Light, Astronomical Observations, and the Sun

Light, Astronomical Observations, and the Sun begins with an examination of the nature of electromagnetic radiation and how it is used to gather information concerning the state of matter, composition, temperature, and motion of stars and other celestial objects. After investigating the nature of light, the focus shifts to astronomical tools and how they are used to intercept and study the energy emitted by distant objects in the universe. The chapter concludes with descriptions of the structure of the sun, some features that occur on the active sun, and the source of the sun's energy

Learning Objectives

After reading, studying, and discussing this chapter, you should be able to:

•Describe electromagnetic radiation and the two models used to explain its properties.

•List and describe the three types of light spectra.

•Explain how light (electromagnetic radiation) can be used to investigate the properties of a star.

•Describe the two types of optical telescopes and list their component parts.

•List and describe the three properties of optical telescopes that aid astronomers in their work.

•Describe radio telescopes and list some of their advantages over optical telescopes.

•List and describe the four parts of the sun.

•Describe several features found on the active sun.

•Describe the source of the sun's energy.

Chapter Summary

•Visible light constitutes only a small part of an array of energy generally referred to as electromagnetic radiation. Light, a type of electromagnetic radiation, can be described in two ways 1) as waves and 2) as a stream of particles, called photons. The wavelengths of electromagnetic radiation vary from several kilometers for radio waves to less than a billionth of a centimeter for gamma rays. The shorter wavelengths correspond to more energetic photons.

•Spectroscopy is the study of the properties of light that depend on wavelength. When a prism is used to disperse visible light into its component parts (wavelengths), one of three possible types of spectra (a spectrum, the singular form of spectra, is the light pattern produced by passing light through a prism) is produced. The three types of spectra are 1) continuous spectrum, 2) dark-line (absorption) spectrum, and 3) bright-line (emission) spectrum. The spectra of most stars are of the dark-line type. Spectroscopy can be used to determine 1) the state of matter of an object (solid, liquid, high or low pressure gas), 2) the composition of gaseous objects, 3) the temperature of a radiating body, and 4) the motion of an object. Motion (direction toward or away and velocity) is determined using the Doppler effect - the apparent change in the wavelength of radiation emitted by an object caused by the relative motions of the source and the observer.

•There are two types of optical telescopes; 1) the refracting telescope which uses a lens as its objective to bend or refract light so that it converges at an area called the focus, and 2) the reflecting telescope, which uses a concave mirror to focus (gather) the light. When examining an image directly, both types of telescopes require a second lens, called an eyepiece, which magnifies the image produced by the objective.

•Telescopes have three properties that aid astronomers: 1) light-gathering power, which is a function of the size of the objective-large objectives gather more light and therefore "see" farther into space, 2) resolving power, which allows for sharper images and finer detail, is the ability of a telescope to separate objects that ate close together, e.g. Pluto and its moon Charon, and 3) magnifying power, the ability to make an object larger. Most modern telescopes have supplemental devices that enhance the image.

•Invisible radio wave radiation is detected by "big dishes" called radio telescopes. A parabolic shaped dish, often consisting of a wire mesh, operates in the same manner as the mirror of a reflecting telescope. Radio telescopes have poor resolution, making it difficult to pinpoint a radio source. To reduce this problem, several can be wired together into a network called a radio interferometer. The advantages of radio telescopes over optical telescopes are that radio telescopes are less affected by the weather, they are less expensive to construct, "viewing" is possible 24 hours a day, they can detect material in the universe too cool to emit visible radiation, and they can "see" through interstellar dust clouds.

•The sun is one of the 200 billion stars that make up the Milky Way galaxy. The sun can be divided into four parts 1) the solar interior, 2) the photosphere (visible surface), and the two layers of its atmosphere, 3) the chromosphere and 4) corona. The photosphere radiates most of the light we see. Unlike most surfaces, it consists of a layer of incandescent gas 300 kilometers (200 miles) thick with a grainy texture consisting of numerous, relatively small, bright markings called granules. Just above the photosphere lies the chromosphere, a relatively thin layer of hot, incandescent gases a few thousand kilometers thick. At the edge of the uppermost portion of the solar atmosphere, called the corona, ionized gases escape the gravitational pull of the sun and stream toward Earth at high speeds producing the solar wind.

•Numerous features have been identified on the active sun. Sunspots are dark blemishes with a black center, the umbra, which is rimmed by a lighter region, the penumbra. The number of sunspots observable on the solar disk varies in an 11-year cycle. Plages are large "clouds" that appear as bright centers of solar activity often directly above sunspot clusters. Prominences, huge cloudlike structures best observed when they are on the edge, or limb, of the sun, are apparently condensations of coronal material gracefully "sliding down" lines of magnetic force back to the chromosphere. The most explosive events associated with sunspots are solar flares. Flares are brief outbursts that release enormous quantities of energy that appear as a sudden brightening of the region above sunspot clusters. During the event, radiation and fast-moving atomic particles are ejected, causing the solar wind to intensify. When the ejected particles reach Earth and disturb the ionosphere, radio communication is disrupted and the auroras, also called the northern and southern lights, occur.

•The source of the sun's energy is nuclear fusion. Deep in the solar interior, at a temperature of 15 million K, a nuclear reaction called the proton-proton chain converts four hydrogen nuclei (protons) into the nucleus of a helium atom. During the reaction some of the matter is converted to the energy of the sun. A star the size of the sun can exist in its present stable state for 10 billion years. Since the sun is already 5 billion years old, it is a "middle-aged" star.