Image Caption: NGC 4414, a typical spiral galaxy in the constellation Coma Berenices, is about 56,000 light-years in diameter and approximately 60 million light-years distant. Credit: NASA/ESA/Wikipedia
What is Astrophysics?
For much of the modern age the term Astrophysics has been used synonymously with Astronomy. This interchange is so common that many textbooks even offer the two as having the same meaning.
However, from a strictly historical perspective there are differences between the two. Astronomy developed as a purely observational science. The first of mankind to look to the heavens did so with their eyes alone. It was not until the 17th century that telescopes were used with any regularity.
Even then, the results of such studies led to the development of orbital mechanics – the laws that governed the motions of stars and planets. In essence, there was very little that could be derived about the actual objects themselves, besides their motion and appearance, from these observations.
The Age Of Astrophysics
At the dawn of the 20th century, though, things began to change. The use of the optical telescope expanded, and spectroscopic measurements could be made of various astronomical objects. These data could tell us more about their composition – and therefore hint about their formation and evolution.
Additionally, the field of radio astronomy was born, followed later by X-ray and gamma-ray observatories. The addition of these wavelengths allowed for detailed studies of objects across the entire electromagnetic spectrum, allowing us to begin asking questions about the physical principles that lead to the formation, evolution and death of stars, gas clouds, galaxies and more.
The simple use of a telescope to study the sky had become a much more involved endeavor, and with it the ability to ask ever more complicated questions. This really gave rise to the field of astrophysics, where the focus was not about optical imaging, but rather the analysis of broad spectrum data sets to explore the foundational questions of our Universe.
Astronomy Or Astrophysics?
With a few exceptions, there is very little that happens that would be strictly considered astronomy. Observational techniques are so advanced, and the purpose and goals of these studies so deep, that they always seek to answer underlying questions of physics.
For this reason, virtually all that we call Astronomy today, is probably more aptly defined as Astrophysics. This can even be seen in the training of the modern Astronomer.
Many, if not most, professional Astronomers – myself included – actually have degrees in physics. While those that do have degrees in the field of Astronomy recall that much of the work to achieve that degree revolved around physics courses, and courses that are founded heavily in mathematical and physical theory.
So are the two terms interchangeable? As definitions of scientific fields, the answer is no. But in the modern context, where the fundamental pursuit of astronomy is all but extinct, the terms now carry essentially the same meaning: The pursuit to understand the Universe around us.