Answer to Question 1
Stars in the central bulge formed first, out of material found early in the Universe's history that was mostly hydrogen and helium. Many stars in the disk are younger, and formed after stellar evolution had enriched the interstellar medium with heavier elements. Since metals were present in the materials that formed disk stars, disk stars have a higher metallicity.
Answer to Question 2
The first postulate states that the laws of physics are the same for all observers, no matter what their motion, so long as they are not accelerated. The word accelerated is important. If either spaceship were to fire its rockets, then its velocity would change. The crew of that ship would know it because they would feel the acceleration pressing them into their couches. Accelerated motion, therefore, is differentthe pilots of the spaceships can always tell which ship is accelerating and which is not. The postulates of relativity discussed here apply only to the special case of observers in uniform motion, which means unaccelerated motion. That is why the theory is called the special theory of relativity.
Magnetic resonance imaging (MRI) image demonstrating a back herniated disk.
Otitis media. This illustration shows an inflamed tympanic cavity, which is the most common source o
Muscles of the lower extremity.
Dermatological changes with myxedema can occur behind the eyes or in the lower leg, as shown here.
The Upper (A) and Lower (B) Respiratory Tracts (2 of 2)