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Astronomy & Astrophysics
Michael Reid

Week 1 The solar system is huge and the vast majority of it is empty (not quite tiny, tiny quantities of gas and dust fills the space) Average distance between Sun and the Earth ~ 1 AU or 150 Billion metres Light year is distance light travels in one year 1 AU ~ 8 light minutes, Black hole ~ 26,000 light years Andromeda Galaxy is 2.5 million light years away (the closest galaxy to Earth) Light travels at finite speed and therefore we cant seeeverything in the universe as it is now. Thus, no matter where we look, we are looking back in time. Stars are huge spheres of super-hot gas (plasma) that produces energy by nuclear fusion (smashing atoms together). The Sun is a fairly typical star. Planets are large bodies which orbits stars. Planets may be various mixtures of rock, ice or gas. 3 main criteria for planets: - Planets orbit stars directly (not other planets) - Planets must be massive enough for their own gravity to compress them into spherical shapes. - Planets must have cleared their orbits. That is, there cant be a lot of other stuff orbiting the parent star in orbits very similar to the planet Pluto is now called a dwarf planet. 5 main dwarf planets at the moment: Pluto, Ceres, Makemake, Haumea and Eris. Moon is a large body which orbits a planet. A nebula is a cloud of gas and dust. We seethe light emitted by the gas (bright parts) and the places where light is blocked by dust (dark parts). It is where stars come from, they form when parts of nebulae are crushed by their own gravity. A galaxy is a city of stars. Large galaxies, like the Milky Way, contain about 100 billion stars, all orbiting a common center of gravity. They are grouped into galaxy clusters. The Milky Way is in a galaxy cluster called Local Group.
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