Imagine a primordial black hole so heavy that it is a blackbody and there are many planets orbiting around it, there is a moon thriving with intelligent life orbiting a gas giant. I am wondering would the intelligent alien species be able to tell a blackhole "Sun" against a fusion furnace?

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    $\begingroup$ I don't quite understand the technical details but this answer (physics.stackexchange.com/questions/130209/…) suggests there are slight differences in terms of emission spectrum. $\endgroup$
    – Daron
    Jan 16, 2021 at 2:48
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    $\begingroup$ There might be other ways to distinguish the two objects than spectrum though. I imagine the black hole will be completely static and uniform, and the sun will have visible currents, geysers, and be hotter in some places than others. $\endgroup$
    – Daron
    Jan 16, 2021 at 2:50
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    $\begingroup$ The visual size and mass are going to be dead giveaways; the larger the black hole, the lower the temperature of its Hawking Radiation. To get Sun-hot, your black hole is going to be asteroidal in mass, and its event horizon, microscopic. $\endgroup$
    – notovny
    Jan 16, 2021 at 3:45
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    $\begingroup$ I note that from their point of view, it will be a question of realizing that most of the stars in the sky are not blackholes but fusion furnaces. $\endgroup$
    – Mary
    Jan 16, 2021 at 4:12
  • $\begingroup$ ...but a Sun-hot, microscopical-sized black hole would have not enough power to even detectable from 1 AU. $\endgroup$
    – Gray Sheep
    Jan 18, 2021 at 16:47

3 Answers 3


Mismatching Spectrum and size

If their "sun" emits the same total energy of light as our Sol, then it will be only:
total luminosity : matches Sol = 5e26 watt
diameter: 1/ 10 000 th of a proton. (1e-24 m diameter)
peak emission: about 4.2e16 electonvolt. That is deep, deep, deeeeeeep cosmic ray, at about 6 millijoule PER PHOTON.

It would also only mass about 1 tonne,
and they would have a few microseconds to observe it before it evaporates.

A black hole large enough the be the gravitational center of a solar system, especially a "black hole so heavy that it is a blackbody" will have virtually nil detectable radiation of its own. Maybe from an infalling accretion disk, but that has a very visible and distinct signature.

Here' a nice tool to play around with, to see the size & mass & luminosity of black holes.


It's really hard to have a black hole involved in a habitable system. As others have pointed out, the black hole you describe would be a tiny point of light, and it would immediately evaporate in a burst of gamma radiation.

Another alternative would be a quasi-star, a massive object heated by a black hole accreting matter at its core. These would be obviously unlike normal stars, being thousands of times as massive as the sun and outputting as much light as a small galaxy. They might provide heat and light to a planet orbiting another star that itself is too dim. However, they would only last 7 million years or so, and since stars move, it's unlikely a planet would end up in the habitable zone for even that long.

A stellar black hole with an accretion disk seems more plausible, though you have the problem of explaining how the planet survived the creation of the black hole in suitable condition for life, and the accretion disk seems likely to produce harmful radiation and to be too unstable a source of heat and light for life to form.

However you arrange it, it seems unlikely for complex life to evolve locally in the time available: the aliens would almost have to themselves be alien to the world they're on, having come from somewhere else. Refugees from the formation of the quasi-star, settling a world that it's made habitable for the next few million years?


Assuming that they follow the same path of scientific evolution that we had, they would notice that its mass is rather large.

How did we determine the mass of the Sun?

We start by determining the mass of the Earth. Because we know the radius of the Earth, we can use the Law of Universal Gravitation to calculate the mass of the Earth in terms of the gravitational force on an object (its weight) at the Earth's surface, using the radius of the Earth as the distance.

Knowing the mass and radius of the Earth and the distance of the Earth from the sun, we can calculate the mass of the sun (right), again by using the law of universal gravitation. The gravitational attraction between the Earth and the sun is G times the sun's mass times the Earth's mass, divided by the distance between the Earth and the sun squared. This attraction must be equal to the centripetal force needed to keep the earth in its (almost circular) orbit around the sun. The centripetal force is the Earth's mass times the square of its speed divided by its distance from the sun. By astronomically determining the distance to the sun, we can calculate the earth's speed around the sun and hence the sun's mass.

A similar approach would lead to estimating the mass of the light emitter.

However be advised: a black hole with peak emission at 400 nm would have a radius of 10 nm, a mass about 1 millionth of Earth and a luminosity of 1 millionth of a Watt. In this world it's probably true that the Sun orbits around the Earth, but it would be very feeble.


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