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This question was inspired by the previous question Would a black hole passing next to a star create a deadly focal point due to gravitational lensing?. I understand, as gravity also acts on light, that any massive object acts as a sort of lens. In the case of our sun, the focal distance starts about 550au, and there are several proposals to send probes there, given we could use the sun as a lens for a huge "telescope". See for example, Hesperides: Solar/nuclear missions to the Sun׳s inner gravity focus and First Stop on the Interstellar Journey:The Solar Gravity Lens Focus.

So I wonder, could the intensity of this effect be enough to melt water and create a sort of habitable zone (perhaps more of a habitable spot) in the opposite direction of a supernova as seen from the sun, by concentrating its light rays. I put the expression habitable zone inside quotes in the title because I don't think that sort of environment would be actually be habitable, as ionizing radiation of the supernova would be concentrated as well, so that would be a extreme environment. Yet, I can think of some interesting observable effects. I think that "habitable spot" would be tiny, but if we were lucky enough for a cometary nucleus pass through it just at the right time, perhapts it could develop a coma that we would be able to observe from Earth. Countless supernovas happened since solar system formation, so I would not be surprised if outer solar system contains a couple remnants of objects roasted this way.

The light of a near passing star would be concentrated too, but I think the effect would be weaker when compared with what a supernova like, say, SN 1054 can achieve.

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Supernovas have a light curve going from normal to peak and back on the order of a month. Hence the spot would go from too cold to habitable and back very quickly. The same is true for stars passing the focal point: their motion (typically tens of km per second) makes the heated point move equally fast.

Habitable zone denotes a region where a planet could have liquid water (and maybe other benign environmental properties). This implicitly assumes that this is a stable state, since the context is planetary orbits and the emergence of life, a process that takes long time. A ultra-brief habitable zone is missing the spirit of the concept, even if it may be correct to state that a certain location is habitable over a short span of time.

As the deadly focal point post discusses, the brightening is also not enormous: to get a habitable spot, you need a fairly nearby supernova or star, so the gain is not enormous.

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  • $\begingroup$ Thank you. About the last point, I think indeed the gain on a nearby passing star would not be that great. But at least the supernova case would have potential to greater gain, as it tends to be farther, and thus to be more point-like. I will wait a bit before accepting, in case other answers show up. $\endgroup$
    – ksousa
    Commented Jan 23, 2021 at 15:47

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