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A planet is orbiting an old star that is on the verge of going supernova. The planet is colonized, so there's no need to worry about the habitability of the planet or evolution of life during the earlier stages of the star's life, only at its current state. The plan is for there to be an attempted evacuation literal hours before the detonation.

Given the semi-rapid and massive changes the star would undergo during this process, how would that affect the actual habitability of the planet?

According to Woosley, Stan, and Thomas Janka. “The Physics of Core-Collapse Supernovae.” Nature Physics 1.3 (2005): 147–154., the luminosity of a star around 15M☉ during its silicon-burning phase would peak around 75000L☉ and the core temperature at 3.3*10^9K, but I imagine that the actual effective temperature would be substantially lower due to the rapid loss of material? And on that note, what result would all of that ejected material have on the planet? Would there already be a basic planetary nebula formed at this point? If that's the case then I would imagine the planet would have already been stripped of its atmosphere, and my question changes to "How fast would a planetary nebula be formed, and is it fast enough that the aforementioned evacuation could be moved up to that point instead and still be suitably dramatic?"

In either scenario, getting a rough idea of some potential effective temperatures for the star just before whichever event occurs would be enough for me to properly calculate just about anything else, but the final mass of the star at that point would also be useful since it would presumably be lower than when the star was born. I'd hate to go through all this trouble just to find out that the planet's orbit was rendered unstable and it flew off into the void.

Apologies if I did anything wrong here— First time posting, plus it's after midnight and I'm tired of scrolling through old journals, so I figured I'd just ask.

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    $\begingroup$ A supernova is an extremely, unbelievably, unimaginably energetic event. There is a well-known XKCD What-If which concludes that (1) a supernova at a distance of 1 AU (= the distance between the Sun and the Earth) would appear more luminous that a thermonuclear bomb detonated in contact with the observer's eye, and (2) even the flux of neutrinos (which are notoriously very small and reluctant to interact with anything) would be lethal at a distance of 2 AU. Nothing survives. $\endgroup$
    – AlexP
    Commented Sep 13, 2020 at 7:37
  • $\begingroup$ Considering such things don't usually operate on a strict schedule, and neither do evacuations, I'd want off weeks before the scheduled stellar redecoration. $\endgroup$
    – NomadMaker
    Commented Sep 13, 2020 at 20:33

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A star big enough for a supernova has a very short lifetime (100 to 500 million years). The first microbes appeared on Earth one billion years after the Sun's formation, and the first trees needed more than 3 billion years to exist.

During the lifetime of this star, the planet wouldn't even be cool enough for aliens to terraform.

The only way I know is a system with two stars. Your planet is orbiting a small star, and from a distance of half a lightyear, there is this giant star. Even with a distance of 4 Lightyears (it would not be one system anymore) the supernova would kill everything on your planet.

Wikipedia Geologic Timescale

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    $\begingroup$ This question is about the period of time before a supernova, on a colonized planet where the evolution of life is unnecessary. $\endgroup$
    – Delta
    Commented Sep 13, 2020 at 18:33
  • $\begingroup$ I'm sorry, development of live was not the question. But i'm still not sure if a rockplanet is colonizable after 500 million years. A small one perhaps. Or far outsite of the habitable area. $\endgroup$
    – Tecnick3
    Commented Sep 13, 2020 at 19:21
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I don't think you'll live to see silicon.

(Disclaimer - I only have half a physics degree, but I think you'll need to evacuate a few years before Silicon.)

The power radiated from a star per unit surface area is proportional to the 4th power of it's thermodynamic temperature (Stefan–Boltzmann law). Mass isn't a factor in the equation for black-body heat radiation.

So, it follows, that a doubling in star temperature will lead to a 16-fold increase in effective temperature. Regardless of mass loss.

I'm assuming no-one would be silly enough to start a settlement in the neon, oxygen, or silicon stage of star evolution - we would do a proper habitability check including analysing the sun. Assuming this is true, the settlement is at least 3.5 years old. The system would've been stable for about 2000 years at this point, and there'd be an established habitability zone.

Assuming your settlement was founded during the stars Carbon stage:

  • Start of Neon stage. Star temp goes from 8.1e8K to 1.6e9K. A 1.98 fold increase. 1.98^4 is 15.22. The Effective temperature has gone up 15 fold.

  • Start of Oxygen stage. Goes up to 1.9e9K. A 2.35 fold increase over carbon. The effective temperature has gone up 30 fold.

  • Entering the silicon stage (3.3e9K. 4 fold temp increase. It's a 275-fold increase in effective temperature).

You would need to do your evacuation quickly, the Neon stage would be dangerously hot for a colony that was formally habitable.

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  • $\begingroup$ The core temperature is irrelevant to life on a planet. What's relevant there is the temperature of the surface (which is only a chilly 5600K in the case of our sun). When a star ramps up its energy production, it will overcompensate with an increase in surface area (it grows), effectively reducing its surface temperature. That's why we call such old stars red giants: They are huge, but their spectrum shifts into the direction of red/infrared. $\endgroup$
    – cmaster - reinstate monica
    Commented Sep 13, 2020 at 10:15
  • $\begingroup$ That said, with every stage of burning, life must choose a planet that's much further out to avoid being roasted alive. $\endgroup$
    – cmaster - reinstate monica
    Commented Sep 13, 2020 at 10:17
  • $\begingroup$ So what I'm getting from this is that the initial state would be a cooler red giant as one would assume, requiring the chosen planet to be closer to stay warm, but then as soon as the neon burning stage hits it's toasty time? But would we already be getting atmosphere stripping mass ejections by that point? $\endgroup$
    – Delta
    Commented Sep 13, 2020 at 14:18
  • $\begingroup$ Hang on, is this the same effective temperature that is usually presented as the given "temperature" of a star? $\endgroup$
    – Delta
    Commented Sep 13, 2020 at 22:21
  • $\begingroup$ Sorry about the break, apparently hitting enter on my phone submits the comment rather than starting a new line. Continued: For instance, the given temperature of another dying star— Betelgeuse— is ~3500K and it is presumably still in its helium burning stage. Going by that logic, wouldn't that mean that it would jump all the way to ~1,435,218K when it reaches carbon burning? That seems a bit high. Am I just misunderstanding something here? $\endgroup$
    – Delta
    Commented Sep 13, 2020 at 22:29

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