Why is it useful to know the density of a celestial object?

Because it is important to know the composition of the object

Density is the ratio of the mass of a sample to the volume occupied by that mass, and expresses how much mass there is per unit volume of a given portion of matter. Density is a physical property of extreme importance in the characterization, identification and use of substances or materials.

the story goes that because of this belief, constantine was able to defeat his foes. constantine became an ardent1 christian. christianity and other religions were tolerated under constantine's rule. constantine also was responsible for the building of a great cathedral in rome.

in 1897, pershing became a tactical officer at west point, and that's where he was given the nickname "black jack." pershing's first name is "john," for which "jack" is a nickname, and he was a white man who commanded black troops. hence "black jack".