Chapter 2 Decoding 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 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.

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 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!

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 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

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.

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 determined by both temperature and surface area. As temperature increases, the energy released by the object increases.

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

Electrons Occupy Specific Orbits 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 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.

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

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.

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

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.

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

Reflecting Telescopes Use a curved mirror to concentrate 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 record very fine image details.