"As we arbitrarily divide the human body into a torso with a head and limbs, so we can conceptually separate the Galaxy into various components. The flying-saucer shape—consisting of the central bulge and the spiral disk—is only the most obvious part of the Galaxy. The spiral disk itself can be subdivided into a thin disk, which rises about 1,000 light-years above and below the galactic mid-plane, and a thick disk, which extends to about 3,500 light-years on either side of the plane. The relative flatness of our galaxy is evident when one considers that the galactic disk is generally thought to be about 120,000 light-years across. Our sun resides in the thin disk about 28,000 light-years from the galactic center... About 200 globular clusters are known, and they appear to be some of the oldest objects in the Galaxy. It took many decades of careful study to tease apart the various regions of the Milky Way, and the process of dissecting out fine-scale subregions continues even today.One of the reasons it’s so difficult is that we cannot measure the properties of all the stars in the Galaxy—they are simply too far away....the different orbits of the stars, their kinematic properties, provide a crucial distinction between stars that belong to different regions of the Galaxy"