There are certain concepts that are important to understand when dealing with the physics of astronomy. In 1862, William Huggins was the first astronomer to use spectroscopy to study the chemical composition are stars and nebulas. This was the birth of the field of science called astrophysics.

Everything we know about celestial objects is through the study of light and other forms of electromagnetic radiation. In addition, merging principles of gravitation and forces also helped to strengthen this field of study.

We already discussed the celestial coordinate system, so we will now focus on more fundamental physical processes.

Of all of the physical concepts, nothing is more important to astronomy than proportions. Much of what we know of the other planets and other stars (and galaxies for that matter) is comparing them to what we know: Earth, Sun and Milky Way galaxy.

For example, the brightness of a star is proportional to its brightness and distance:

We can use this tool to help compare brightness of any star to another, however to be practical it is best to compare to a know object - like our Sun. When comparing two stars, this is the form the proportionality will take:

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