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We present a novel interpretation of the previously puzzling differentbehaviours of stellar populations of the Milky Way's bulge. We first show, bymeans of pure N-body simulations, that initially co-spatial stellar populationswith different in-plane random motions separate when a bar forms. The radiallycooler populations form a strong bar, and are vertically thin andpeanut-shaped, while the hotter populations form a weaker bar and become avertically thicker box. We demonstrate that it is the radial, not the vertical,velocity dispersion that dominates this evolution. Assuming that early stellardiscs heat rapidly as they form, then both the in-plane and vertical randommotions correlate with stellar age and chemistry, leading to different densitydistributions for metal-rich and metal-poor stars. We then use ahigh-resolution simulation, in which all stars form out of gas, to demonstratethat this is what happens. When we apply these results to the Milky Way we showthat a very broad range of observed trends for ages, densities, kinematics andchemistries, that have been presented as evidence for contradictory paths tothe formation of the bulge, are in fact consistent with a bulge which formedfrom a continuum of disc stellar populations which were kinematically separatedby the bar. For the first time we are able to account for the bulge's maintrends via a model in which the bulge formed largely in situ. Since the modelis generic, we also predict the general appearance of stellar population mapsof external edge-on galaxies.

Separation of Stellar Populations by an Evolving Bar: Implications for the Bulge of the Milky Way