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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.

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    $\begingroup$ 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. $\endgroup$ – Florin Andrei Mar 31 '15 at 1:28
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    $\begingroup$ This depends on what you mean by "now". The concept of simultaneity gets a bit slippery once you consider relativity. $\endgroup$ – Mark Mar 31 '15 at 10:11
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    $\begingroup$ @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 ;-) $\endgroup$ – Steve Jessop Mar 31 '15 at 10:39
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    $\begingroup$ @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" $\endgroup$ – gigatexal Apr 1 '15 at 23:58
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    $\begingroup$ @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.) $\endgroup$ – Harry Johnston Apr 2 '15 at 20:32
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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.

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    $\begingroup$ 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. $\endgroup$ – Harry Johnston Mar 31 '15 at 3:20
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    $\begingroup$ 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. $\endgroup$ – asawyer Apr 2 '15 at 19:39
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    $\begingroup$ @asawyer: no dreadfully sophisticated reason, it simply seemed a reasonable guess as to the question's intent given the way it was phrased. $\endgroup$ – Harry Johnston Apr 2 '15 at 20:25
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No, I think that most stars huge enough to form black holes, need a few millions of years to do so from birth to death. Since we can only see stars in the Milky Way up to a few tens of thousands of light years away, only about 1% of them should've collapsed to a black hole since they emitted the light we now see here.

But many of the same kind of giant stars observed in the Andromeda galaxy, which is 2+ million light years away, might have turned into black holes "today". Because their light travel time is comparable with their lifetime. If one could observe such young giant stars more than 0.1 billion light years away, one could confidently say that ALL of them have turned into black holes now as the light of their past starlife reaches us.

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    $\begingroup$ -1 wrong. Bigger stars burn faster than small ones. $\endgroup$ – AJMansfield Mar 31 '15 at 1:18
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    $\begingroup$ @AJMansfield I'm not seeing anything in this answer that suggests the opposite...? $\endgroup$ – senshin Mar 31 '15 at 4:54
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    $\begingroup$ The question was "is it possible", not "is it probable", so you shouldn't start with an all-out "No". For the lest sentence, however... $\endgroup$ – vsz Mar 31 '15 at 8:16
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It is safe to say that a decent number of the stars we see are dead today. Space is infinite so it is a safe assumption that there are stars that are over a billion light years away that did not live a billion years. Therefore they will have died before their light reaches Earth. Additionally, the larger the mass of the star the quicker it burns its fuel, so the stars that will become black holes are less likely to still be alive. I don't have specific examples of stars that are likely to become black holes in the "future" but it is exceedingly likely. I put future in quotes because it happened 500 million years ago but the star is 501 million light years away so the fact that it is collapsing into a black hole won't reach us for another million years. It's hard to explain conceptually.

As an example: Star A is 400 million light years away. This means it takes 400 million years for the light to get here. We are seeing the star as it was 400 million years ago. If Star A officially "formed" (started fusion in its core) 350 million years ago and collapsed into a black hole 10 million years ago we would not yet see the light from the star in the first place, but it would already be a black hole. In 50 million years we would see the star start to form, and in 390 million years we would see the star collapse into a black hole.

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    $\begingroup$ Also I can't comment on the other answer yet (yay low rep) but I'd like to point out that it's rather incorrect. We can see millions of light years away unless you're talking literally by your own eyes and even then I'm pretty sure we can see outside the milky way and more than tens of thousands of light years just with our eyes. $\endgroup$ – Robert Wertz Mar 31 '15 at 0:33
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    $\begingroup$ 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. $\endgroup$ – Harry Johnston Mar 31 '15 at 2:42
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    $\begingroup$ @Harry Johnson Thank you for the correction, I had no idea that we were so limited. Spent too much time staring at the Hubble images and not enough using my own eyes I guess. I think the non "eyes" aspect still deserves consideration though. $\endgroup$ – Robert Wertz Mar 31 '15 at 4:06
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    $\begingroup$ Fair enough; it's a valid interpretation of the question, though my guess is that the OP meant naked-eye visible. There also seems to be considerable disagreement between different sources as to which stars are naked-eye visible; e.g., Wikipedia says that Rho Cassiopeiae is visible at circa 8200 light years, contrary to the source I gave earlier. $\endgroup$ – Harry Johnston Mar 31 '15 at 5:38
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https://xkcd.com/1342/ and http://www.explainxkcd.com/wiki/index.php/1342:_Ancient_Stars

https://xkcd.com/1440/ and http://www.explainxkcd.com/wiki/index.php/1440:_Geese

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?

For some stars, yes it could be. There are certain stars that we can individually observe, and that we believe could go supernova more-or-less at any moment. Betelgeuse is probably the most famous example. It's 640 light years away, it could go type II supernova any time (expected within the next million years, but could conceivably happen within the next 640 years as we observe it, such that it's already happened). It might become a black hole when it does (although a mere neutron star is also on the cards, I believe, and probably more likely).

