The prefix astro is a combining form that relates to stars or celestial bodies. It also refers to space-related activities that take place outside Earth's atmosphere. The prefix is used in compound words such as "astronautics" and "astrophysics."
Astro refers to outer space, including not only stars but also the the planets, other objects in space and even structures that are shaped like stars.
Astro means star in words such as "astrophysics." It means "celestial body" in words such as "astronomy," and "outer space" in words such as "astronaut."