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AST101H1 (417)
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Lecture 3

# Lecture 3 - Tour of the Solar System - September 17.docx

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Department
Astronomy & Astrophysics
Course
AST101H1
Professor
Michael Reid
Semester
Fall

Description
September 17, 2013. Lecture 3 – Tour of the Solar System  Nearest large galaxy to Milky Way is Andromeda o 2.5 million ly away  Light travels at a finite speed o Things we see in the sky are not as we are seeing them, we see them as they were in the past o E.g., takes 8 minutes for light from the sun to reach Earth, so if we look at the sun we are looking at it 8 minutes in the past o No matter where you look, you are looking back in time o E.g., takes 3 billionths of a second for light to reach our eyes from a light bulb if we stand a meter away o Light we see from Andromeda is 2.5 million years old, some of the stars giving off that light could already be dead and gone o Light from distant galaxies can be billions of years old  Lookback time = how far into the past we are seeing based on how long the light takes to reach us  Stars: balls of plasma which produce energy by nuclear fusion o Plasma = hot hot gas, ionized  Planets: rocky, icy, or gassy, and they don’t produce much energy o Mainly solids and liquids o Planets orbit stars and only planets o In 2006, the International Astronomical Union (IAU) accepted a new definition of the term “planet”, adding one new criterion to the traditional definition:  1. Planets orbit stars directly (not other planets).  2. Planets must be massive enough for their own gravity to compress them into spherical shapes.  3. Planets must have cleared their orbits. That is, there can’t be a lot of other stuff orbiting the parent star in orbits very similar to the planet. o Pluto can’t be a planet because its orbit is full of other objects (mainly comets)  Eris (orbits mainly outside of Pluto) is another that goes through all those comets  Pluto is a dwarf planet  The IAU recognizes only five dwarf planets in our solar system when there are potentially thousands  Dwarf planet: meets the first two criteria for a planet, but not the third (hasn’t cleared its orbit)  Moon: any large body which orbits a planet  Asteroid: a chunk of ice and rock which is not big enough to crush itself into a sphere  Nebula: is cloud of gas (bright) and dust (dark); Greek for “cloud”  Solar system: consists of a star orbited by planets, comets, asteroids, moons, dust, etc. o The inner solar system: Mercury, Venus, Earth and Mars o The outer solar system: Jupiter, Saturn, Neptune, Uranus, lots of comets and dwarf planets o Terrestrial planets: small, rocky, thing or not atmosphere, few or no moons  Earth, Mercury, Venus, Mars o Jovian planets: large, liquid and gas, small rocky core (probabl
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