7. how do scientists use electromagnetic emissions of stars to determine their composition, age, and stage of development?

The Fraunhofer lines of stars are caused by the emitting of light or of absorbing it at particular wavelength from a particular element which determines the composition, age and development stage of stars.

Explanation:

Through the study of electromagnetic emission of stars it is now possible for scientists to know about the composition, development and age of a star. As the Fraunhofer line exhibited from stars are caused by the elements that emits and absorbs light at a particular wavelength, the composition of the star can be determined by it.

Through the composition, the star age is known to the scientists now. By studying the spectra wavelengths one can know the temperature of it and this temperature determines their age.  

when atoms of different types of elements join together, they make molecules called compounds. water consists of compound molecules made up of 2 hydrogen atoms and 1 oxygen atom. this is why it's called h2o. water will always have 2 times the number of hydrogen atoms as oxygen atoms.