Just to be a little picky…
“Nova” and “Supernova” are different events, despite the similar name. A supernova is what we’re generally talking about here; the explosion of a massive star at the end of its lifespan. A nova is a phenomenon that occurs in binary stars and is probably better explained by someone smarter than me:
Its impossible to know which individual star is about to metamorphise …
But the rate of supernova in the milky way is one in 50 years.
So around 10 or 20 … so if you pick the 20 most likely candidates in the milky way, you’d be largely correct.
“A decaying aluminum-26 nucleus emits a gamma-ray photon at a characteristic energy of 1,809 kiloelectron volts. The gamma rays can travel across our galaxy virtually unimpeded by gas or dust.”
However the article notes that the supernovae haven’t been seen lately, the last was observed in 1680 …
One wonders is this because the dense core blocks the light whereas the gamma rays get through… There’s enough supernovae debris to say “one every 50 years”… But like trees in the forest - do they all go at the one time, due to all being hit by the same storm, or equiv. ? Perhaps collision with nebulae ? supernovae begat supernovae ? Chain reaction. OR perhaps they are going off in a random way, and the lack of observation may just be because of something obscuring the visible light. Because the article mentions the rate of growth of stars in the milky way …they are growing and getting fat… which primes them to turn nova.
As usual, there’s a wiki list of supernovae candidates. Unfortunately, that list doesn’t give any indication of how likely that the star will go boom. Some are much more likely than others.
I’m not an astronomer, but high on my list, perhaps even higher than eta Carinae, is rho Cassiopiae.
Betelgeuse is another possible candidate. Estimates vary considerably, but I’ve seen some as low as sometime in the next thousand years, and it’s about 650 ly away.
Beetle Juice
Beetle Juice
Beetle Juice
It’s GQ, so if I may be pedantic… “reality” isn’t defined as you’re using it here. Possible quantum mechanical fuckery aside, the speed at which information propagates cannot exceed the speed of light. We have no way to communicate with beings within the star’s immediate reference frame, nor any way to see that it no longer exists, nor can we travel near it to witness its existence in any reasonably immediate timeframe. So, while said star may have ceased to exist in its own reference-frame, it continues to exist in ours.
No, it’s perfectly fine to state that the star has ceased to exist in our reference frame even though we still see its light. There will be other reference frames in which it still exists, but those reference frames won’t typically be of any particular interest to the problem. The only reference frames of interest are the one where we, the observer, are at rest, and the one where the star is at rest, and those are typically going to be so close that it’s not worth distinguishing between them.
I’m with Chronos on this one, but it’s an interesting question. Did the Battle of New Orleans take place after the War of 1812 was over? Yes and no. It seems obvious that it was part of that war – Andrew Jackson just hadn’t received the message yet. Yet, we have no trouble also saying it took place after the war had ended.
Sorry if my analogy is a poor one, but it seems appropriate and helpful to me, anyway.
I defer to your scientific knowledge, Chronos, but I don’t understand how we could know that a star had ceased to exist while its light was still arriving as usual. Obviously, once we see it go supernova, we know it’s gone, but how could we know, except as a statistical likelihood, before then?
I was also under the impression that when speaking in terms of interstellar scale, it doesn’t really make much sense to speak as though there is a universal “now,” as the OP seems to do. Buckminster Fuller claimed that Einstein coined the term “non-simultaneous universe,” which in my mind has always suggested the notion of multiple, independent frames of reference that may not always agree about the order in which events occured, but are nevertheless correct from their point of view.
So I would answer the OP by saying, there undoubtedly are stars whose incoming light path to us will, within the next few years, reveal that they have died. But we have no way, and can never have any way, of knowing which ones they are before the light bearing that information arrives here.
A second vote for Rho Cassiopeiae. To have a statistically high chance of exploding while the light is still travelling to Earth, the candidate star needs to be as far away as possible; if you want to be able to see it as well the star needs to be very bright. Rho Cass ticks both those boxes. Eta Carina is even better but it can’t be seen from the North.
For simplicity’s sake, let’s imagine a star is N light years away where N is some number greater than the number of years any star can “live”.
And let’s say this particular instant of time on earth we call time t.
It’s natural to think the star is “really” gone, and that at time t there is no star there.
