1. Chapter 2 Decoding the Hidden Messages in Starlight

2. Light Takes Time to Travel
When close to Jupiter, the moons appeared to eclipse “too early.”
When far from Jupiter, the moons appeared to eclipse “too late.”
Light takes time to travel the extra distance!
c = 300,000 km/s
During Romer’s time, navigators intended to use the eclipses of Jupiter’s moons as a clock of sorts, with very carefully calculated tables of eclipses for each of the moons, for every day. What Romer discovered was that the tables were “slow” when Earth was close to Jupiter, and “fast” when Earth was farther away. For instance, if the table said that Io was to eclipse at 10 P.M. on a certain day, when Earth was close to Jupiter it would eclipse at 9:52 P.M., but when it was farther away, it would eclipse at 10:08 P.M. The error would oscillate according to the relative distances between the planets.

3. Glowing objects, like stars, emit an entire spectrum of light.
The Sun emits energy that:
Your eyes can see
Your skin can feel
Damages your DNA

4. Sunlight Is a Mixture of All Colors
Prisms don’t “add” colors to the sunlight.
Each color light “bends” as it passes through the material.

5. Light Travels in Waves
Water waves show diffraction, addition, and canceling.
So does light! A wave!

6. Electromagnetic Radiation
A “field disturbance”
Both electric and magnetic properties
A spectrum of waves, varying in wavelength and frequency

7. Our Eyes See Only Some of the Spectrum of Light
Half of this image was taken with a “visible light” camera, the other half was taken with a “UV camera.”
Bees can see designs on the petals!

8. As Frequency Increases, Wavelength Decreases
f is the symbol for frequency
Hertz = 1 wave per second
λ is the symbol for wavelength
f = c / λ

9. Light Has Properties of Both Waves and Particles
Shorter wavelength
Higher frequency
Higher energy
More “particle-like”
Longer wavelength
Lower frequency
Lower energy
More “wave-like”
Light Has Properties of Both Waves and Particles

10. Infrared light can pass through interstellar clouds that visible light cannot.
If our eyes can only see some parts of the spectrum, there must be things we can’t see.
Infrared light can pass through clouds of dust and gas.

11. Objects emit specific amounts of light, revealing their temperatures.
Wien’s Law: The higher the temperature, the more intense the light and the shorter the wavelength….

12. How much energy a star emits is determined by both temperature and surface area.
As temperature increases, the energy released by the object increases.

13. Identifying Chemical Substances Using Spectral Lines
The light from a burning chemical makes a special, unique pattern when it passes through a prism.

14. Electrons Occupy Specific Orbits within Atoms
Each orbit is a specific energy state.
Electrons “leap” between orbits.

15. Electrons “leap” when they absorb the perfect amount of energy.
Electrons “fall” and emit that same specific amount of energy.

16. Kirchhoff’s Laws
Law 1: A hot, opaque body or a hot, dense gas produces a continuous spectrum—a complete rainbow of colors without any spectral lines.

17. Kirchhoff’s Laws
Law 2: A hot, transparent gas produces an emission line spectrum—a series of bright spectral lines against a dark background.

18. Kirchhoff’s Laws
Law 3: A cool, transparent gas in front of a source of a continuous spectrum produces an absorption line spectrum —a series of dark spectral lines among the colors of the continuous spectrum.

19. Kirchhoff’s Laws
The wavelengths absorbed by the gas exactly match the wavelengths emitted by the gas.

20. Spectra Also Reveal Motion
An object’s motion through space is revealed by the precise wavelength positions of its spectrum of light.
The Doppler Effect

21. Exploiting the Doppler Effect
The wavelength we observe
The velocity of the object, toward or away from us =
The wavelength
we “should” observe
The speed of light

22. Telescopes Gather Light
Telescopes aren’t primarily used to magnify stars.
Light-gathering power is directly related to the size of its objective lens― the gathering area.

23. Refracting Telescopes
Use a lens to concentrate incoming light at a focal point
Magnifies near objects

24. Reflecting Telescopes
Use a curved mirror to concentrate incoming light at a focal point.
More durable, and can be made bigger and less expensive.

25. Adaptive Optics
Computers compensate for turbulence in the atmosphere.

26. Telescopes in Orbit
Detect light that does not penetrate the atmosphere
Near infrared ultraviolet X-ray….

27. Looking toward the center of the Milky Way using the best of Earth-based and space telescopes

28. Charge-coupled devices record very fine image details.