Answer to Question 1
The spiral arms are home to spiral tracers, including OB associations, emission nebulae, and young open star clusters. All of these objects are young objects, and it is the OB associations that make the arms so clearly visible. If all of the objects that trace the arms are young, they must have formed near their current location; hence spiral arms must be active sites of star formation.
Answer to Question 2
Earth rotates eastward, and friction with the ocean beds drags the tidal bulges slightly eastward out of a direct EarthMoon line. These tidal bulges are massive, and their gravitational field pulls the Moon forward in its orbit. As a result, the Moon's orbit is growing larger by about 3.8 cm a year, an effect that astronomers can measure by bouncing laser beams off reflectors left on the lunar surface by the Apollo astronauts.
The Rotation Curves of Four Spiral Galaxies
Formation of Karstlands
Formation of Acid Rain
Mechanisms of active and passive immunity.
Bone Formation
Teens who date in early adoles-cence are more likely to become sexually active