Universe Today
Ever since physicist Freeman Dyson first proposed the concept in 1960, the “Dyson sphere” has been the holy grail of techno-signature hunters. A highly advanced civilization could build a “sphere” (or, in our more modern understanding, a “swarm” of smaller components) around their host star to harvest its entire energy output. We know, in theory at least, that such a swarm could exist - but what would it actually look like if we were able to observe one? A new paper available in pre-print on arXiv, and soon to be published in Universe from Amirnezam Amiri of the University of Arkansas digs into that question - and in the process discloses the types of stars that are the most likely to find them around.
Perhaps unsurprisingly, one of those types is a Red Dwarf. The most abundant type of stars in the Milky Way, they burn through their nuclear fuel incredibly slowly making them extremely long lived. With estimated lives in the trillions of years - far longer than the current lifetime of the universe - they are also relatively small compared to our own Sun. A Dyson swarm could be built around 0.05 to 0.3 AU away from its surface, with relatively low cost of material.
White dwarfs are arguably even better for material costs, and represent the second type of star that it’s worth tracking. These are compact, dead remnants of stars like our Sun, which have shrunk down to have incredibly small radii - around 1% of their original star. In this scenario, a Dyson swarm could be built just a few million kilometers away from the surface of the star, alleviating much of the engineering challenge of build a supermassive structure around a larger star. They also radiate energy steadily for billions of years, essentially creating an effective long-lived power source.
*The H-R diagram used to classify stars. Credit - ESO*
But what would stars surrounded by such megastructures actually look like? Astronomers typically use a tool called the Hertzsprung-Russell (H-R) diagram to classify stars based on their temperature and luminosity. However, since a Dyson sphere would block all of a star’s natural light, it would completely change where on the diagram it would fall. Energy can neither be created nor destroyed, so the sphere itself would have to emit the exact same amount of radiation away from itself as the star is putting into it. It just does it in the form of heat, or infrared light instead. So a Dyson sphere can really be thought of as a shell that absorbs a star’s light, does something useful with that energy, and then emits it as heat.
In doing so, it is shifting the location of the star entirely to the right - where lower temperatures are mapped on the diagram. The luminosity itself doesn’t change at all, it is simply shifted to the infrared, and since H-R diagrams use bolometric luminosity (i.e. the luminosity over all of the spectra), it would appear in the same vertical place on the diagram as whatever its host star is, whether that’s a red or white dwarf.
But the key take away is how much further on the right the star would go. A typical red dwarf, which inhabits the lower right hand corner of a H-R diagram, has a surface temperature of around 3000K degrees. A Dyson sphere surrounding a star would have a temperature down to 50K - two orders of magnitude lower. There are no natural stars in this area, making any such object highly interesting as a potential Dyson swarm candidate.
Fraser explains the concept of a Dyson Sphere.One further factor contributing to the possibility of an object being a Dyson swarm is a lack of dust. A star without a Dyson sphere would typically show a spectral line for silicate emission that is commonly associated with dusky disks. However, radiator panels don’t have any dust surrounding them, so they would look remarkably “clean” to a spectrograph monitoring them.
One thing to note - in the “swarm” methodology, there would likely be gaps between some of the solar collectors, or varying thickness in certain parts of the swarm. This is intended to make the material requirements actually physically possible - modern calculations show that, even with relatively small radii, an actual full Dyson sphere is physically impossible. In the case where there were these small gaps, the star would behave exceedingly erratically, with non-natural light curves as the structure rotates.
Since infrared is the specialty of the James Webb Space Telescope, it is well placed to monitor for these kinds of structures. But even older telescopes like WISE are being actively used to search for them. In May 2024, a paper highlighting work from Project Hephaistos identified seven strong Dyson sphere candidates (all red dwarfs) out of a catalogue of 5 million stars. One was eliminated as a possible source, as there was a supermassive black hole aligned perfectly in the background around the star, explaining the anomalous readings. But that still leaves five more potential candidates that are worth some closer observation. This new paper will add another tool to astronomers’ understanding of what to search for to one day find one of these elusive technosignatures.
Learn More:
A. Amiri - Dyson spheres on H-R diagram
UT - Astronomers are on the Hunt for Dyson Spheres
UT - Will Advanced Civilizations Build Habitable Planets or Dyson Spheres
UT - High-Resolution Imaging of Dyson Sphere Candidate Reveals no Radio Signals
