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Astrophysics is a specialty that combines two fields of science: astronomy and physics. Astrophysicists use the principles of physics to study the solar system, stars, galaxies, and the universe. How did the universe begin? How is the universe changing? These are the types of questions astrophysicists try to answer through research and experimentation. Physicists may also be concerned with such issues, but they use physics to study broader areas such as gravity, electromagnetism, and nuclear interactions. There are approximately 19,960 physicists and astronomers employed in the United States.