Final answer:
The thickest parts of the Milky Way Galaxy are associated with the thick disk, which consists of older stars with hotter orbits that take them further from the galactic plane.
Step-by-step explanation:
The question about the thinnest and thickest years seems to refer to the thin and thick disks of the Milky Way Galaxy, and the stellar halo. The topic under discussion is the distribution of star ages in these different components of our galaxy, which provides insight into the galactic formation process.
Thick disk stars are generally older than those in the thin disk, which suggests that the thick disk formed earlier in the history of the Milky Way. The thickness of these disks is a result of the stellar vertical motions: stars in the thick disk have hotter orbits that take them further from the galactic plane.
The thin disk stars are younger and their orbits are cooler, keeping them closer to the galactic plane, making the disk thinner. The ages of stars are critical in understanding the evolutionary history of the Milky Way Galaxy.