Chapter 21: Light, Astronomical Observations, and the Sun
I. The study of light
A. Electromagnetic radiation
1. Visible light is only one small part of an array of energy
2. Electromagnetic radiation includes
a. Gamma rays
b. X-rays
c. Ultraviolet light
d. Visible light
e. Infrared light
f. Radio waves
3. All forms of radiation travel at 300,000 kilometers (186,000 miles) per second
B. Light (electromagnetic radiation) can be described in two ways
1. Wave model
a. Wavelengths of radiation vary
1. Radio waves measure up to several kilometers long
2. Gamma ray waves are less than a billionth of a centimeter long
b. White light consists of several wavelengths corresponding to the colors of the rainbow
2. Particle model
a. Particles called photons
b. Exert a pressure, called radiation pressure, on matter
c. Shorter wavelengths correspond to more energetic photons
C. Spectroscopy
1. The study of the properties of light that depend on wavelength
2. The light pattern produced by passing light through a prism, which spreads out the
various wavelengths, is called a spectrum (plural: spectra)
3. Types of spectra
a. Continuous spectrum
1. Produced by an incandescent solid, liquid, or high pressure gas
2. Uninterrupted band of color
b. Dark-line (absorption) spectrum
1. Produced when white light is passed through a comparatively cool, low pressure gas
2. Appears as a continuous spectrum but with dark lines running through it
c. Bright-line (emission) spectrum
1. Produced by a hot (incandescent) gas under low pressure
2. Appears as a series of bright lines of particular wavelengths depending on the gas
that produced them
4. Most stars have a dark-line spectrum
5. Instrument used to spread out the light is called a spectroscope
D. Doppler effect
1. The apparent change in wavelength of radiation caused by the relative motions of
the source and observer
2. Used to determine
a. Direction of motion
1. Increasing distance – wavelength is longer ("stretches")
2. Decreasing distance – makes wavelength shorter ("compresses")
b. Velocity – larger Doppler shifts indicate higher velocities
II. Astronomical tools
A. Optical (visible light) telescopes
1. Two basic types
a. Refracting telescope
1. Uses a lens (called the objective) to bend (refract) the light to produce an image
2. Have an optical defect called chromatic aberration (color distortion)
b. Reflecting telescope
1. Uses a concave mirror to gather the light
2. No color distortion
3. Nearly all large telescopes are of this type
B. Detecting invisible radiation
1. Photographic films are used to detect ultraviolet and infrared wavelengths
2. Most invisible wavelengths do not penetrate Earth's atmosphere, so balloons, rockets,
and satellites are used
3. Radio radiation
a. Reaches Earth's surface
b. Gathered by "big dishes" called radio telescopes
1. Large because radio waves are about 100,000 longer than visible radiation
III. Sun
A. One of 200 billion stars that make up the Milky Way galaxy
B. Only star close enough to allow the surface features to be studied
C. An average star
D. Structure can be divided into four parts
1. Solar interior
2. Photosphere
a. "Sphere of light"
b. Sun's "surface" – actually a layer of incandescent gas less than 500 kilometers thick
c. Grainy texture made up of many small, bright markings, called granules,
produced by convection
d. Most of the elements found on Earth also occur on the Sun
e. Temperature averages approximately 6000 K (10,000°F)
3. Chromosphere
a. Just above photosphere
b. Lowermost atmosphere
c. Relatively thin, hot layer of incandescent gases a few thousand kilometers thick
d. Top contains numerous spicules – narrow jets of rising material
4. Corona
a. Outermost portion of the solar atmosphere
b. Very tenuous
c. Ionized gases escape from the outer fringe and produce the solar wind
d. Temperature at the top exceeds 1 million K
E. Solar features
1. Sunspots
a. On the solar surface
b. Dark color is due to a cooler temperature (1500 K less than the solar surface)
c. Follow an 11-year cycle
d. Large spots are strongly magnetized
2. Plages
a. Bright centers of solar activity
b. Occur above sunspot clusters
3. Prominences
a. Huge arching cloud-like structures that extend into the corona
4. Flares
a. Explosive events that normally last an hour or so
b. Release enormous quantities of energy
c. Eject particles that reach Earth in about one day and interact with the atmosphere
to cause the auroras (the Northern and Southern Lights)
IV. Solar interior
A. Cannot be observed directly
B. Nuclear fusion occurs here
1. Source of the Sun's energy
2. Occurs in the deep interior
3. Nuclear reaction that produces the Sun's energy is called the proton-proton reaction
a. Four hydrogen nuclei are converted into a helium nuclei
b. Matter is converted to energy
c. 600 million tons of hydrogen is consumed each second
4. Sun has enough fuel to last another five billion years
* Notes take from Tarbuck & Lutgens Website