Dr. Aaron Titus | Department of Physics, High Point University
PHY1050      Astronomy of Stars, Galaxies, and the Cosmos
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Chapter 11

Read Chapter 11 from Discovering Astronomy.


WebAssign -- Chapter 11. Answer homework questions covering Chapter 11.


Almost everything we know about the universe has been learned by studying light.

Sure, we've intercepted and studied atomic nuclei, called cosmic rays, that have sped across many light-years of space to collide with our outer atmosphere.

We've used spacecraft to collect particles from Sun (Genesis mission) and from comets (Stardust mission) and bring them back to Earth.

We've landed on Moon and brought back lunar rocks.

We've sent rovers Spirit and Opportunity to roll around on Mars, study its surface, and radio back to us the data they collect.

We've sent the probe Huygens to land on one of Saturn's moons, Titan, and study its surface.

We've sent a probe to land on an asteroid and study its surface (the Near Shoemaker mission).

We've sent an impactor to crash onto the nucleus of a comet as an orbiter studied the debris thrown off (Deep Impact mission).

We've studied meteors, asteroids, and Mars rocks that have struck Earth.

But besides the few missions within our solar system that have actually collected samples or landed on the surface of a planet, moon, comet, or asteroid, everything we know about our solar system, galaxy, and universe has been learned by studying light.

Even the spacecraft sent to fly by or orbit Mercury, Venus, Mars, Jupiter, Saturn, Uranus, and Neptune measured characteristics of the sunlight reflected from the surfaces.

As a result, it's essential for you to understand how we use light to know:

  • the composition of stars (what stars are made of)
  • the temperature of stars (how hot stars are)
  • motion of stars (how fast and in what direction are stars moving)

But before we can understand what light tells us about stars, we have to know what light is. So, we'll discuss two models of light--light as a wave and light as a particle. We'll also discuss how atoms absorb and emit light.