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If a star accretes enough mass it collapses into a black hole (I am not a physicist or an astronomer so I guess this is the simplified version).

But what would happen if a forming planet continued to accrete mass (*)? What transformations would it undergo as it mass increases? Would it become a star? Would it still be a planet until it becomes a black hole? Or something else entirely?

If the answer is different depending if the planet is solid or gas, please consider and explain both possibilities.

(*) hypothetically, let's ignore other problems like how would it do that, or if it would be able to maintain orbit or fall into the star, etc.

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    $\begingroup$ What type of mass is added? Hydrogen? Iron? Rubber ducks? Plutonium? $\endgroup$
    – SE - stop firing the good guys
    Commented Oct 31, 2016 at 13:10
  • $\begingroup$ @Hohmannfan since this is a hypothetical question, I don't have an answer for your (valid) question. Guess I expect an answer along the way "if you add that that happens, if you add that that happens". It's just a curiosity of mine. Would a planet become a sun, an ever increasing ball of rocks or some other interesting thing. I am sorry I am cannot be more helpful, it's not like I am writing a novel where a planet is continuously bombarded with iron or something like that. What I am trying to say is I have no reason to pick hydrogen over rubber ducks or vice-versa. $\endgroup$
    – bolov
    Commented Oct 31, 2016 at 13:45
  • $\begingroup$ It's incredibly easy to find a few astronomy sites which will tell you that planets which gain mass grow into brown dwarfs and thence to becoming a star. $\endgroup$
    – Carl Witthoft
    Commented Oct 31, 2016 at 14:22
  • $\begingroup$ Stars can't accumulate enough mass to become a black hole, as they accumulate more mass they grow larger. Stars become black holes when they form and collect enough Iron in their core. Similarly, as noted, a planet accumulates enough matter it becomes a star. This may stop being true when hydrogen stops being the most abundant element in the universe, but for now, this is universally true. $\endgroup$
    – userLTK
    Commented Oct 31, 2016 at 16:55
  • $\begingroup$ Hi Bolov. Sure, if a planet "weighs more" it becomes a small star. (If our own Jupiter was a bit bigger it would be a star.) Your question is so general, that no more specific answer can be given. And sure, once something is a star, if it becomes even bigger and bigger, in time it will likely be a black hole. You can very easily go to Wikipedia and read a tremendous amount of popular-science level topics on "star formation". (I love popular-level astronomy, so go enjoy!) $\endgroup$
    – Fattie
    Commented Oct 31, 2016 at 17:35

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It only matters if that mass is iron or not. The state doesn't really matter as it will turn to plasma sooner or later due to gravitational energy transformed to heat.

If a planet continues to accrete mass, if it would be any element before iron in the periodic table, it will - given enough mass - become a star.

If it is iron, it will collapse down to a neutron star or black hole if the mass is enough.

If it is anything heavier than iron, it either does the same as with iron or will go critical and explode in a nuclear fission reaction.

Since most mass in the universe however is either hydrogen or helium, usually the first thing is what happens.

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  • $\begingroup$ There's also the Chandrasekhar problem to deal with. A planet of Iron above 1.4 (maybe 1.2-1.3) solar masses would overcome it's electron degeneracy and rapidly condense, but in condensing weird things would happen and some (perhaps much) of the material could bounce off or get blown off the rapid shrinkage. Type 1A supernovas don't leave neutron star remnants, they're too small. I don't know if a pure Iron/metalic white dwarf could leave a Neutron star or not. $\endgroup$
    – userLTK
    Commented Oct 31, 2016 at 20:54
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    $\begingroup$ If it is all iron (which is pretty unlikely) it will not go supernova if it reaches the Chandrasekhar limit. The supernova trigger in that case is triggered by enough mass compressing the core of a white dwarf to initiate a run-away fusion reaction with the remaining oxygen, nitrogen and carbon within the dwarf. If it is the hypothetical iron, it will not fuse due to iron having the highest binding energy of any element and simply cannot fuse to anything with more. It will accrete iron until it overcomes electron degeneration pressure and then collapse to a neutron star. $\endgroup$
    – Adwaenyth
    Commented Nov 3, 2016 at 6:18
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    $\begingroup$ Wow... ran out of space... anything between carbon and iron might result in a supernova though once it reaches the Chandrasekhar limit and not leave a stellar remnant as the energy will just blow the dwarf apart. But usually it would be hydrogen or helium falling onto that object which would result in it becoming a star anyways and going through the main line. Accreting mass like this is actually the normal way of stellar evolution. $\endgroup$
    – Adwaenyth
    Commented Nov 3, 2016 at 6:24
  • $\begingroup$ You can edit your answer if you'd prefer rather than comments, but I think you're probably right on the runaway fusion reaction making all the difference. $\endgroup$
    – userLTK
    Commented Nov 3, 2016 at 7:23

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