Furthermore, when we observe a distant galaxy it is certain (as far as our theories are concerned) that some of the stars contributing to the light from that galaxy have since become black holes. However, since we aren't resolving individual stars in those galaxies even with our best telescopes, you could have an argument whether we're "seeing" the stars or not.

is it safe to say that most of the stars are already dead that we see?

Stars in our galaxy, the Milky Way, no. It's only around 100k light years across, it contains 100 billion - 1 trillion stars, of which we can individually "see" only a small proportion. Most of those are very close, the furthest around 1500 light years, and most of those are not supernova candidates at all, let alone black hole candidates. Even allowing for our best telescopes, we still only resolve a small proportion of stars individually.

The galaxy probably experiences roughly one supernova every 50-100 years. So even if all of those were visible stars (which they aren't: even of the largest stars in the galaxy only a small proportion can be seen with the naked eye), and all the supernovae resulted in black holes (which they don't), then at most 15-30 naked-eye-visible stars could already be black holes, and only up to 1000-2000 total stars in the whole galaxy regardless of visibility are in the duration between going supernova, and earth entering the future light cone of that supernova. This extremely loose upper bound is by no means "most of the stars that we see", and in fact the chances are that none of the stars you see when you look up at the night sky are already black holes. It might well be that none of them ever will be.

It's also unlikely that any star you observe with the naked eye is dead, since their lifetimes are of the order of 10 million years (for the largest) to billions of years, and their distances from earth are 1500 light years or less. But there are a few stars, like Betelgeuse, that we know are near the ends of their life and that therefore might be "already dead" in this sense.

Using telescopes we can see some individual stars in nearby galaxies, or at least indirectly observe them since they're lighting up nebulae that we see, up to a few tens of millions of light years away (these tend to be blue supergiants, I think). Of these, those on course to turn into a black hole may well have lifetimes less than or equal to their distance from us. It's not necessarily the case that all or even most of them become black holes, I'm not sure what's typical of the types of star in question, but for those stars you can say that many or most are already black holes. The most distant individually-resolved stars are all dead one way or another.

If you allow for us "seeing" each star in a very distant galaxy, even though we can't resolve any of them individually before they supernova, then the numbers change again, but my instinct is that it's not the case that "most" of the observable universe is already collapsed into black holes. But the stars may well be dead even though they aren't black holes. We can see galaxies 10+ billion light years away, which is about the entire lifetime of a star like the Sun. To come up with "most", though, you have to firstly agree what counts as a star and secondly survey what kinds of star those galaxies contain. If you end up concluding that most stars are (by comparison with the Sun) tiny little red-brown things with lifetimes in the tens of billions of years, then most stars aren't already dead no matter how far away the galaxy they're in :-) On the other hand, the brighter objects that contribute most of what we observe via telescope in the most distant galaxies, sure, certainly die in far less than a few billion years.

Note that in special relativity the concept of "simultaneous" is a bit difficult to pin down. But I've followed the convention that if something is X light years away, then anything we observe from it between now and X years from now, has "already happened". We're not necessarily entitled from where we're standing to say that it's "already happened", since it's not in our past light cone, but good enough :-)

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Not all stars that die become black holes. Only the really, really massive ones, the "giants" they call them. They are rare. None of the stars visible to the naked eye from Earth are massive enough to be black holes. Scientists can tell the mass of a star using the Hertzsprung–Russell diagram.

It is a possible scenario however, just not on Earth.

It's possible because the light emitted from a living star will travel forever until it is absorbed by matter in it's path. So, a star can be dead for eons before the light is finally absorbed by something—like an alien planet and alien eyes—far away and long after the light-bearing star has reached black hole status. It's just that specific to Earth's situation the visible stars are not black holes because their masses aren't great enough.

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    $\begingroup$ My research suggested that Xi Persei, P Cygni, and Rho Cassiopeiae are examples of visible stars that would one day eventually become black holes. However, there seems to be some disagreement between different sources over what constitutes a "visible" star. Can any experts chime in here? $\endgroup$ – Harry Johnston Mar 31 '15 at 5:36
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It is possible. Say Mankind formed 195,000 years ago (oldest humanoid fossil found) and a very massive star closer than 195,000 light years away went supernova or even imploded into a black hole with in 195000 years, there is a chance that some humanoid may have seen it. Historically people have documented seeing supernovas in the past and if any of those would have been massive enough stars, then it may a black hole formation that was witnessed. However no black holes are located in the areas described by the descriptions made by the witnesses but seeing one for is possible, just hope its not very close to our system. The resulting GRB might ruin your day.

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    $\begingroup$ If the massive star that imploded was more than 195000 light years away or imploded sooner in years than its distance in light years we would not have seen its death yet. For example if a star 19000 light years away imploded 18000 years ago we would not be able to detect its change for a 1000 years. $\endgroup$ – JMC Sep 25 '16 at 19:15

protected by Sir Cumference Sep 26 '16 at 20:23

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