Could it be that all the stars we see in the sky no longer exist?
Sure, if everything we think we know about stellar evolution is completely wrong.
I don't think so. There must be some universal gravitational equilibrium for our solar system to exist especially at the edge of a spiral galaxy. Some of these star may not exist but I think the majority do.
If the closest known star next to our Sun is, as it has been measured, four and half light years away from our planet, could it be, therefore, that within four and half years we will discover that we are alone in the universe?
As in somebody turned off all the stars at once? If so, then the answer is no. If every other star in the Universe quits shining tomorrow, the only thing we'll notice in 4.3 years will be that Alpha Centauri has vanished. The other stars will continue as before. After 6 years, Barnard's Star (the next closest) would vanish, then the next closest, then the next...
The only way we could ever see the Universe 'go dark' all at once would be if all of the stars around us had been disappearing in an inward-moving pattern (a star a million light years away disappeared a million years ago while one 10 light years away disappeared 10 years ago.)
As suggested above, the physics of stellar evolution indicates the answer is no. It takes millions/billions of years for stars to burn through their fuel. During that timeframe, they go through different phases. So, by seeing what phase the star is in, you can get an idea how old it is and how much longer it will be burning.
The stars we see in the night sky (by eye) are all "local" (relatively close compared to the size of the galaxy). Close enough that the time lag in our view of their current condition (based on the travel time of the light) is small compared to the stellar evolution stages. In short, most of them will last longer than the time it takes for their light to reach us. Perhaps the answer to your question would be 'yes' if all the stars we see in the night sky were in their final active stage. There are a few which could have gone supernova by now, but it's unlikely.
Even if they do burn out/explode, their remains would still be there for huge amounts of time afterwards.
Any way the closest star alpha centurai is invisible to naked eye. So there is a huge probability that there can be many dim stars b/w alpha and us. We cannot say with 100% sure that it is the nearest star ?
Alpha Centauri, being the brightest star in the Centaurus constellation, is very visible to the naked eye.
Perhaps you are thinking of Alpha Centauri C, or Proxima, Which is the smallest and dimmest star of the Alpha Centauri system and is at present the closest of the three stars.
I don’t know whether stars nearest to the sun than Proxima Centauri can be still found. However, in our neighborhood within 10 pc there have been discovered lots of stars in the last years. You can read more about this in the http://www.chara.gsu.edu/RECONS/ [Broken] homepage.
Unless a star was someohow "special" (a brown dwarf or something), we'd absolutely see/detect it if it was closer than 4.5ly.
edit: Actually, hasn't there been some speculation that there might be a brown dwarf step-sister of our sun? Or is that a pX hoax I'm thinking of?
Would that little brown companion star be Nemesis?
Since its only idle speculation and exists only in my head, it can be whatever I want it to be. I call it "Bob". However, it can be said with some certainty that it is not the "Nemesis" or "Planet X," or whatever that crackpots have been claiming for the past several years. That star doesn't exist (either).
Why do you have to call it Bob? Can't you call it Quincy, or Aloysius, or something?
Are you referring to http://www.binaryresearchinstitute.org/BRI-Evidence.pdf [Broken]? Their paper seems to be the one most often referred to. I thought it was interesting.
Of course, it's main obvious drawback is that we haven't actually found a companion star.
Edit: Okay, once I actually read the article, it's pretty thin. The author's background as investment banker and venture capitalist probably don't add to the credibility of his theory, either. An interesting idea, anyway.
Teegarden's star - very close?
http://www.chara.gsu.edu/RECONS/new.SO025300.5+165258.htm [Broken] of the status of Teegarden's star (SO025300.5+165258). While it's unlikely to pip many in the RECONS 'nearest 100', it does illustrate that there's still a lot to learn about our immediate neighbourhood, even in terms of the stars!
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