Grimble Gromble wrote:

Also referring to memory, which, as I always remind everyone, is very dodgy,
I have a vague recollection that when the velocities of nearby stars are
compared, the stars essentially fall into two groups. Stars in our group
move pretty much in the same direction and speed as the Sun, while the other
group of stars travel in a direction and speed that is common to them and
different from ours. I believe there were other factors such as age and
composition that distinguished the two groups? I apologise if this is not
the case, however, like most things, I cannot remember my source.

You're probably thinking of the "Population I" _vs_ "Population II"
classification. The former stars, including our Sun, are part of the
galactic disk, having been born from its clouds of gas and dust, and
orbit the galactic centre pretty much in a plane. The latter group,
mostly older stars that are evolving out of the main sequence, form a
spherical 'halo' around the Galaxy, with orbits that tend to
intersect the disk at steep angles, and make up most of the globular
clusters. Arcturus (Alpha Boötis), a fairly nearby orange giant, is
one of the most prominent examples of a Population II star, and
because of the high inclination of its path through the galactic disk
it exhibits the largest proper motion of any first-magnitude star,
cutting across the 'stream' in which the Sun and its contemporaries
are moving.

--
Odysseus