Can't be gravitational, due to the fact that atoms are way too small to have a gravitational pull for something of similar size.
The strong nuclear force is the answer you're looking for.
Astronomy can best be described as a "<span>a. study of objects beyond earths atmosphere", although the subject of astronomy can be much more complicated. </span>
Parietal cells within the stomach lining secrete hydrochloric acid that lowers the pH of the stomach. A low pH (1.5 to 2) activates pepsin. Acetylcholine, gastrin, and histamine stimulate the proton pump in parietal cells to release hydrogen ions and decrease pH.
Mitosis produces all animal and plant cells
