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Hypothetically, let's say we had a gas giant that continued to accrete mass. I've heard that the cores of gas giants are electron degenerate. So if the planet continued to accrete mass and the core mass went beyond the Chandrasekhar limit, what would happen?

In white dwarfs, the result is dependent on the composition. Carbon-oxygen white dwarfs will undergo carbon fusion, leading to a type 1a supernova. Oxygen-magnesium-neon white dwarfs will undergo rapid oxygen fusion, leading to a rapid ignition and supernova but leaving behind a neutron degenerate core.

So would the composition of a gas giant's core play a similar role, if it went beyond the Chandrasekhar limit? What would happen if, say, Jupiter somehow accreted a core mass beyond the limit?

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Gas giants like Jupiter consists mainly of Hydrogen and some Helium. If you gradually add mass to these planets then core temperatures will rise gradually and a stage will come where they will ignite like normal stars. Unlike white dwarfs where accretion of mass leads to type 1A supernova.

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  • $\begingroup$ Not neutronization? $\endgroup$
    – Sir Cumference
    May 18, 2016 at 15:13
  • $\begingroup$ @SirCumference Nope, planet cores won't convert protons+electrons into neutrons. Adding hydrogen will just give it fusion material so that once it becomes big enough, it will turn into a star. If you start with a Jupiter mass planet, you would have to increase its mass by 75 times before it had enough to ignite and become a star. $\endgroup$
    – RichS
    May 18, 2016 at 15:42
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    $\begingroup$ @RichS But it only has to increase 13 times to fuse deuterium and 65 times to fuse lithium, right? In other words, it would become a brown dwarf before a star, yes? $\endgroup$
    – called2voyage ♦
    May 18, 2016 at 16:13
  • $\begingroup$ @called2voyage Ah, yes, the lower mass limit for fusing Deuterium is assumed to be 13 Jupiter masses. So adding 13 Jupiter masses of Deuterium to a gas giant will turn it into a brown dwarf. Not sure where you will get all that D! $\endgroup$
    – RichS
    May 19, 2016 at 6:06
  • $\begingroup$ What if you add matter at a slow enough rate that the core temeprature never goes above a few hundred kelvin. ( I know this will require timescales far longer then the current lifetime of the universe but whatever.) $\endgroup$
    – blademan9999
    Sep 20, 2021 at 5:40
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@Knu8 was right that adding mass to a gas giant will turn it into a star long before the gas giant could become a white dwarf or neutron star. But that works if you add fusionable material such as hydrogen. If you add something that can't fuse into a heavier element, such as Iron or Tin, the matter just keeps accreting onto the gas giant until the planet collapses under its own weight until electron-degeneracy pressure holds it up. That's when it becomes a white dwarf.

Thanks @Knu8 for the right answer. I gave you an upvote for that. I posted a separate answer to describe what happens if you add heavier elements.

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    $\begingroup$ And then as you continue, it will collapse and explode as some sort of core-collapse supernova... $\endgroup$
    – ProfRob
    May 18, 2016 at 20:16

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