Theory · Intermediate · 1:50

Dyson swarm

Catching a whole star's light: how a civilization could build it, and how it would look

Theory, not observation. Nothing here has ever been seen. Each idea comes from real physics and says how it could be made, and what it would look like. Light is bent as the equations say it would be; the things themselves are drawn.

Below are the 8 chapters of this simulation in Cosmic Scale: each one's caption, then the fuller explanation the app shows under “Read more”. Colours, arrows and lines mentioned in the text belong to the 3D scene in the app.

Open Cosmic Scale then the What if tab

1. Catching all of a star's light

A star sends nearly all its light into empty space. In 1960 Freeman Dyson suggested that a civilization short of energy would catch it, and that its star would then look different. The Sun gives out 2 × 10¹³ times what humanity uses.

The idea has an older source: Olaf Stapledon's novel Star Maker (1937), where the stars of a whole galaxy are ringed with traps for their light. Dyson said it gave him the idea. His own point was practical: a civilization that used a star's energy would have to give it back as heat, and heat is something telescopes can see.

The Sun pours out 3.8 × 10²⁶ watts. Humanity uses about 2 × 10¹³. On the scale Carl Sagan drew from Nikolai Kardashev's, where each whole step is a factor of ten billion in power, that puts us at about 0.73. A civilization using all of its star's light would be at 2.

No such civilization is known. Everything here is imagined from physics: how one could be built, and what it would look like.

2. Not a shell, a swarm

A solid shell round a star would not work. It would have to bear about a trillion pascals, some thousand times what steel can, and it would drift into the star. What Dyson meant was a swarm of separate collectors, each on its own orbit.

A thin shell at the Earth's distance is squeezed by the Sun's pull with a pressure of about GMρ/2r: for a shell of aluminium's density, around 1.2 trillion pascals, however thin it is. Steel gives way at about a billion. No material we know comes near.

Even a perfect shell would be unstable. Inside a uniform shell gravity cancels, so nothing keeps the star at the centre: any drift towards one side grows, until the star meets the wall. Dyson himself called a solid shell mechanically impossible and pictured a loose swarm of bodies on independent orbits.

So the picture here is a swarm of panels, each a mirror or a collector on its own orbit round the star. The panels are drawn thousands of times bigger than real ones, and the star bigger than it is, so that both can be seen.

3. Where the material would come from

A sphere as far out as Earth has an area of 2.8 × 10²³ m². All of Mercury's mass, spread over it, would be a little over a kilogram a square metre. In 2013 Armstrong and Sandberg sketched how self-copying factories could take a planet apart to build it.

Mercury holds 3.3 × 10²³ kg. Spread evenly over the sphere that is about 1.2 kg on every square metre, far more than collectors as thin as foil need, so a fraction of the planet would do. About two thirds of it is metal, good for building.

The scheme of Stuart Armstrong and Anders Sandberg starts small: a first factory on Mercury builds more factories, which build more, so that the work doubles again and again. Finished collectors are launched into orbit round the Sun and start giving power back to feed the next round.

The figures are arithmetic. The self-copying factories are an idea, far beyond what we can make, and how long it would take depends on how fast they could multiply.

4. How it would grow

Each panel is tiny: covering the sphere with panels a kilometre across would take 2.8 × 10¹⁷ of them. The star dims slowly at first and faster as the swarm thickens. Here the panels come out one after another.

Panels of a square kilometre are small beside a star. A thousand hardly dent its light and a trillion do little; only when there are hundreds of quadrillions does the star dim for good. Growth by copying gets there faster than one would think.

The chart in the corner is the share of the star's light caught as panels are added, overlaps counted. At first it rises in step with their number, then flattens as new panels fall into the shadow of old ones. It never quite reaches all.

A swarm need not be finished to be useful: half a swarm catches about half the light, and a civilization could stop anywhere along the way. The panels here are drawn far bigger than real ones.

5. From far away

From far off you could not see the panels. You would see a star dimmer than its kind should be and brighter in the infrared than it should be: the swarm's waste heat, at about 280 K, brightest near 10 micrometres.

Whatever a swarm catches, it must give back: no civilization can get rid of heat except by radiating it. A swarm at the Earth's distance would sit at about 280 K, a little colder than a room, and glow most strongly at a wavelength near 10 micrometres, in the mid-infrared.

The chart shows the star's own light, brightest near half a micrometre, and the swarm's heat. They are the same curve, the same power, moved along the wavelengths by a factor of twenty: the light is not lost, only moved from the visible to the infrared.

That is how one would search: for a star whose visible light is low and whose infrared is high. The red glow drawn round the swarm shows where the heat is; infrared cannot be seen.

6. The view from inside

Inside the swarm you would see the star through a scatter of panels. Each glints where it catches the light, and each that crosses the star dims it for a moment. Day would flicker as they pass.

From inside, the panels' bright faces turn towards you. Each is a mirror or a collector, catching the star's light and glinting in it, so the sky is full of them: flecks of light, or small dark shapes against the star.

A panel crossing the star's face dims it briefly, as a planet does in a transit. With many of them the flicker would never stop, and if the swarm were thick the star would be a red-gold glow seen through gaps.

The star here is drawn bigger than it would be and the panels far bigger. A real panel is a speck at any distance, and a real swarm would be much sparser in the sky than this picture.

7. The star that dims

In 2015 astronomers reported a star, KIC 8462852, that dimmed by as much as 22% in irregular dips. One idea was a swarm of alien structures. Later colour measurements pointed to dust, which dims blue light more than red.

The star, called Boyajian's star or Tabby's star after Tabetha Boyajian, who led the team that reported it, dipped by up to 22% for days at a time, in no regular pattern. Jason Wright and Steinn Sigurdsson listed a swarm of alien structures among the possible causes in 2016.

In 2018 Boyajian's team watched the dips in several colours. They dimmed blue light more than red, as dust grains smaller than a thousandth of a millimetre would, and as a swarm of solid panels would not, since panels block all colours alike. Dust is now the leading explanation.

It is still an odd star. But the lesson stayed: a real swarm would dim a star equally at every colour, and glow in the infrared, so a search must check both. The clumps here are drawn.

8. Has anyone seen one?

No. Surveys have searched millions of stars for the infrared glow of a swarm. In 2024 Project Hephaistos found seven red dwarfs with an unexplained excess, but dust or background galaxies may explain it. A Dyson swarm stays an idea.

Project Hephaistos, led by Matías Suazo, sifted about five million stars in three big sky surveys (Gaia, 2MASS and WISE) for stars too faint in visible light and too bright in the infrared. It found seven red dwarfs that its filters could not explain.

Natural explanations are possible: faint background galaxies or dust near the stars could account for the excess. Until the sources are looked at more sharply, the seven are candidates for a puzzle, not a discovery.

Nothing so far needs a Dyson swarm to explain it. It stays a guide to what energy use on a huge scale would leave behind, and a reminder that the star to search is one that looks too dim and too warm.

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