Chapter 5: Light

Question 1

Who first measured the speed of light correctly? What was the technique? Understand Roemer’s approach

Answer 1

• Measured by Roemer when observing Jupiter’s moons, got ~225 mil m/s
• Realized that apparent differences between the predicted
  and observed orbital motions of Jupiter’s moons depend on the distance between Earth and Jupiter. He used these observations to measure the speed of light.
• When Jupiter moon disappears behind Jupiter, light must travel 4.2 AU to reach Earth (when Earth is closer to Jupiter) but an additional 2 AU when Earth is farther away

Question 2

EM Spectrum: longest to shortest wavelength

Answer 2

Question 3

Main wavelengths of opticle light in the visible part of the spectrum

Answer 3

Roy G. Biv (longest red to shortest violet)

Question 4

What are the 2 reasons why astronomers have to build space telescopes?

Answer 4

1. Overcome atmospheric absorption & distortion
2. Increase resolution

Question 5

What is spectroscopy? Why is the discovery that the spectrum of light is related to chemistry important to astronomers?

Answer 5

• Spectroscopy is the dispersion of an object’s light into its component colours (energies)
• You can dissect & analyze the object’s light and you can determine the physical properties of that object
• Find temperature, mass, luminosity, composition

Question 6

What are the 3 kinds of spectrum that astronomers see in the sky?

Answer 6

1. Continuum spectra (stars emit continuous radiation)
2. Line spectra (Nebulae emit only discrete radiation)
3. Absorption (Hot Star through warm glas cloud, continuum of star minus lines the nebulae emits)

Question 7

Doppler Shift

Answer 7

Question 8

What does Blueshift and Redshift mean?

Answer 8

• Doppler determines velocity
• Blue means toward you, Red means away from you

Question 9

Given the rest wavelength of an emission line (eg hydrogen) and observed a different wavelength, could you tell what direction the source is moving?

Answer 9

Hydrogen emission lines are observed in moving objects. Lines ordered the same way but appear as different colours!

Question 10

Emission & Aborption

Answer 10

Emission means giving off light and absorption means taking in light

Question 11

Recombination & Ionization

Answer 11

• If an electron recombines with an atom it’s called recombination, emits light
• If an electron is ejected, the atom is ionized, absorbs light

Question 12

What colour of stars are hotter? Cooler?

Answer 12

• Blue: 12,000^o K
• Yellow: 5,800^o K
• Red: 4,500^o K