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Are there any objects out there that have since vanished, but because of their distance and the travel time for light, can be seen with the naked eye? Or are those things only visible with magnification? How much magnification at least?

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Could You explain what You mean with "vanished"? – Georg Feb 16 '11 at 21:31

Betelgeuse is obviously a naked eye object. It is a red supergiant about 600-700 light years from Earth. Its mass is almost certainly large enough that it will end its life in a supernova explosion and then fade into obscurity quite quickly after that.

There is significant uncertainty about when this will happen - basically any time over the next half a million years. There is unlikely to be much warning it will happen, certainly not on a timescale of tens or hundreds of years, so it is possible that Betelgeuse has exploded and vanished and we are going to find out about this sometime in the next 600 years or so.

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Most famously, the "pillars of creation" part of the Eagle Nebula that appeared in the famous space telescope pictures, are thought to have probably been blown away by now. They are 6000 some lightyears away, and strong interstellar winds are eroding them, so if we could be instantly transported there we might find them gone.

Also high mass stars have short lifetimes of only a few million years. And these are the ones we can see in distant galaxies. So in a galaxy say a hundred million light years away, the brightest stars we see should by now have all died (presumably replaced by new ones).

We can detect gamma ray bursts from several billion light years away (and they only last at most a few minutes).....

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Do supernovas count? They could be visible with naked eye for a few days.

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Exactly. For example, this 10-year-old girl… has seen (as the first person) an object that's been vanished for 240 million years - a supernova - but it took 240 million years for the light to get to her eyes and make her a star on TV. ;-) She saw it by naked eye, and she could, but it was still helpful to make a photography and compare them by PC programs. – Luboš Motl Feb 16 '11 at 21:56
Ah, that's where I knew it from. I searched for it in a number of blogs I follow but couldn't find it. – MBN Feb 16 '11 at 22:02
@Lubos, that might be an answer, but "marienbad" asked for an object which vanished and "can" (not could!) be seen with naked eye. – Georg Feb 16 '11 at 22:05

There's no way to know. The only way to tell whether an object has vanished is by the fact that its light (or other EM radiation) stops reaching us.

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or possibly by gravitational lensing of other objects' light by the vanished object. (Again, guess it depends on what vanished means, sort of like Clinton's "It depends on what the meaning of "is" is.") – Gordon Feb 16 '11 at 22:06
I guess, theoretically, we could see two white dwarves on a collision course, and infer that we will observe their collision, and that they will then collapse and vanish. But I don't know of any such systems. (Though I don't know about the total inspiral time of the Hulse-Taylor binary pulsar as compared to its light travel time) – Jerry Schirmer Feb 16 '11 at 22:08
And I just looked it up and the answer is no--the inspiral time for the Hulse-Taylor Binary is 300,000,000 years, and the system is 21,000 light years from us. – Jerry Schirmer Feb 16 '11 at 22:10
But you can - at least theoretically - know that a core-collapse supernova is on its way by the neutrino flash, and predict when you'd see the actual explosion in EM radiation (but the neutrinos are obviously not visible to the naked eye). The actual explosion is a bit delayed by the speed of sound in the exploding star, where the neutrinos can escape almost instantly. Also, when you see a supernova, you are pretty safe to assume that the object has already vanished :-) – Thriveth Jun 6 '13 at 14:30

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