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Since the Sun is the only star we know to have life in its planetary system, I came to wonder about if it was replaced by, say, VY Canis Majoris How far would we have to be so that we would still be functioning normally as if it was really the Sun? Could such a thing even be possible?

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  • $\begingroup$ Are you asking about whether life could form, or whether it could survive, if placed there? $\endgroup$ – HDE 226868 Jun 18 '16 at 17:18
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This is actually pretty simple. We just have to calculate the stellar flux, which is the luminosity of the star divided by the surface area of a shell with the radius of the planet's orbit. In other words: $$F=\frac{L}{4\pi r^2}$$ Now, let's denote the Sun's luminosity as $L_{\odot}$, the current radius of Earth's orbit as $r_i$, the luminosity of VY Canis Majoris as $L_{\text{VY}}$, and the new radius of Earth's orbit as $r_f$. We need to set the fluxes equal, in order to keep conditions on Earth the same. Therefore, $$F_{\odot}=F_{\text{VY}}\to\frac{L_{\odot}}{4\pi r_i^2}=\frac{L_{\text{VY}}}{4\pi r_f^2}$$ The $4\pi$s cancel, and, given that Wikipedia gives VY Canis Majoris's luminosity as about 270,000 times that of the Sun, we get $$\frac{L_{\odot}}{r_i^2}=\frac{270,000}{r_f^2}\to r_f=\sqrt{270,000}r_i\simeq520r_i$$ Therefore, Earth would have to be 520 times as far away - 520 AU - for conditions to be such that we could "function normally", as you put it.

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Without going into any mathematical details, I can tell you this: no, life on a planet around CM is not possible. The very existence of a planet around CM is highly unlikely. If a planet is found circling CM, a lot of astrophysicists will be staying up late, scratching their heads, wondering how to explain this scenario.

The first and foremost reason of no life around CM is that it is not a normal star but a red hypergiant. This means that the time when it had an active core (where fusion took place), its size would have been much smaller than what it is now. This also means that its luminosity and gravitational interaction with its planets would also have been different than what would be expected of now.

So that if there was any planet in the goldillock zone around it, that planet would have definitely roasted to cinders when CM turned hypergiant. Since the hypergiant phase of stars doesn't last long enough, there is zero chance that life could have formed and evolved on any possible planet around CM after it turned hypergiant.

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  • $\begingroup$ Couldn't the star simple capture a rogue planet? Also, the habitable zone still exists; it's simply much further out. $\endgroup$ – HDE 226868 Jun 18 '16 at 17:19
  • $\begingroup$ Nope. According to wikipedia, CM has an average density of 5-10 **mg**/m$^3$ which is far far too less to capture a planet to make it orbit around itself. If anything, the planet would simply fall into CM, considering it's monstrous size and (probably) low escape velocity. $\endgroup$ – Youstay Igo Jun 18 '16 at 17:25
  • $\begingroup$ What does density have to do with it? $\endgroup$ – HDE 226868 Jun 18 '16 at 17:26
  • $\begingroup$ what-if.xkcd.com/68 $\endgroup$ – Youstay Igo Jun 19 '16 at 5:24
  • $\begingroup$ I'm sorry; I don't see the relevance there to the situation at hand. $\endgroup$ – HDE 226868 Jun 19 '16 at 13:40

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