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Suppose someone dropped a black hole into our lovely Sun a few million years ago. It was big enough (far bigger than that) from the start to eat matter faster than radiating it away, and kept growing and growing. At some time in the future, it would consume most of the star and the change would be obvious to most. But there should be some less noticeable changes before that, something that we could detect with all the various instruments, earthbound and in space, directed at the Sun.

So, that is my question: what would be the first thing we would notice? Would it be increase in x-rays from accretion? Would it be some change in size or temperature due to slightly disrupted fusion? Or maybe something else? To be honest, I have no idea how to even start tackling this question, so any pointers are welcome.

  • Assume current-day measurement tech. It is acceptable to include currently planned missions/projects (or those on their way, like Solar Orbiter). Nothing beyond year 2050 though.
  • All that matters is that we notice something is not normal with our Sun, not figuring out that it's the black hole inside it. Anything that makes us go "Huh, stars don't do that" is preferable to things that have reasonable explanations, even if rare.
  • Bonus if you'll also calculate how much time we would have left till the Sun is gone, but I can calculate that on my own (thanks to HDE 226868).
  • Notice that since the black hole was added long ago, any measurements of Sun's mass included it, and since accretion doesn't destroy mass, gravity (probably) won't change in any noticeable way.
  • I do not care how we arrived to the current situation. All the question is about is: we detect something today because of a black hole growing inside the Sun. What would that something be?
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  • $\begingroup$ possible duplicate worldbuilding.stackexchange.com/q/74809/30492 $\endgroup$ – L.Dutch - Reinstate Monica Oct 20 '19 at 4:30
  • $\begingroup$ to avoid being locked as duplicate, define the black hole... the previous question and answer are only valid for that specific blackhole $\endgroup$ – V. Sim Oct 20 '19 at 4:52
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    $\begingroup$ @L.Dutch This is specifically about the case when the black hole is big enough to start accretion, as opposed to that question. The only answer to the linked question pretty much says "nothing happens" which is the opposite of what happens here. $\endgroup$ – Alice Oct 20 '19 at 4:54
  • $\begingroup$ @V.Sim I can't define it, it depends on answers to this question. If we can't detect a 1% $M_s$ blackhole, you can say it started there, if we can, then maybe 0.1%? 0.01%? As far as I can understand, I don't think hole's starting point and past history matter much, all I'm asking about is its final few thousands? millions? hundred millions? years. $\endgroup$ – Alice Oct 20 '19 at 5:00
  • $\begingroup$ To answer this question we first need to know what the minimum size is that allows the BH to eat more mass than it radiates. There is a question on the physics stack and one of mine that deals with this problem but they have no answer. Its not just that the BH needs to run into enough mass to keep going, that mass also has to overcome the pressure the Hawking radiation exerts to actually reach the BH. We dont know where that point is (or not on this site anyway). That said, the added weight would likely be detected first assuming the BH is on the border of being able to suck in enough mass. $\endgroup$ – Demigan Oct 20 '19 at 6:57
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When you said "big enough", you pointed to a question that mentions a black hole of about 6 $\times$ 108kg. The answer marked as correct in that question says that matter will mostly be unable to interact with the black hole at all.

Someone asked the scientists at Cornell how long a black hole one order of magnitude more massive than yours would take to consume the Earth. Cristopher Springob, a PhD in the subject, provided the following answer:

(...) A black hole that weighs a billion tons would have an event horizon that's only about 10-15 meters. So it would be so small that it would really only eat particles that happened to run into it, which wouldn't happen very often. If you were to plant it in the center of the Earth, it would just sit there forever, never consuming enough matter for anyone to notice.

If instead of setting it in the Earth's core, you were to drop it from the surface of the Earth, it would sink down through the middle, pop out the other side, and slide back and forth through the Earth for all eternity. If you assume that the black hole would only consume atoms that it happens to run into, then I calculate that it would take about 1028 years for it to consume the entire Earth, far longer than the age of the Universe. This assumes that the black hole wouldn't lose any mass due to Hawking radiation. If you factor that in, it would probably never consume the whole Earth.

... Which is consistent with the answer to the first question you linked. And though the sun may be very dense at its core, it is not dense enough to change the situation. We will never notice this black hole. In fact there might be a lot of those in the sun right now and we would never know.

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    $\begingroup$ I thought it was obvious from the following words that I mean a black hole way bigger than the one in the linked question, as I specifically said "big enough to eat matter faster than radiating it away". But I clarified the question nonetheless. $\endgroup$ – Alice Oct 20 '19 at 5:36
  • $\begingroup$ At the mass mentiomed in my answer it will matter faster than it radiates. But at an abismally slow rate. "it would really only eat particles that happened to run into it, which wouldn't happen very often (...)" $\endgroup$ – The Square-Cube Law Oct 20 '19 at 6:06
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    $\begingroup$ Look, that's not what the question is about. I ask what would we see when the black hole is big enough for us to notice something. Saying "well for some black hole "when" is "never"" doesn't answer my question. $\endgroup$ – Alice Oct 20 '19 at 6:31
  • $\begingroup$ @Alice if so then you need to specify your black hole's mass in another question. $\endgroup$ – The Square-Cube Law Oct 20 '19 at 14:19

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