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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 did not the same processes happen in both the disk and the halo? That would mean that a lot of Population II halo stars will 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?

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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 higher-mass star formation (whose stars live shorter); while the halo can only really produce lower-mass stars (which 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.

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Perfect; you totally cleared up my thoughts on that point. Thanks! – Gigi Giles Apr 7 '12 at 19:17
Happy to help! If you wouldn't mind one-upping my response, or 'checking it' (as the correct answer) --- I'd appreciate it :) – DilithiumMatrix Apr 7 '12 at 22:33

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