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  • Where would an Earthlike planet need to be so it's still in the 'Habitable Zone' of a hypergiant Star, compared to the distance of the Earth from the sun?
  • Could complex life develop or survive on such a planet?

It's not necessary to answer the second question, as it is not really related to Astronomy much. But if someone could really answer that, I would be grateful.

Some background

I was watching this video of Kurzgesagt ( that's a YouTube channel.) which was released only some days ago, and the video spoke about the comparison of the Sun to some of the largest types of stars out there. The video also mentions Giants and of course, Hypergiants.

This made me think about what would happen if our very own Sun, turned into a Hypergiant.

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    $\begingroup$ I've edited quite radically, to make the about real stars and not about the sun (which can't change to a hypergiant) This is to make the question on topic and answerable. I've removed the third question, which was probably unanswerable. $\endgroup$ – James K Sep 29 '20 at 5:24
  • $\begingroup$ This is relvant, if not an actual duplicate astronomy.stackexchange.com/questions/20610/… $\endgroup$ – James K Sep 29 '20 at 5:38
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I note that in old style space opera type stories, such as Star Trek, it is common to mention and visit habitable planets orbiting around types of stars which should not have habitable planets for various reasons. Thus one could assumed that in such stories hypothetical super advanced aliens have moved habitable planets into orbit around those stars, or have terraformed planets orbiting those stars to make them habitable.

Assuming that the Earth could be moved into orbit around a hypergiant, such as by creating an artificial wormhole in front of the orbiting Earth so that Earth enters the mouth of the wormhole in our solar system and emerges from the other mouth near the hypergiant star, it is fairly simple to calculate the proper orbital distance.

Simply multiply the distances of the inner and outer edges of the Sun's circumstellar habitable zone, by the luminosity of the new star relative to the Sun, to find out how close or far from that new star the inner and outer edges of the star's circumstellar habitable zone are.

it's very, very simple.

Except that I have seen a list of about a dozen estimates and calculations of the inner or outer edges, or both, of the Sun's circumstellar habitable zone, and some of them differ a lot from others.

https://en.wikipedia.org/wiki/Circumstellar_habitable_zone#Solar_System_estimates[1]

As I point out in my answer to this question:

https://worldbuilding.stackexchange.com/questions/186020/how-big-can-the-habitable-zone-be-in-a-g-type-binary-system/186104#186104[2]

The broadest possible range (combining different estimates) for the Sun's circumstellar habitable zone is about 481 times as wide, and about 1,436,139,559 kilometers wider, than the narrowest possible range (combining different estimates) for the Sun's circumstellar habitable zone.

So instead of calculating the extent of a hypergiant's hypothetical circum stellar habitable zone, we can assume that the Earth could orbit the hypergiant at Earth's exact distance from the Sun, one Astronomical Unit, or AU, multiplied by the Hypergiant's luminosity relative to the Sun.

Because of the inverse square law, if an object is moved to twice as far away from a source of light, it will receive only a quarter of the light it once received, and if the object is moved to half the distance from the light source it will recieve four times the light it once received.

If a star is twice as luminous as the Sun, a planet at a distance of 1.41 AU will receive as much light from it as Earth gets from the Sun.

If a star is 64 times as luminous as the Sun, a planet at a distance of 8 AU will receive as much light from it as Earth gets from the Sun.

If a star is 100 times as luminous as the Sun, a planet at a distance of 10 AU will receive as much light from it as Earth gets from the Sun.

If a star is 1,000 times as luminous as the Sun, a planet at a distance of 31.622 AU will receive as much light from it as Earth gets from the Sun.

If a star is 10,000 times as luminous as the Sun, a planet at a distance of 100 AU will receive as much light from it as Earth gets from the Sun.

Since the list of the few dozen most luminous stars known includes luminosities between 1,000,000 times that of the Sun and at least 6,400,000 times that of the Sun, a planet in orbit around such stars woudl have to orbit at least 1,000 AU from the star, and up to at least 2,529.822128 AU from the star, to recieve exactly as much radation from those stars as Earth receives from the Sun.

https://en.wikipedia.org/wiki/List_of_most_luminous_stars[3]

Of course, because those stars have different spectral types than they Sun, different percentages of their radiation will be in the visual spectrum as compared to infra read and ultra violent parts of the spectrum. So that will have some effect on the planetary temperatures which I have not calculated.

And if you want to learn more about what type of stars would be suitable for having habitable planets, you might want to read Habitable Planets for Man by stephen H. Dole, 1964:

https://www.rand.org/content/dam/rand/pubs/commercial_books/2007/RAND_CB179-1.pdf[4]

And this article and its sources for more recent research:

https://en.wikipedia.org/wiki/Planetary_habitability#Suitable_star_systems[5]

Ir you are wondering how many habitable planets could orbit within the circumstellar habitable zone of a star, you might want to go to the PlanetPlanet site and check the Ultimate Solar System section. The Ultimate Solar System is dedicated to designing solar systems with the most possible habitable planets. And some of them are very impressive and correspondingly improbable in the number of habitable planets they have:

https://planetplanet.net/the-ultimate-solar-system/[6]

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  • $\begingroup$ So, (if I am doing my maths right) Eta Carine, the star that is about 4,000,000 times the mass of our Sun (more on that here), then Earth will have to be about 2000 AU away from it to receive as much light from it as it gets from our Sun. $\endgroup$ – Infinity Milestone Oct 1 '20 at 10:42
  • $\begingroup$ @AyushBhatt Eta Carinae A has a mass of about 100 times that of the Sun, and Eta Carinae B has a mass of about 30 to 80 times the mass of the Sun. Together they have a luminosity about 6,000,000 times that of the Sun, and so Earth would have to orbit about 2,449.48 AU from them to have the same temperature as it actually does 1 AU from the Sun. en.wikipedia.org/wiki/Eta_Carinae $\endgroup$ – M. A. Golding Oct 1 '20 at 16:08
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As in the case of Betelgeuse, it isn't really meaningful to talk about the habitable zone of a hypergiant. These are massive, short-lived stars that don't last long enough for terrestrial planets to form: their evolution is measured on timescales of only a few million years. Their progenitors are very hot O-type stars that are highly destructive to protoplanetary discs, eroding them with extreme ultraviolet and X-ray emissions. Even if you did manage to form a terrestrial planet there, there wouldn't be sufficient time for the magma ocean to cool before the star undergoes a supernova or collapses to form a black hole. The intrinsic heat of the planet would likely keep water as superheated steam in a rock vapour atmosphere. Furthermore, these stars undergo extreme rates of mass loss, which would make the space weather environment very hostile to maintaining an atmosphere over long periods of time. This is not an environment where you would find Earth-like planets or life.

As for the question of what would happen if the Sun turned into a hypergiant: it won't because (fortunately for us) it is not massive enough to do so. Such a scenario is in the realm of fantasy so it isn't really possible to say: once you're invoking wizards and magic sufficiently advanced AIs with the technology to do this then all bets are off.

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  • $\begingroup$ Hmmm.....Interesting. @antispinwards, what would happen if the Sun doesn't change into a Hypergiant, but we exchange the Sun with a Hypergiant? $\endgroup$ – Infinity Milestone Sep 29 '20 at 13:41
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    $\begingroup$ @AyushBhatt - then the part of my answer about wizards and magic applies: that would put you well into fantasy land no matter what sciency-sounding words you dress such a scenario up in. Such an operation is not remotely feasible in the real world. $\endgroup$ – user24157 Sep 29 '20 at 13:58

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