What do the spectral types mean?
The sequence goes O,B,A,F,G,K,M,L,T and relates to temperature, hottest (O) to coldest (M). M stars are only red-hot, G stars are like the Sun, and O stars are blue-hot.
Masses, brightness and number usually vary with spectral type too. M stars are the least massive, dimmest, and yet most common, while O stars are the most massive and brightest... but they are so rare there is only one B star within 25 parsecs (Regulus, 23.7 parsecs), and no O stars.
The L and T classes are for objects smaller than stars and larger than planets. They're very cool, very red, extremely dim, and (unexpectedly) rare. WD are white dwarfs, which are the blue-hot remains of dead stars.
How many of these can I see?
All of the B, A, F, and G-type stars should be visible without a telescope from somewhere on the Earth. A few of the closest K stars can be barely seen without a telescope, but you'll need one to see any of the M stars.
Is this everything within 25 parsecs?No. There are only 2011 star systems in this plot but 50 systems within 5 pc of the Sun. If we assume the space density is the same, there should be 6250 systems within 25 pc (Volume: 5x5x5=125). This is only 32% of what's probably out there.
This is also out of date. Ten years of discovery has gone by since NStars was last seriously updated; the last updates of any kind were made in 2006.
Finally, there are some objects here that should not be. For instance, an eagle eye will notice FOUR B stars in this plot (one is within 10 parsecs) rather than one; these objects have bad distance measurements and should not have made it into the database.
Why do you count systems, not stars?
Current star formation theory expects that star systems form as systems, not as stars. One collapsing cloud = one mention here. On a more practical note, all the members of the multiple systems fall well within the plotted dots.
Why isn't star (x) on there?
They are not in the NStars database; either they weren't known in 2001, or they didn't meet the database's inclusion criteria.
How was this made?
The NStars database (courtesy of Dr. David Koerner), a custom IDL program, and a mix of Imagemagick and GIMP to assemble the frames.
May I use this image elsewhere?
Yes, but please credit A. Riedel and RECONS.
Can I get the list?
It used to be available here: http://nstars.nau.edu/ but the site is now dead and the existing list I have is not terribly helpful. It's out of date, and we're making a better one anyway. Eventually we WILL publish the new data.
I'm having trouble seeing it.
It's rotating clockwise, and viewed from 30 degrees above the equatorial plane.