On the lateral surface of the human brain, the central sulcus separates the frontal lobe from the parietal lobe. The lateral sulcus separates the frontal lobe from the temporal lobe. The frontal lobe bottom can be divided into a lateral, polar, orbital (above the orbit; also called basal or ventral), and medial part.
The age of an individual star can vary; they were all made at different times and have different life spans. Stars can range from a few million to a several billion years old. The oldest star yet discovered, HE 1523-0901, is an estimated 13.2 billion years old.
The young O and B stars used to trace the spiral structure are less than 0.1 Galactic year in age, whereas the oldest stars in the halo are 40 to 70 Galactic years in age. From the dynamics of the Galaxy, we know that relatively little change can occur in the Galaxy's structure in periods of less than a few Galactic years.