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Do galaxies have habitable zones the same as stars do? Say in a galaxy with a very active nucleus producing a lot of heat and radiation, would there be a point at which no star's planets could harbor life do to the effects of the black hole? Also would there possibly be a habitable zone for galaxy clusters? If there were many galaxies with extremely active galactic nuclei condensed closely together would this possibly also hinder the evolution of life?

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The Sun has done more than a dozen tours around the Milky Way while there was life on Earth. If there is a galactic HZ it doesn't seem to be small. Icy objects with subsurface oceans might be common and they are, like the oceans on Earth, pretty well shielded from harmful radiation. I don't think that planets care too much about the galactic scale, it's so empty out there. They worry more about comets and their own star. –  LocalFluff Mar 23 '14 at 18:01

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stars themselves, like our sun does, have a heliosphere. the size of this heliosphere would vary depending on the size/strength of the star and it's position in the galaxy. and what happens outside the heliosphere has usually little effect inside, in terms of radiation. so, the presence of a black hole or an agn near the star doesn't matter as long as the star itself is strong to create a large heliosphere (in which the planets reside). we can talk about the problem case by case but i don't know of a general galactic version of the goldilocks zone.

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Besides Gamma Ray Bursts ;) –  Envite Mar 25 '14 at 10:06

The zone, you mean, in galaxies would be very unstable - huge changes in electromagnetic flux (something like explosions) because of more dynamic changes of the environment there (than in the outer rim of galaxy: black hole, jets, etc...) - so there is considered lower probability of life in the center of galaxies (or Galaxy). But in fact I think we have to few information about galaxies to seriously think about probability of life on free planets close to the center of the Galaxy (the more other galaxies).

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