It depends on what kind of physics you studied. Many universities combine their physics and astronomy departments, so you can earn a PhD in physics and do a dissertation in any field of astronomy. That's what I did. My PhD in physics makes me eligible for physics jobs and the astronomy research makes me eligible for jobs in astrophysics. The field depends on what you studied, but pretty much any field would work with a PhD in physics.
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You would need a degree in some field of astronomy too. It would be easier to just get a degree in astrophysics.
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Hi. Theoretical astrophysics would be a good candidate.