What exactly is the difference between astronomy and astrophysics?  This is a topic that has come up a few times recently in conversation, so it has been on my mind.  There’s no one right answer to this question – every astronomer/astrophysicist will have a varying opinion.  It’s a fun discussion to have, so I thought I’d start it off.  If you have different definitions for astronomy and astrophysics, leave it in the comments.

For me, the distinguishing characteristic between astronomy and astrophysics is the emphasis on the data.  Astronomy is about observations and data – from designing an observing plan to careful reduction of data.  The goal is to observe interesting sources in order to garner new knowledge. Astrophysics is the application physics of to understand astronomical sources.  Here, the emphasis is on detailed calculations to explain the specifics of various astronomical processes.

Those are the over-arching definitions I use, but they are rather vague.  And, as you can probably easily see, there is a grey area between astronomy and astrophysics – when do you go from doing astronomy to astrophysics?  Below is an example with my differentiation.

  • I’m observing for ALFALFA?  Astronomy.
  • Flagging ALFALFA data to mark bad spots with interferences? Astronomy.
  • Extracting and measuring the basic properties of ALFALFA sources?  Astronomy.
  • Calculating time scales for different processes that can disrupt gas clouds?  Astrophysics.  (Also, what I should be doing right now.)

The terms astronomy and astrophysics are often used interchangeably because of this smooth transition from one to the other – most astronomers/astrophysicists do both astronomy and astrophysics.  (There are some theoretical astrophysicists who never work with data.  I’m sad for them although they seem happy.)  Personally, I consider myself an astronomer; for me, the driving aspect (and fun part) of my studies is observing and working with data.  I do astrophysics in order to understand sources I observe, but I don’t consider myself an astrophysicist.

As for astronomy and physics, astronomers have an old joke:
What do you say when you’re sitting on a plane and the person nexts to you asks what you do?  If you feel like talking, say you’re an astronomer.  If you feel like being left alone, say you’re a physicist.