DepartmentAstronomy & Astrophysics
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•The solar system is huge and the vast majority of it is empty, tiny quantities of gas
and dust fill the space.
•1 Astronomical Unit is the average distance between Earth and the Sun = 1.5 x
•1 light year = 9.46 x 10^12 km = distance light travels in one year
•1 AU = 8 light minutes
•The Milky Way Galaxy is our galaxy as seen from Earth
•The Andromeda Galaxy is 2.5 million ly away, largest galaxies to Milky Way.
•If light moved at infinite speed, you’d see everything in the universe as it is now.
•Light travels at a finite speed, so it takes time to reach us from wherever it is
•Thus, no matter where you look, you are looking back in time.
•You are seeing things the way they were when the light left them.
•When you look at the sun, you are looking at it 8 minutes ago.
•When you look at Andromeda, you are looking at it 2.5 million ly ago.
•Stars are hue 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
rocky, gaseous, or icy.
•The asteroid Eros can’t be a planet because it’s not a sphere.
•Pluto can’t be a planet because its orbit is full of other objects.
•There are five known dwarf planets in the solar system: Pluto, Ceres, Makemake,
Haumea, and Eris
•Comet is icy and Asteroid is rocky
•A nebula is a cloud of gas and dust
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