current observational limits on the existence of Dyson spheres/swarms/rings?

(Asked by: username_0 on 2013-09-30)

A [Dyson sphere/swarm/ring](https://en.wikipedia.org/wiki/Dyson_sphere) is a hypothetical structure an extra-terrestrial entity would construct to collect a large fraction of its host star's light, and would likely generate a fairly strong infrared signature that could potentially be detected by modern infrared surveys (such as [WISE](http://www.nasa.gov/mission_pages/WISE/main/index.html)). Do we have any meaningful constraints on the existence of these structures within the Milky Way (or beyond) from these surveys? What sorts of natural phenomena could be confused with such structures, if they existed?

**Tags:** observational-astronomy, extra-terrestrial

> **Comments:**
> **username_1**: Why would it be 'fairly strong'? Relative to other IR sources, like the star it would be around, wouldn't it more likely be insignificant in comparison? Like the hard-to-pin-down vague boundary of the corona of a red hyper giant?  
> **username_2**: Since the surface collects visible light and then radiates the energy in the infrared, you would probably see 2 superimposed black body curves, one for the host star and one for the object which peaks in the infrared. (Too bad I can't attached an image.)  
> **username_3**: @username_2 If it was a full-enclosed Dyson sphere, why would there be a black body curve for the host star?  
> **username_2**: @username_3 A Dyson sphere would only have one black body curve with the peak in the infra-red. I was thinking about a Dyson ring when I posted my statement.  

---

## 1 answers

### Answer by username_4 on 2014-08-04. Score: 7 | Accepted answer

Reading [Dyson's original argument](http://adsabs.harvard.edu/abs/1960Sci...131.1667D) gives some useful information. He says that such a sphere would have a surface temperature of 200-300 Kelvin, as it would reradiate some of the energy it absorbs at infrared wavelengths. That's a temperature comparable to cool [Y-type brown dwarfs](https://en.wikipedia.org/wiki/Brown_dwarf#Spectral_class_Y). If we can assume a [black body approximation](https://en.wikipedia.org/wiki/Black_body), and a radius of about 1 AU, we get a [luminosity](https://en.wikipedia.org/wiki/Stefan%E2%80%93Boltzmann_law#Temperature_of_stars) for a 200 K sphere of
$$L\_{\text{DS}}\simeq4\pi\sigma R^2T^4\sim10^{25}\text{ Watts}\simeq0.1L\_{\odot}$$
That's substantially more luminous than a cool brown dwarf of the same temperature. Additionally, [Planck's law](https://en.wikipedia.org/wiki/Planck%27s_law) gives a peak black body wavelength of about 14,500 nm, placing the emissions in the mid-infrared range, as expected. Relatively hot brown dwarfs should be expected to have high infrared emissions. In other words, a Dyson sphere should look something like, from a distance, a large, luminous brown dwarf.

Dyson spheres, in the classic picture of a solid shell enveloping a star, are actually unstable, so it's unlikely that they would ever be built. We should expect other variants - Dyson swarms, rings, bubbles, etc. - that are composed of large arrays of smaller objects are much more likely to be created.

Fermilab used data from the [Infrared Astronomical Satellite (IRAS)](https://en.wikipedia.org/wiki/IRAS) to look for Dyson spheres as [a secondary mission](http://home.fnal.gov/~carrigan/infrared_astronomy/Fermilab_search.htm), searching a band in infrared wavelengths centered around 300 K (see [Carrigan (2009)](http://iopscience.iop.org/article/10.1088/0004-637X/698/2/2075/meta;jsessionid=8CC048BBAB18C31EE60139CC33C9AC98.c4.iopscience.cld.iop.org)), covering 100 K to 600 K. The study started with 250,000 sources over 96% of the sky, then whittled them down to about 6,000 based on temperature range and flux. 17 "ambiguous" candidates were finally left. This is, to my knowledge, the most complete survey yet undertaken.

There have, of course, been [others](http://home.fnal.gov/~carrigan/infrared_astronomy/Other_searches.htm), most using IRAS data. [WISE](https://en.wikipedia.org/wiki/Wide-field_Infrared_Survey_Explorer) and [2MASS](https://en.wikipedia.org/wiki/2MASS) also contributed data. Very few candidates have been found, and it has been estimated that any within about 1 kpc in reasonable temperature ranges (less than 400 K) should have been among these.

There are indeed natural sources that could prove to be false positives. Carrigan lists several:

* Dust shells around stars possibly undergoing mass loss
* Stars embedded in dusty areas, including dense nebulae
* [Mira variables](https://en.wikipedia.org/wiki/Mira_variable), which undergo mass loss towards the end of their lives
* Planetary nebulae
* [AGB stars](https://en.wikipedia.org/wiki/Asymptotic_giant_branch), which may create circumstellar envelopes, as well as post-AGB stars

> **Comments:**
> **username_5**: I think a Dyson sphere is impossible, because nothing can stabilise it against gravitational collapse into the star. A ring rotating at the right rate is better, but a completely different type of thing and can only capture a tiny fraction of the stars energy.  
> **username_4**: @username_5 I agree with you about that. I also think that, gravity neglected, it would be incredibly difficult for a civilization to build any of the above - if I were an astronomer, I wouldn't search for them. But for the purposes of this answer, I decided to just ignore that.  
> **username_5**: No serious astronomer does. Those who do, do only because they can market that well with the public and hence get some funding.  
> **username_4**: @username_5 No kidding. I would expect something like this maybe out of SETI, but not of anyone working at somewhere like Mauna Kea. Especially given how hard it can be to get timeslots.  

---