But there is no simultaneous t across two distant places.
If we could fly to that distant place at the speed of light, the journey would take zero time (so my watch would still say it’s time t) but we might arrive and find a new star there. Who’s to say which reference frame is correct?
Or have I misunderstood?
Probably the main difficulty with answering this question is that we haven’t studied any stars that went supernova before they did. All supernovae in the telescope-era have been in other galaxies. Even the close one in the LMC, Sanduleak -69° 202, was not studied even though it had been cataloged. It was a bluish star and not thought to be on the verge of exploding.
So we only have a rough idea of what a pre-supernova looks like and how it acts.
We don’t know, of course, and won’t until we see the light. But once we do see the light, we’ll be able to say “A-ha, that means that the star has now been dead for 642 years”, or whatever.
Like I said, there are reference frames where that star has not yet exploded. And those reference frames are exactly as “correct” as any other. But they’re not interesting, and there’s no reason we would use them.
Let’s try an analogy.
In the early days of American colonialism, a man leaves his country to settle in the New World. His brother sends a letter every month. Each letter takes about a year to arrive. After twenty years, the brother writes that his health is failing and he will probably be dead within a year. But he will keep writing until the end. When the colonist receives this, he knows that his brother is already dead. But there are still ten or eleven more letters in transit. He will still get news of his brother for months after the brother died. And he will know that these are letters from a dead man.
Sure, and the reference frame where we’re traveling at c and the universe is squished to a pancake is interesting.
But what makes the reference frame where humans are fixing Y2K bugs while there’s no LG5221 (or whatever the next star to go pop is called) the right one?
Why is that considered the reality?
It’s not the right one, but it is one we use constantly for almost everything that matters and most often call “reality”. It’s the one where we, the debating observers, are not moving.
How far away does something have to be before you start questioning someone calling this homey reference frame the default “reality”?
Thanks, Peter, but believe it or not, I’m not quite as stupid as I seem. 
IF we somehow had accurate information that a particular star was going to die in the (relatively) near future, we might know that it had “already” died, even though its light was still arriving as usual. But as dtilque points out, we haven’t studied supernovae closely enough to know if there are warning signs, and if so, what they are. Even so, that’s still not knowledge that it has definitely died, only that it’s not feeling well, and could go at any moment. (“I feel fine!”)
In your analogy, a traveler who could go faster than the letters might bring news of the brother’s death before some of the last letters. In Star Trek and a lot of other sci-fi, people can travel and send messages faster than the speed of light. But in our universe, the speed of light is the absolute speed limit, as far as we know, for transmitting information. So absent supernova warning signs (which even if we knew them, might not provide accurate estimates of the imminent end within a margin of plus or minus many millennia) we cannot know that any particular star has “already” gone supernova.
On the surface of a small planet where dying brothers send written messages by horseback, it is reasonable to believe that the concepts of simultaneity and a “universal now” are meaningful. And they are, until we need to do things like build a Global Positioning System or bounce signals off satellites that are significant fraction of a light-second away.
But as someone once said, the universe is big, really big. And the speed of light is really slow relative to the size of the universe. This means that many common-sense attitudes about time and simultaneity don’t work on that scale. In a relativistic universe, two observers can see two events in a different order, yet both are “right.”
So ISTM that saying that a star went supernova “1,000 years ago” is in a certain way treating the universe as if it were a small planet and light as if it were a hand-written letter. There are implicit assumptions in the statement that are simply not appropriate in that context. It is human nature to think in those terms, and although it is only in more serious scientific contexts that the inaccuracy would matter, I think that here at SDMB we should be trying to open our minds to bigger (if more confusing) truths.
The thing is the Universe and the planet example aren’t that much different as the only set of reference frames of interest qualitatively give the same ordering in time between events of interest and quantitatively don’t differ much in the perceived time intervals between those events.
Again, it’s not a matter of which reference frame is “right”, or is the “true reality”: They all are, and if you say that in the reference frame of some ultra-high-energy cosmic ray the star has not yet died, then you’d be correct. But I would then ask “Why are you telling me the timing in that cosmic ray’s reference frame? That’s not useful to me. All of my clocks and calendars are sitting in my lab, not riding that cosmic ray.”.