A Soviet astrophysicist sorted civilizations into three tiers by the energy they command - a planet, a star, a whole galaxy. It doubles as a search strategy, and when astronomers flagged seven stars as possible Type II megastructures, the trail led somewhere ordinary.
Ask an astronomer how advanced an alien civilization might be, and many will not reach for warp drives or ray guns. They tend to ask a blunter question: how much energy can it command?
That is the core of the Kardashev scale, a ranking proposed in 1964 by the Soviet astrophysicist Nikolai Kardashev. In a paper titled “Transmission of Information by Extraterrestrial Civilizations,” he sorted possible civilizations into three tiers by the energy they can harness. According to the scale, a Type I civilization masters all the energy available on its home planet, a Type II captures the full output of its star, and a Type III commands the energy of an entire galaxy.
The appeal is that it sidesteps guesswork about alien machinery. We cannot picture what devices another species would build, but physics lets us estimate a power budget - and, crucially, gives telescopes something concrete to look for.
Kardashev put rough numbers on each rung. As Universe Today summarizes his 1964 figures, a Type I civilization runs on about 4 x 10^19 ergs per second, a Type II on roughly 4 x 10^33, and a Type III on some 4 x 10^44 - each step a staggering leap, a factor of at least a hundred billion, over the one below.
An erg is a tiny unit of energy, but the exponents are the point: a star pours out roughly a hundred trillion times more power than reaches a single planet from it. Kardashev’s insight was that energy leaves a fingerprint. A civilization does not have to broadcast a greeting for us to notice it; the sheer scale of its energy use might give it away.
The astronomer Carl Sagan later suggested filling in the gaps with a continuous scale rather than three hard steps, since a real civilization would sit somewhere between the rungs. By that logic, humanity has not yet reached Type I. We still let most of the sunlight and other energy reaching Earth go uncaptured, so we do not yet master even our own planet’s budget. On this scale, in other words, we are the aspiring case, not the advanced one.
The scale is not just bookkeeping. It doubles as a search strategy, and the clearest target sits at Type II.
In 1960, the physicist Freeman Dyson pointed out that a civilization desperate for power might wrap its star in a shell or a loose swarm of collectors, a structure later nicknamed a “Dyson sphere.” Here thermodynamics does the informing. Any starlight those collectors absorb has to go somewhere, and it comes back out as waste heat. So a star being harvested would look strangely dim in visible light while glowing in the mid-infrared, the band we feel as warmth.
That makes a Dyson sphere one of the tidiest technosignatures on offer - a sign not of alien biology but of alien engineering, and one an ordinary infrared telescope can hunt. It is a different bet from the classic approach of listening for a deliberate radio signal, which is closer to what Kardashev himself had in mind. If Type II or Type III civilizations were common, their heat and their broadcasts should be hard to miss, which is part of why their apparent absence feeds the Fermi paradox, the puzzle of why a universe so old and vast looks so quiet.
Recently, a team ran exactly the hunt Dyson imagined. Project Hephaistos, led from Uppsala University, combed roughly five million stars catalogued by the Gaia, 2MASS, and WISE surveys, all within about 300 parsecs of the Sun, for that odd infrared excess. In 2024 it reported seven candidates - small, dim M-dwarf stars glowing with waste heat at temperatures of roughly 100 to 300 kelvin, close to what a partial Dyson sphere might produce.
Then came the careful part. A 2026 follow-up, which widened the analysis to ten objects, found that the stars themselves look entirely ordinary for their class, with no irregularities. More tellingly, new observations from the James Webb Space Telescope showed that at least two of the candidates sit directly in front of very red background galaxies - distant, dust-choked galaxies whose glow blends with the star’s light and mimics the signature of a megastructure.
The researchers are measured about it. They acknowledge they still cannot fully explain every infrared excess, but the leading explanations are now mundane: circumstellar dust, or a background galaxy hiding in the same patch of sky. Further Webb and ALMA observations should settle each case.
None of that rules out advanced civilizations, and it does not diminish the Kardashev scale as a yardstick. To date, no survey has turned up a confirmed Type II or Type III technosignature. What the scale offers is not proof that star-eaters exist, but a clear, physics-based idea of what their footprint would look like - and the discipline to notice when a promising glow turns out to be dust.
The phrase now anchors UAP hearings, a federal law, and a Pentagon office - and it is carefully broader than 'extraterrestrial.' Here is what 'non-human intelligence' actually covers, and why NASA and the Pentagon still say there is no proof behind it.
Tabby's Star spent a decade as the most famous 'alien megastructure' candidate in astronomy. A July 2026 study spotted a single symmetric transit in TESS data that fits a giant planet on a three-year orbit. It is suggestive, not confirmed.
For decades, sightings clustered near bases and nuclear sites fed a theory that something is monitoring our weapons. NASA's 2023 study team found the pattern mostly reflects where the cameras, radars, and trained observers are - a reporting bias, not a beacon.
