Galactic Evolution. Illustration showing the evolution of a galaxy from a gas cloud (upper left), clockwise, to a flat disc galaxy (middle left).This image is 500 x 533 pixels; the original measures 4,000 x 4,262 pixels.
The current scenario on the formation of the Galaxy is that the Galaxy formed from a giant, slowly rotating cloud of gas that collapsed under the influence of gravity. During the collapse, fragments of this cloud broke off creating smaller clouds that themselves collapsed to form stars and globular clusters (these are spherical compact star associations comprising of between 100,000 and 1,000,000 stars on average). As the remainder of the giant cloud contracted further, it started flattening up into a disc, because of rotation. This final contraction lasted billions of years, during which stars started to form also in the disc.
This scenario provides a natural explanation of why there is a significant age difference (about 3 billion years) between the galactic disc and the galactic halo.