1. Ancient Greeks believed the Earth was at the center of the universe, but Copernicus proposed the sun was at the center instead. 2. Kepler modified Copernicus' model, proposing that planets move in ellipses rather than perfect circles. Galileo also made observations that supported Copernicus, such as phases of Venus and moons orbiting Jupiter. 3. When stars run out of hydrogen fuel, they evolve in different ways depending on their mass - less massive stars become white dwarfs, more massive stars explode as supernovae.

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