On Nearby Stars

The above video is something I made for my old research group, the Research Consortium on Nearby Stars (pi: Todd Henry).  I think it does a fairly good outlining what we know about the nearest stars, and the majesty of the whole thing is pretty well uplifted by that soundtrack (from Clarence Yapp).

Everything you see in the video is what’s in the RECONS 25 parsec database; the only things I added were exact positions for the planets (the planets themselves are in there, although the only note about the Earth is that it’s “Mostly Harmless”) seen at the very beginning, and I made up that Oort cloud because it looked cool.

Aside from what you can see in the video, there are a few things I’d like to point out.

The old and famous Gliese & Jahreiss Catalog of Nearby Stars contained, in its unintentionally final preview of the third edition (c. 1991), 3803 stars in a difficult-to-count number of systems (they kept changing how they kept track).  The new catalog contains 3069 objects in 2167 systems, even though more than 20 years of new discoveries have passed, including the discovery of every single brown dwarf and planet in the database.  Why are there fewer systems? Our standards are higher now.  Every single system in the new catalog has a parallax (see previous posts) with an error of less than 10 milliarcseconds (25% distance error at most).  The old catalogs included other less reliable methods, and many of its stars have turned out to not actually be within 25 parsecs.

Is this every nearby star?  Nope.  If you take the value for the number of stars at 5 parsecs (51) and assume that density holds for all of space, you’ll find that at 25 parsecs there should be 6250 systems, not 2167.  We’ve only found about 1/3 of everything, by parallax at least.  Most of what’s left to find and get parallaxes of are really dim and cold M dwarfs and brown dwarfs.  You can see this in the spectral type segment when the redder objects are all clustered near the center, i.e. closer to Earth where they’re easier to see and measure distances to. The list is also missing a few famous recent discoveries like Luhman 16 and the planets around Kapteyn’s Star, just because they weren’t there by the time RECONS froze the catalog for testing (and imminent release).

Why are there so few M dwarfs?  Everyone cites Henry et al. (2006) as saying that 75% of all nearby stars are M dwarfs, but if you run the numbers in the video, it’s just higher than 53%.  Well, those numbers, plus the white dwarfs, brown dwarfs, and planets, add up to only 2394 objects.  We don’t know (or at least RECONS hasn’t found records of) the spectral types of 675 known objects, and then there’s the other several hundred completely missing systems full of stars that I mentioned above.  Most of those are going to be M dwarfs.

Is Castor really a sextuple?  Yes, it seems to be.  Alcor and Mizar, a visible pair in the big dipper that’s actually a single gravitationally bound system of SIX stars, just barely misses the 25 parsec cut; they’re at something like 25.1 parsecs.


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