Astronomy 1021 Lecture Notes - Lecture 1: Astronomical Unit, Solar System, Cosmic Calendar
16 Jan 2019
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Star: a giant ball of hot gas that creates and emits its own radiation through nuclear fusion. Planet: a celestial body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and dominates its orbit. Moon: a moon is defined to be a celestial body that makes an orbit around a planet. Solar system: a solar system is the collection of planets, asteroids, and moons that occupy a space around a star and are held within the gravitational pull of the star. Galaxy: a large grouping of stars (solar systems) galaxies are found in a variety of sizes and shapes. Our own milky way galaxy is spiral in shape and contains several billion stars. Some galaxies are so distant their light takes millions of years to reach the earth. Nebula: a cloud of dust and gas in space, usually illuminated by one or more stars.
