The basics of astronomy can be understood by looking at the prominent objects in the sky, such as the Sun and the Moon. Understanding how things at our planet get influenced by Sun and Moon is the first baby step one can take in astronomy. Ever wondered why planets, Sun, and our Moon are spherical? Large objects experience immense gravitational force. So the entire mass in such large objects gets evenly spread around the center, causing the round shape. Ever wondered why Sun is a star but Earth and Moon are not? Any object that is 0.08 solar masses or more experience a very high gravitational force. This results in the nuclear reaction in their core, making them stars. None of the planets or Moons in our solar system are close to that mass. So we have only one star in solar system, that is our Sun.