If population I and II stars and the galactic halo?

Question

If population I and II stars get well mixed into the Galactic halo, can we use studies of the Galactic halo to tell us about the origin of the Milky Way Galaxy? Why or why not?

Answer 1

OK, your teacher is getting a bit weird with this one.

Pop I does not really mix into the galactic halo.  The oldest Pop I stars are in a thick (and ever thickening) disk that does overlap with the halo somewhat, but in most of the volume of what is called the halo, there are few, if any  Pop I stars.  A few "runaway stars" perhaps, from broken-up binaries.

The "stupid" answer is to say that, no, if Pop I and Pop II mix, it would wash out the historical information about the origin of the Milky Way.

But there are ways of distinguishing Pop I and Pop II stars from their spectra---Pop II are low metallicity (small amounts of elements heavier than helium).  Even if Pop I and Pop II where spatially well-mixed, they would still be kinematically seperable in phase space.  Stars do not really "mix" in phase space (3 positions and 3 velocities)---there's a theorem in stellar dynamics that says this is impossible for systems with no 2-body collisions---so your teacher's premise is flawed.  Studying their phase space distribtion as a function of metallicity would likely yield a good deal of valuable information about the history of the Milky Way.

Answer 2

like machi ootoo