PHY134 Introductory Astronomy The Solar System 1

When Light Meets Matter Dense objects absorb light energy or reflect it. How much absorbed can depend on wavelength – dyes. Can learn composition from reflected spectrum Light scatters off tenuous matter (Rayleigh 1871) Scattering decreases with wavelength: blue scatters more than red 2

Scattering on Earth Atmosphere scatters blue light making sky glow blue and Sun appear yellow When we get more scattering – when Sun low in sky – lose green to scattering leaving Sun red 3

Scattering and Refraction Moon haloRainbow 4

Line Spectra Fraunhofer 1814: Sun’s spectrum has gaps Kirchoff-Bunsen 1859: Tenuous gas emits line spectrum Atoms and molecules emit/absorb at characteristic wavelengths when heated or ionized Line spectrum yields chemical composition At higher pressure and density lines broadened 5

Solar System - Inventory Mostly, Solar system is an average ( ) main sequence star Sun contains about of the bound mass Radiation heats planets Solar wind – charged particles streaming from Sun at high energy carry in 6

What is Out There Eight planets orbit Sun in slightly eccentric elliptic orbits Radius Orbits near ecliptic match Sun rotation 7

Two Kinds of Planets Four Inner planets within 1.6 AU – Dense – Small – Rocky Four Outer planets from AU – Less dense – Huge – Fluid 8

What Else? Lots of MoonsRings 9

Craters 10

Asteroids 11

Farther Out 12

of mass is Sun of rest is Jupiter, Saturn Hydrogen Helium Metals What it’s Made Of 13

Questions Why are all planet orbits circular and in a plane? Why aren’t comets’? Why are planets and large Moons round? Why aren’t asteroids? Why are inner planets small, rocky, dense while outer planets are large, fluid, light? Why aren’t asteroids a planet? What is the story with Pluto? What are rings? Why are Saturn’s different? What made all the craters? Where did it go? Why do comets fall into inner Solar System? Why do asteroids fall into near-Earth orbits? If orbits can change – will planet orbits? Have they? Where did it all come from? When? 14