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Chapter 21

Light, Astronomical Observations,


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 km ( 186,00 mps )

B. Light


1. As waves

a. Wavelengths of radiation vary

1. Radio waves measure up to several kilometers long

2. Gamma rays 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. As particles

a. 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)

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

Spectroscopy continued

4. Most stars have a dark-line spectrum

5. Instruments used to spread out the light is called spectroscope

D. Doppler

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 - wave-length 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 0 telescopes

1. Two basic types

a. Refracting telescopes

1. Uses a lens ( called the objective ) to bend light (refract) the light to produce an image

2. Light converges at an area called the focus

3. Distance between the lens and the focus is called the focal length

4. The eyepiece is a second lens used to examine the image directly

5. Have an optical defect called chromatic aberration (color distortion)

Telescopes continued

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

2. Properties of optical telescopes

a. Light-gathering power

1. Larger lens ( or mirror ) intercepts more light

2. Determines the brightness

b. Resolving power

1. The ability to separate image and finer detail

c. Magnifying power

1. The ability to make an image larger

2. Calculated by dividing the focal length of the eyepiece

3. Can be changed by changing the eyepiece

4. Limited by atmospheric conditions and the resolving power of the telescope

5. Even with the largest telescopes, stars (other than the sun) appear only as points of light

B. Detecting invisible radiation

1. Photographic films are used to detect ultraviolet and infrared wavelengths

2. Most invisible wavelengths do not penetrate Earthís atmosphere, so balloons, rockets, and satellites are used

3. Radio radiation

a. Reaches Earth Ďs surface

b. Gathered by " big dishes " called radio telescopes

1. Large because radio waves are about 100,00 longer than visible radiation

2. Often made of a wire mesh

3. Have rather poor resolution

4. Can be wired together into a network called a radio interferometer

5. Advantages over optical telescopes

a. Less affected by weather

b. Less expensive

c. Can be used 24 hours a day

d. Detects material that does not emit visible radiation

e. Can "see " through interstellar dust clouds

6. A disadvantage is that they are hindered by man-made radio interference

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. " Sunís surface " - actually a layer of incandescent gas 300 km (200 miles) thick

c. Grainy texture made up of many small, bright markings, called granules, produce by convection

d. Most of the elements found on Earth also occur on the sun

e. Temperature averages approximately 6000 K ( 10,000 degrees F )


3. Chromosphere

a. Just above photosphere

b. Lowermost atmosphere

c. Relatively thin, hot layer of incandescent gases a few thousand km thick

d. Top contains numerous spicules - narrow jets of rising material

4. Corona

a. Outermost portion of the solar atmosphere

b. Very tenuous

c. Ionozed 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 center, the umbra, surrounded by a lighter region, the penumbra

c. Dark color is due to a cooler temperature ( 1500 K less than the solar surface )

d. Follow an 11 year cycle

e. Large spots are strongly magnetized

f. Pairs have opposite magnetic poles

2. Plages

a. Bright centers of solar activity

b. Occur above sunspot clusters


3. Prominences

a. Huge arching cloudlike structures that extend into the corona

b. Condensations of materials in the corona

4. Flares

a. Explosive events that normally last an hour or so

b. Sudden brightening above a sunspot cluster

c. Release enormous quantities of energy

d. Eject particles that reach Earth in about one day and interact with the atmosphere to cause the auroras ( the northern and sourthern lights )

IV. Solar interior

A. Cannot be observed directly

B. Nuclear fusion occurs here

1. Source of the sunís energy

2. Occurs in the deep interior

3. Nuclear reaction that produces the sunís 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

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