Chapter 2Decoding the Hidden Messages in Starlight

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

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

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.

Light Travels in Waves

• Water waves show diffraction, addition, and canceling.

• So does light! A wave!

Electromagnetic Radiation

• A “field disturbance”• Both electric and magnetic properties• A spectrum of waves, varying in wavelength

and frequency

Our Eyes See Only Some ofthe 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!

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

Light Has Properties

of BothWaves and

Particles

Shorter wavelengthHigher frequencyHigher energyMore “particle-like”

Longer wavelengthLower frequencyLower energyMore “wave-like”

Infrared light can pass through interstellarclouds 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.

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….

How much energy a star emits is determinedby both temperature and surface area.

As temperature increases, the energy released by the object increases.

Identifying Chemical SubstancesUsing Spectral Lines

The light from a burning chemical makes a special, unique pattern when it passes through a prism.

Electrons Occupy SpecificOrbits within Atoms

• Each orbit is a specific energy state.

• Electrons “leap” between orbits.

• Electrons “leap” when they absorb the perfect amount of energy.

• Electrons “fall” and emit that same specific amount of energy.

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

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

Kirchhoff’s LawsLaw 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.

Kirchhoff’s Laws

The wavelengths absorbed by the gas exactly match the wavelengths emitted by the gas.

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

Exploiting the Doppler Effect

The wavelengthwe observe

The velocity of the object, toward or away from us

The wavelength we “should” observe The speed of light

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Telescopes Gather Light

Light-gathering power is directly related to the size of its objective lens― the gathering area.

Telescopes aren’t primarily used to magnify stars.

Refracting Telescopes

• Use a lens to concentrate incoming light at a focal point

• Magnifies near objects

Reflecting Telescopes

• Use a curved mirror toconcentrate incoming light at a focal point.

• More durable, and can be made bigger and less expensive.

Adaptive Optics

Computers compensate for turbulence in the atmosphere.

Telescopes in Orbit

• Detect light that does not penetrate the atmosphere–Near infrared ultraviolet X-ray….

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

Charge-coupled devices recordvery fine image details.