Final answer:
The disk and halo of the Milky Way are classified as separate parts of the Galaxy due to their differences in overall shape, main components, and the types, ages, compositions, and orbits of their stars.
Step-by-step explanation:
The disk and halo of the Milky Way are classified as separate parts of the Galaxy due to their differences in overall shape, main components, and the types, ages, compositions, and orbits of their stars.
The disk of the Milky Way is a flat, spiral-shaped structure that contains dust, gas, and young and old stars. It is the main component of the Galaxy and has a disc-like shape.
The halo, on the other hand, is a spherical structure that contains populations of very old stars. These stars have low abundances of elements heavier than hydrogen and helium and have highly elliptical orbits randomly oriented in direction.
The stars in the disk are classified as population I stars and are young, metal-rich, and orbit the Galaxy's center at high speed. The stars in the halo are classified as population II stars and are old, metal-poor, and spend most of their time far above or below the plane of the Galaxy. The main difference between these two populations is their age and metal content.