<p>FWIW, a few categories:</p>
<p>Astronomer : teacher or writer on the general subject, planetarium lecturer, etc.,
Astrophysicist : physicist who studies stars or stellar evolution
Cosmologist : physicist or mathematician who studies the origin/evolution of the Universe
Planetary Geologist : scientist who studies geology of planets and moons</p>
<p>Start with the math (calculus) and physics, then specialize.</p>