Astronomy vs. Astrophysics vs. Space Science

The following was prepared by Christopher Palma, The Pennsylvania State University

The term astronomy is definitely older than astrophysics.  For as long as people have been studying the sky they have been doing astronomy.  Much of what was (and is) considered astronomy was concerned with cataloging the contents of the Universe, measuring their positions, and studying their motions.

The field of astrophysics is often described as having begun when the technique of spectroscopy was used to reveal the nature of astronomical objects (for a detailed history, see, e.g., Osterbrock 1995 ApJ 438, 1).  With observations of spectra allowing the chemical composition of objects to be determined, entirely new types of research questions opened up for investigation.

Today, students studying "astronomy and astrophysics" will learn both observational techniques used in conventional astronomy and the underlying physical theories about the nature of the objects they study.  Thus, most practicing astronomers are happy to call themselves either astronomers or astrophysicists, and the difference between the two really no longer exists.  If anyone does distinguish between the two, a purely observational work (e.g., a survey of a large patch of sky with a dedicated telescope) is more likely to be referred to as a work of astronomy, while a purely theoretical discussion about the origin of the Universe would be more likely to be referred to as astrophysics.

Although the planets and other bodies in the Solar System have been long subjects of astronomical research, as technology has allowed us to roboticly explore many of them, the research questions being explored have also changed.  Today, for example, the types of questions being investigated about Mars are very much like the detailed analyses geoscientists are interested about on Earth.  So the field of "planetary science" has really split off from astronomy, and Solar System studies are conducted by scientists different than those who study stars, galaxies, and the Universe itself.