Population I and II stars

Question

I have been thinking about the formation of the galaxy. I can easily understand that old, low-metallicity stars are in the halo, but I'm missing something when it comes to the disk.

If slowly-rotating gas settled into the disk, formed stars, then did the disk at one time consist of Population II stars which have since exploded?

Why didn't the same processes happen in both the disk and the halo? That would mean that a lot of Population II halo stars would have exploded and formed plenty of Population I halo stars... but without disk dynamics perhaps not many would form? How about the missing Population II disk stars, why are they not still around like the halo clusters?

Answer 1

Keep in mind that there is no clear, rigid distinction between Pop. I,II and III stars... its a rough distinction that Pop. III stars formed from primordial element abundances, Pop. II stars formed from material mixed with ejecta of supernovae from Pop. III stars; and Pop. I stars from ejecta of Pop. II stars.

The additional important factor is that massive stars die younger.

In the galactic disk, the densities (and general conditions) are conducive to star formation, while the halo tends to have much less cold-gas which is necessary for star formation. Thus, in disks where there is star formation, there can still be massive stars (that live shorter); while the halo contains almost entirely older stars which must be less massive (because lower-mass stars live longer). Additionally, old stars in the disk have time to migrate (or be ejected) out of the disk --- while there aren't many methods of injecting halo stars into the disk.

From these effects, you end up with a natural, strong bias towards younger stars (Pop. I) in the disk, and older (Pop. II) stars in the Halo. This does not mean there are no stars from the opposite populations mixed in.

To my knowledge, the distribution of stars and their ages is completely consistent with population synthesis simulations, and there don't seem to be an excess of 'missing' Pop. II stars from the disk.

Answer 2

The main premise of your question is questionable. There are plenty of population II stars found in the disk of our Galaxy. The halo population has roughly spherical symmetry - but that means that most of them do not have "disk kinematics".

The second part of your question is why aren't population I stars found in the halo. The reason certainly is not that there were no massive stars formed - for example we know of plenty of neutron stars in population II globular clusters. The reason is more likely that star formation requires raw materials and there wasn't enough raw material in the detritus of supernova explosions and massive stellar winds from population II stars in the halo. The gas from which stars form is mostly primordial and merely polluted by the products of stellar evolution. By the time a large number of population I stars in the halo had lived and died, most of the gas in the halo would have sunk into the disk or been expelled from the relatively shallow gravitational potential by supernovae.

Note though that some enrichment (by massive stars) must have occurred in the halo because the tag "Population II" actually covers stars with quite a range of metallicities - anything between about a tenth and a millionth of the solar metallicity. Many of those stars contain chemical elements (e.g. europium) that can only have been produced by the r-process in supernovae or as the result of colliding neutron stars (produced by supernovae).

The disk (and bulge) of the Galaxy is more metal-rich than the halo because that's where most (by orders of magnitude) of the star formation took place and continues to take place.