Is it possible that some stars are already black holes yet we see light emitted from before becoming a black hole?

  • For stars we see that are burning fuel at a fast rate, that are very bright, (or any star for that matter sufficiently far away from us) could it be that they are already black holes (the sufficiently large ones) and we are just seeing the light emitted during it's main phase? So, if so, is it safe to say that most of the stars are already dead that we see?



    Edit: I appreciate all the answers. This has been a fascinating read. I also hate having to select a "the answer" when I gleaned something from all of them.


    No, the vast majority of stars visible with the naked eye are still "alive and kicking" today. It's quite unlikely that any star you see now has blown up already. Possible counter-examples are giants such as Betelgeuse, but even then the probability is not that great. The reason is - most naked-eye stars are quite close to us, so light doesn't take that much time to reach us, maybe a few years to a few hundred years. That's a very, very narrow time window at the cosmic scale.

    This depends on what you mean by "now". The concept of simultaneity gets a bit slippery once you consider relativity.

    @Mark: I've made that point now in my answer too. But if you take a "safe" definition of "already dead" to mean "dead in our past light cone" then the answer is (a) boring, "none of them, since a black hole is not in the main phase" and (b) clearly not what the questioner is referring to, they've merely expressed themselves using words whose jargon meaning in relativity differs from what is clearly intended ;-)

    @Mark This is true. I guess I was thinking if the observer (ooh! me, pick me!) were say within a safe viewing distance of the star would it already be dead and we (us?) on earth are seeing the remnants of the light that left it while it was still "alive"

    @Mark: for stars visible with the naked eye, it doesn't really make that much difference. I think the answer is going to be pretty much the same regardless of whether you choose an observer stationary relative to Earth, to the star in question, or to the galaxy as a whole. (Even taking GR into account isn't going to matter much; the answer will be the same for any reasonable choice of spacelike surface.)

    Betelgeuse, Orion's right shoulder is expected to explode in a supernova within the next million years. Some reputable sources say within 100,000 years. The estimates could be off. The star is about 640 LY away, so unless the estimates are way off, It has not yet exploded.

  • It depends what you mean by "see". I assume that you mean with the naked eye. Let's do some rough calculations to estimate the odds.



    The farthest single star visible to the naked eye is Deneb, estimated distance around 1550 light years. Most stars visible to the naked eye are much closer. Since even the sort of star that forms a black hole lives a few million years, we would expect perhaps as many as one sufficiently large star in a thousand to have formed a black hole during the time their light was on the way to us.



    There are about 5000 stars visible to the naked eye under good conditions, but only a hundred or so massive enough to form black holes. So on that basis, the odds are that there is at most one such star, and probably none.



    To estimate this another way, consider that there is on average one supernova in a galaxy per century. Comparing the distance to the farthest visible star with the diameter of the galaxy I estimate we can see at most 1/2500th of the Milky Way, so we should expect only one visible star to go supernova every 250,000 years. Dividing that by the distance in light years to the farthest visible stars, we get only about one chance in a hundred that any of the visible stars have gone supernova during the time their light was on the way to us.



    One caveat is that I've gotten these figures from disparate sources, based on Google searches. However, it seems clear that the odds are not good. Hopefully one of the experts can provide better data.


    Note that this estimate depends on what you consider to be visible to the naked eye. According to Wikipedia, P Cygni is visible to the naked eye at an estimated 6000 light years. Using my second calculation, that reduces the odds to about 1 in 7.

    Why limit to naked eye? I'm sure a consumer level 8 inch reflector would see a lot more then 5k stars naked eye on a clear night.

    @asawyer: no dreadfully sophisticated reason, it simply seemed a reasonable guess as to the question's intent given the way it was phrased.

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Content dated before 7/24/2021 11:53 AM

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