It’s almost certain that we have not discovered all stars out to 20 light years. In fact, just last year, some astronomers discovered a star (named LHS 1723) that is about 18 light years away. This is the web page of the RECONS project which has as a mission to discover stars in the solar neighborhood and discovered that star (see their press release from Jan 2002 where they say it’s 20 light years away, but give a parallax corresponding to 17.8.).
Not true. Proxima, as seen from Earth, is quite a ways away from the other two.
The Nemesis theory is interesting. I read Muller’s book about it just recently. It seemed convincing to me, but then again I have no background in the technical details involved, so if he was fudging figures or omitting conflicting evidence, I wouldn’t pick up on it.
Here are some quotes from his webpage that might be pertinent:
If I understand the description of the Hipparcos catalog correctly, they’ve measured the parallax of 118,000 stars, that’s ALL of them up to ~magnitude 9, and not found nemesis.
Hipparcos Catalogue:
Number of entries 118 218
Entries with associated astrometry 117 955
Entries with associated photometry 118 204
Mean sky density ~ 3 per square degree
Limiting magnitude V ~ 12.4 mag
Completeness Up to V = 7 .3 - 9 .0 mag
Ive heard Nemesis can be anythng from a planet slightly larger then jupiter to a burnt out steller core - so the size difference is massive. Either way it wouldn’t emit much of it’s own light and not detectable w/ our current tech.
As far as other stars coming close to us, here’s an old thread that will be of interest. Note especially the discussion about the star Gliese 710 and how a million years from now it’s going to make a close approach to the Sun (give or take a light year or so).
To quote manhattan: RUN!
As far as Nemesis. I don’t think that we’ve ruled out the possibility that it could be a brown dwarf. A brown dwarf the age of the sun will be very cool and dark.
Burnt out stellar cores are seen all the time, they’re called white dwarfs, another class of observed dwarfs are called brown dwarfs and haven’t got enough mass to be stars. A planet slightly larger than Jupiter is verging on a brown dwarf which we have detected. If this nemesis is on a highly eccentric orbit with our sun, it still means it has to be fairly close (Oort Cloud-like distances). That there’s an unobserved object that’s larger than the size of Jupiter out there with a proper motion is rather unlikely. People have been looking for such things for more than 100 years.
I have also seen at least one paper that argues that there is a threshhold limit for stable orbits of large “binaries” of planetary systems… that a nemesis would either eject the planets from the system or itself be ejected. In short, it’s not entirely out-of-the-question that nemesis exists, but I would be willing to bet against it.
A point of clarification, by the way, about the Alpha Centauri system. If you look up in the sky at the constellation Centaurus, the brightest thing you’ll see in that constellation is [symbol]a[/symbol] Cen. [symbol]b[/symbol] Cen is the second-brightest thing in the constellation, and is in a completely different place. Now, what we see as [symbol]a[/symbol] Cen is actually two stars, comparable to the Sun (if I recall correctly, one is type F and one is type G). The larger of the two is [symbol]a[/symbol] Cen A and the smaller is [symbol]a[/symbol] Cen B, not [symbol]b[/symbol] Cen which is a completely different star (or system; I don’t know if it’s a binary). Then, there’s Proxima Centauri, which is a red dwarf (type M, much fainter than the Sun) which happens to be close to the other two. It’s probably a member of the [symbol]a[/symbol] Cen system, which would make it [symbol]a[/symbol] Cen C, but some astronomers think that it’s not bound to the system and just happens to be nearby. Either way, it’s the closest known star to the Sun.
For those interested in learning about the Alpha Centauri system, Here’s a site with lots more info. Note for example, that there are hints that Proxima may have a large companion, possibly a brown dwarf.
I just ran across this article, which I’m kind of surprised nobody referenced before.
“New Neighbors for the Sun,” by Richard Talcott in the May 2002 issue of Astronomy magazine.
12 new stars (seven individual stars, a double star and a triple star system), eleven of them white dwarves, one of them a red dwarf, have been found within 10 parsecs (32.6 light years) of Earth. They were found by a Chilean observatory, and more may yet be found because the southern sky has not been so thoroughly mapped as the northern sky.
The article didn’t say exactly how far away any of the individual stars were.
The Todd Henry mentioned in that Astronomy article is the main guy of the RECONS project, whose page I gave earlier in the thread (Here it is again so you don’t have to go looking for it.)
Because the page uses frames, I can’t give a link to their Jan 2002 press release, but here’s an interesting quote:
BTW, you or that Astronomy article have an error. The dozen new stars are 11 red dwarfs and 1 white dwarf, not the other way around.
Another general version of the OP’s question is this: Is it possible that there are things that we don’t know or know about? Well, of course. In fact, most advances in knowledge are not enormous leaps but refinements - not revolutionary, but evolutionary. In this case, we know a good deal about the world we live on, its nearest neighbors, etc. Each year, we discover more about what we already know and even add a little bit, maybe even discovering small, previously unknown stars. That means that there is more to be known, but it’s unlikely that we’ll discover that there are, for example, lots of invisible monsters inhabiting the space between the stars. I used to have a sign on my science classroom wall: “…As far as we know…” and I referred to it plenty - to indicate the transitory and evolutionary notion of knowledge. So, of course there could be stars and galaxies we haven’t discovered. Yet. Except for the Great Master of the SDMB, no one knows it all.